Food Chemistry (Biochem 361) - Third Year PDF

Food Chemistry (Biochem 361) Third Year  Contents No Topic Page 1 Definition and history of food chemistry 1 2 Chemical Aspects of Food Components and 6 Additives in Quality and Safety 3 Balanced Diet...

Food Chemistry (Biochem 361) Third Year  Contents No Topic Page 1 Definition and history of food chemistry 1 2 Chemical Aspects of Food Components and 6 Additives in Quality and Safety 3 Balanced Diet 24 4 Additional diet is required during pregnancy 34 and lactation 5 Vegetarian diets & Nutrition Calculations 36 6 Body Composition Methods 38 7 Nutrition Calculations& Obesity 49 8 Exercise regularly to maintain ideal body 56 weight 9 Pre-cooking processes & Processed foods 60 11 Food poisoning 66 11 Food adulteration 75 12 Chemical analysis of constituents of 81 diet 13 MCQ questions 85 14 Practical part 97 15 References 131  Introduction Definition Foods may be defined as natural products, fresh or processed, which are consumed by human beings for their nourishment. Foods are composed of various chemical constituents including mainly carbohydrates, lipids, proteins, vitamins, minerals, and enzymes. All these chemical constituents when acting alone or when interacting with others play an important role for the changes of the physical and chemical properties of foods and food products during their production, handling, processing, storage, and distribution. The science of food therefore studies all these properties in foods. Food science is a discipline in which biology, physical sciences, and engineering are used to study the nature of foods, the causes of their deterioration, and the principles underlying food processing. Food science is thus a multidisciplinary subject which involves chemistry, microbiology, biology, engineering, as well as biotechnology, and all these different fields are interrelated. Chemistry is the science of the composition of matter and the changes in composition that occur under changing conditions). The developments in various branches of chemistry such as organic chemistry, analytical chemistry, and physical chemistry were considered as essential to the advances of food chemistry. Importance of Food Chemistry Food chemistry is one of the major disciplines of food science which is mainly focusing on changes in the composition and chemical, physical, and functional properties of foods and food products during various stages from farm to fork. Such changes would ultimately affect the quality attributes and sensory characteristics of foods as well as the safety aspect. Therefore, the application of food chemistry mainly focuses on improving food quality and safety for the consumers. Thorough knowledge of chemistry, biochemistry, botany, zoology, physiology, and molecular biology is the key criteria to understand the food chemistry.  Food chemistry does not only concern with the composition of food raw materials and end products but also with the desirable and undesirable reactions which are controlled by a variety of physical and chemical parameters.  There are extensive studies on the chemical composition of foods, micronutrients, contaminants, and additives as well as mechanisms of various reactions which affect the quality and safety of foods.  Food chemistry can provide an in-depth understanding of the principles and mechanisms involved in the various reactions that happened during food preparation, processing, and storage.  A thorough understanding of all these chemical reactions that occur in foods would allow us to maximize the production or preservation of health-promoting compounds while minimizing the formation of harmful substances during food handling and preparation. Some typical examples of these chemical reactions include:  Enzymatic and nonenzymatic browning (Maillard reaction)  lipid oxidation  Starch hydrolysis  Formation of trans-fatty acids  cross-linking and denaturation of proteins   Gel formation  starch retrogradation  Toughening and softening of meat texture, degradation of vitamins, development of food flavor and off-flavors  Formation of carcinogenic compounds during cooking  Migration of various chemicals from packaging materials to food and their interactions, as well as those measures taken to reduce deterioration of foods during handling, processing, and storage. History of Food Chemistry At some point in the distant past, our ancestors who were hunter-gatherers had harnessed fire and learned to cook raw foods and started to experiment primitive methods to preserve excess foods by trial and error. These events gave birth to the earliest knowledge of food science in which the chemistry of food was the major area. Later, as foods became more abundant due to agriculture and farming, people were motivated to make their diets more interesting and palatable and had the luxury to enjoy the variety of food. Without knowing the principles behind, our ancestors started to explore different cooking methods and treatments to make foods more digestible, to remove toxins, to improve its taste, and to keep it longer. Hence, traditional cooking and food preparation methods were developed in different regions and handed down to later generations. It is difficult to trace the origin of food chemistry, but it is closely related to the development of the different branches in modern chemistry and also biochemistry. One of the major areas in food chemistry is the study of food composition in terms of the different chemical components. It was not until in the eighteenth and nineteenth centuries that the quest for understanding of the chemical nature of our food by organic chemists had emerged. Firstly, the chemical structure and properties of macronutrients like carbohydrates, proteins, and lipids were characterized. This was followed by the discovery of various vitamins in the early twentieth century. Research on the biosynthesis of both macro- and micronutrients and their metabolism has also linked up food chemistry and food biochemistry together during the twentieth century. The chemical analysis of food (so-called wet chemistry) was developed in parallel with the study of food components at a similar time due to the demand to expose the malpractices of adulteration by food suppliers. With the advances in modern analytical instrumentation in the 1960s, food analysis has moved from the classical techniques of wet chemistry to sophisticated instrumental techniques using spectroscopy and spectrophotometry. The development of physical sciences in the field of texture and image analysis has allowed the sensory aspects of foods to be described in a more quantifiable manner and the physical characteristics of foods to be more well defined. Dietary goals 1. Maintenance of a state of positive health and optimal performance in populations at large by maintaining ideal body weight. 2. Ensuring adequate nutritional status for pregnant women and lactating mothers. 3. Improvement of birth weights and promotion of growth of infants, children and adolescents to achieve their full genetic potential. 4. Achievement of adequacy in all nutrients and prevention of deficiency diseases. 5. Prevention of chronic diet-related disorders. 6. Maintenance of the health of the elderly and increasing the life expectancy. Dietary guidelines Right nutritional behavior and dietary choices are needed to achieve dietary goals. The following 15 dietary guidelines provide a broad framework for appropriate action: 1. Eat variety of foods to ensure a balanced diet. 2. Ensure provision of extra food and healthcare to pregnant and lactating women. 3. Promote exclusive breastfeeding for six months and encourage breastfeeding till two years or as long as one can. 4. Feed home based semi solid foods to the infant after six months. 5. Ensure adequate and appropriate diets for children and adolescents, both in health and sickness. 6. Eat plenty of vegetables and fruits. 7. Ensure moderate use of edible oils and animal foods and very less use of ghee/ butter/ vanaspati. 8. Avoid overeating to prevent overweight and obesity. 9. Exercise regularly and be physically active to maintain ideal body weight. 10. Restrict salt intake to minimum. 11. Ensure the use of safe and clean foods. 12. Adopt right pre-cooking processes and appropriate cooking methods. 13. Drink plenty of water and take beverages in moderation. 14. Minimize the use of processed foods rich in salt, sugar and fats. 15. Include micronutrient-rich foods in the diets of elderly people to enable them to be fit and active.  Chemical Aspects of Food Components and Additives in Quality and Safety Water 1.1 Introduction Water is relatively small inorganic molecule, but organic life is highly dependent on this tiny molecule. It is the only substance on the earth that occurs abundantly in all three physical states (gas, liquid and solid). Water is essential for life as: (1) regulator of body temperature (2) solvent (3) carrier of nutrients and waste products (4) reactant and reaction medium (5) lubricant and plasticizer (6) stabilizer of biopolymer conformation (7) facilitator of the dynamic behavior of macromolecules (e.g. catalytic activity). Most of the fresh foods contain large amounts of water. It is one of the major component in composition of many foods. Each food has its own characteristic amount of this component. Effect of water on structure, appearance and taste of foods as well as their susceptibility to spoilage depends on its amount, location, and orientation. Therefore, it is essential to know its physical properties. Water has unusually high melting point, boiling point, surface tension, permittivity, heat capacity, and heat of phase transition values. Other unusual attribute of water include expansion upon solidification, large thermal conductivity compared to those of other liquids, moderately large thermal conductivity of ice compared to those of other nonmetallic solids. Water is essential to life. Water (H2O) contains strong covalent bonds that hold the two hydrogen atoms and one oxygen atom together. The bonds between oxygen and each hydrogen atom are polar bonds. The outer-shell electrons are unequally shared between the oxygen and hydrogen atoms, the oxygen atom attracting them more strongly than each hydrogen atom. As a result, each hydrogen atom is slightly positively charged and each oxygen atom is slightly negatively charged. Therefore they are able to form hydrogen bonds. The nature of hydrogen bonds allows water to bond with other water molecules as well as with proteins, pectin, sugar, and starches. Water is important as a solvent or dispersing medium, dissolving small molecules to form true solutions and dispersing larger molecules to form colloidal solutions depending on their particle size and solubility. Water Holding Capacity Term generally used to describe ability of a matrix of molecules to physically entrap large amounts of water in such a way that prevents exudation of the water. The food matrices that entrap water in this manner include pectin and starch gels and tissue cells of plant and animal. This physically entrapped water does not flow from food even when they are cut or minced. But this water behaves almost like pure water during food processing operations like drying, freezing, etc. it is also available as a solvent. Thus, bulk flow of this water is restricted, but movement of individual molecules almost remains same as that of water molecules in a dilute solution. Impairment in this entrapment of water (i.e. holding capacity) of foods has a significant effect on quality of food. Some of the typical examples are oozing out of liquid from gel (syneresis) and exudation of liquid on thawing of frozen foods. Water is an essential constituent of many foods. It may occur as an intracellular or extracellular component in vegetable and animal products, acting as a dispersing medium or solvent in a variety of products, as the dispersed phase in some emulsified products such as butter, and as a minor constituent in other foods. Water is the major component of many foods, with its content varying from food to food. The water content of some food and food products is as follows: meats (50–82 %), fruits (80–95 %), vegetables (70–95 %), beer (90 %), milk (84–86 %), bread (30–35 %), butter (16 %), milk powder (4–5 %), and anhydrous milk fat (0.5 %). Water in the proper amount, location, and orientation profoundly influences the structure, appearance, and taste of foods and their susceptibility to spoilage. Water activity (a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water) has a profound effect on the rate of many chemical reactions (e.g., hydrolytic reactions, nonenzymatic browning, lipid oxidation, color reaction) in foods and on the rate of microbial growth. Carbohydrates Carbohydrates come in various shapes and sizes, from small sugar molecules to complex polymers containing thousands of simple sugar units. Important food carbohydrates include simple sugars, dextrins, starches, and nonstarch polysaccharides including celluloses, hemicelluloses, pectins, and gums. Carbohydrates are the most abundant food component and the most important energy source (4 kcal/g) in our diet. Carbohydrates are defined as polyhydroxy aldehydes or ketones and their derivatives. The latter ones are an essential source of fiber in the diet. Carbohydrates are important constituents of foods not only because of their nutritive values but also because of their functional properties. Carbohydrates, especially polysaccharides, can be used as sweeteners, thickeners, stabilizers, gelling agents, and fat replacers. They are being used in a wide spectrum of convenience foods. The intensity of sweetness of sugars varies depending on the specific sugar (e.g., fructose is the sweetest whereas lactose is the least sweet). The high water solubility of sugars can make syrups easily. However, sugars form various crystals as water gets evaporated or depending upon the saturation of the solution. Depending upon the type of product, sugar crystallization is desirable (e.g., candy making) or undesirable (e.g., sweetened condensed milk, ice cream). A high concentration of sugar reduces the water activity in certain food products like jams, jelly, and sweetened condensed milk which prevents the growth of spoilage microorganisms. The addition of sugar makes food more viscous and provides body and mouthfeel to foods. Nowadays, consumers are more health conscious, but the replacement of sugar by nonnutritive sweeteners could not totally satisfy consumers who demand food body fullness. Hence, the addition of various gums or starches is necessary to get the desired body of the product. The most common energy source of microorganisms is carbohydrates which are metabolized into various components like lactic acid, acetic acid, pyruvic acid, and carbon dioxide. Sugar alcohols such as mannitol, sorbitol, and xylitol are obtained from the reduction of the carbonyl group to a hydroxyl group of the reducing sugar. These sugar alcohols are not readily fermented by microorganisms and are used in chewing gums because of their tooth decay prevention and can be used to replace sugar in noncaloric foods. The reducing sugar, having a free carbonyl group, undergoes reaction with free amino acid groups of protein leading to the formation of various favor and color compounds (e.g., melanoidins – a brown pigment). The interaction of the carbonyl group with amino acids is known as Maillard reaction which can impart a brown color to food products such as bakery products, UHT milk, and milk powder during their production and subsequent storage period. Maillard reaction may also lead to the formation of various toxic compounds and loss of the available lysine. On the other hand, at extremely high temperatures, sugars alone can be decomposed to produce brown color compounds, and the reaction is popularly known as caramelization, which is a nonamino acid type of browning. Moreover, lactose undergoes thermal degradation with the production of various organic acids such as formic acid, lactic acid, pyruvic acid, levulinic acid, and acetic acid which can lower the pH of milk during heat treatment leading to heat coagulation of milk and milk products. Lipids Food lipids are esters formed by fatty acids and glycerol and are commonly known as triglycerides or triacylglycerides. They are a heterogeneous group of naturally occurring substances which are sparingly soluble in water but soluble in organic solvents such as ether, chloroform, acetone, and benzene. Up to 99 % of the lipids in plant and animal materials are consisted of triglycerides known as fats and oils. At room temperature, fats are solid while oils are liquid. The fatty acids in triglycerides can be saturated and unsaturated, depending on the number of carbon carbon double bonds in the hydrocarbon chain. Lipid is a principal dietary component of energy source and reserve which provides (9 kcal/g) energy. The lipid content of foods can range from very low to very high in vegetable and animal products such as haddock (0.1 %), cod (0.4 %), barley (1.9 %), milk (4–6 %), chicken (7 %), cheese (25–30 %), butter (80 %), and ghee/butter oil (99.5 %). Lipids also carry the fat-soluble vitamins A, D, E, and K as well as provide essential fatty acids for our body. Fatty acids such as monounsaturated fatty acid and conjugated linolenic acid (CLA) have beneficial effects to human health such as a decreased risk of coronary heart disease. Apart from its energy and nutritional values, lipid plays an important functional role in foods by providing mouthfeel, palatability, texture, and aroma. Lipids provide either tenderization (e.g., oil pie crusts) or flakiness (puff pastry) that imparts distinct characteristics to a food product. However, health- conscious consumers often demand food products that have reduced fat, low-fat, or no-fat formulations produced by substituting lipids with a variety of fat replacers derived from carbohydrates, proteins, or fats. The fatty acid profile of individual fats and oils is unique, and hence the measure of various parameters related to fatty acid composition (e.g., melting point, saponification number, iodine value, amount of short-chain fatty acids, Reichert Meissl and Polenske value, refractive index) is often used to check adulteration of lipids in the food products. The melting point of fat is usually not sharp but is within a certain range due to variation in low-, medium-, and high-melting triglycerides. Milk fat has a wide melting range (e.g., 40 _C to +40 _C), whereas chocolate fat has a narrow melting range (close to body temperature) to allow its melting in our mouth. Fat has the ability to form different crystals upon cooling. Controlled crystallization of lipids can improve the functional properties in foods because small fat crystals can give a smooth texture to products like butter and anhydrous milk fat. Food lipids are subjected to a number of chemical reactions that would affect their quality and applications. Lipid oils can be modified by hydrogenation process to reduce the number of double bonds to be used as solid fats in margarine. Interesterification of lipids can be used to produce more spreadable butter. Fats and oils undergo various rancidification reactions. There are different types of rancidity that can take place in food products having high fats and oils: hydrolytic (due to action of lipase/lipolysis), ketonic (due to growth of penicillin mold), and oxidative (due to chemical reaction with oxygen) rancidity. Limited lipolysis is essential in certain varieties of cheese for the production of typical flavors but can also be detrimental in anhydrous milk fat which leads to the production of short-chain fatty acids giving a butyric and offensive smell. The oxidative deterioration of fats/oils can lead to the formation of various off-flavors as well as generation of various toxic compounds. Certain fats/oils that have undergone repeated deep-frying processes can form harmful chemicals including various epoxides, free radicals, and other toxic compounds, causing deterioration in the quality of food products. Proteins Proteins are common constituent of all biological materials, without which life is not possible. They are essential constituent of all living cells. A complex nitrogenous organic compound – a polymer of amino acids - therefore defined as high molecular weight polymers of low molecular weight monomers known as amino acids, which are linked together by peptide bonds. Proteins are polymers of some 20 different amino acids joined together by peptide bonds (primary structure). The amino acid composition establishes the nature of secondary and tertiary structures. These, in turn, significantly influence the functional properties of food proteins and their behaviour during processing. Classification of Proteins Proteins have been classified in many ways. Generally they are classified on the basis of composition, shape of molecules and solubility. On the basis of composition On the basis of composition proteins are classified into three groups viz. simple proteins, conjugated proteins and derived proteins. 1. Simple proteins These are the proteins which consist of only amino acids – They do not contain other class of compounds. 2. Conjugated proteins These are the proteins which consist of amino acids as well as other class of compounds.They are further classified into six subgroups. Table: Conjugated proteins Sr. Class Other compound Example No. present 1 Chromoprotein Coloured pigment Haemoglobin 2 Glycoprotein Carbohydrate Mucin (in saliva) 3 Phosphoprotein Phosphoric acid Casein (in milk) 4 Lipoprotein Lipid Lipovitelin (in egg yolk) 5 Nucleoprotein Nucleic acid Viruses 6 Metalloprotein Metal Ciruloplasmin (Cu) 3. Derived proteins They represent various stages of hydrolytic cleavage of simple or conjugated proteins. e.g. proteoses, peptones, peptides, etc. On the basis of shape of molecules On the basis of shape of molecules, proteins are classified into two main groups viz. fibrous proteins and globular proteins. 1. Fibrous proteins-Fibrous proteins are long and thread or ribbon like and tend to lie side by side to form fibers. They are generally insoluble in water as the intermolecular forces in these proteins are rather strong. They serve as the chief structural material of animal tissues. Examples are keratin, myosin, collagen etc. 2. Globular proteins-Globular proteins are spheroidal in shape. They are generally soluble in water or aqueous solution of acids, bases or salts as intermolecular forces in these proteins are relatively weaker. These proteins are generally involved in physiological processes of the animal body. Examples are enzymes, some hormones, haemoglobin, etc. On the basis of solubility On the basis of solubility proteins are classified into the following groups. 1. Albumins- These proteins are soluble in distilled water, dilute salt, acid and base solutions. Examples are lactalbumin, egg albumin. 2. Globulins- These proteins are insoluble in distilled water, but soluble in dilute salt, acid and base solutions. Examples are serum globulins and β- lactoglobulin in milk, myosin and actin in meat. 3. Protamine and Histones- These proteins are highly soluble in distilled water. These are small molecules, stable to heat (i.e. not coagulated by heat). Protamine soluble in NH4OH, whereas histones are insolubleNH4OH. 4. Glutelins- These proteins are insoluble in distilled water and alcohol but soluble in dilute acid and base solution. Examples are glutenin in wheat,oryzenininrice. 5. Prolamins- These proteins are insoluble in distilled water, but soluble in dilute acid, dilute base and 70-80% alcohol. Example are zein in corn, gliadin in wheat. 6. Scleroproteins- These proteins are insoluble in most of the solvents like water, dilute acid, dilute base, dilute salt solution etc. They are generally fibrous proteins serving structural and binding purposes. Examples are collagen, elastin, keratin. Proteins play a central role in biological systems. They are utilized in the formation and regeneration of body muscle. Certain specific proteins serve as enzymes, while others serve to provide functions in metabolic regulations. The energy provided by proteins is 4 kcal/g. Proteins are polymers of different amino acids joined together by peptide bonds. Because of the various side chains that are linked to different amino acids, proteins have different chemical properties. Proteins have four types of structure including primary, secondary, tertiary, and quaternary structure. These structures are stabilized by peptide bonds, hydrogen bonds, disulfide bonds, hydrophobic interactions, ionic interactions, and van der Waals interactions. Knowledge of protein conformation and stability is essential to understanding the effects of processing on food proteins. Food proteins may be defined as those that are easily digestible, nontoxic, nutritionally adequate, functionally useable in food products, and available in abundance. Traditionally, milk, meats (including fish and poultry), eggs, cereals, legumes, and oilseeds have been the major sources of food proteins. Several factors, such as content of essential amino acids and digestibility, contribute to the differences in the nutritive values of proteins. Cereal proteins from wheat, maize, rice, and barley are richer in methionine but are very low in lysine, while legume proteins are deficient in methionine but are higher in lysine content. The nutritional quality of a protein that is deficient in an essential amino acid can be improved by mixing it with another protein that is rich in that essential amino acid. Food proteins of animal origin are more completely digested than those of plant origin. Proteins of animal origin, such as milk (caseins), egg, and meat proteins, are widely used in fabricated foods. These proteins are mixtures of several proteins with wide- ranging physicochemical properties, and they are capable of performing multiple functions. Plant proteins (e.g., soy and other legume and oilseed proteins) are used to a limited extent in conventional foods. Proteins have many useful functional properties in foods such as hydration, emulsification, gelling, and foaming. Therefore, they can be used as thickeners, binding and gelling agents, as well as emulsifiers or foaming agents. Proteins generally have a great influence on the sensory attributes such as texture, flavor, color, and appearance of foods. For example, the textural and curd-forming properties of yogurt are due to the unique colloidal structure of casein micelles; the sensory properties of bakery products are related to the doughforming properties of wheat gluten; and the succulent characteristics of meat products are largely dependent on muscle proteins. Proteins are vulnerable to many chemical reactions that would affect their nutritional and functional properties. For example, proteins can undergo several chemical alterations involving lysine residues when exposed to high temperatures and alkaline pH. Such alterations reduce their digestibility. The reaction of reducing sugars with ε-amino groups (e.g., Maillard browning) also decreases digestibility of lysine and produces certain toxic compounds. Fermentation of proteins leads to the production of various bioactive peptides which have health benefits. Moreover, excessive proteolysis in cheese is responsible for the production of peptides with bitter taste. Various processing treatments can lead to various chemical reactions involving protein-protein and protein-lipid interactions affecting both functional properties and nutritive values of food proteins. Minerals Minerals usually refer to elements other than C, H, O, and N that are present in foods. The mineral material may be present as inorganic or organic salts or may be combined with organic material. Major minerals include calcium, phosphorus, magnesium, sodium, potassium, and chloride. Trace elements include iron, iodine, zinc, selenium, chromium, copper, fluorine, lead, and tin. Minerals play important roles in both living organisms and foods. Minerals are chemically inert to heat, light, oxidizing agents, and extreme pH. Minerals can, however, be removed from foods by leaching or physical separation. The most important factor causing mineral loss in foods is milling of cereals and rice. Hence fortification is generally carried out in certain foods to compensate for the loss of iron. Calcium and phosphate are present in colloidal form with casein micelles, but they can be removed and transferred to the soluble aqueous phase during acidification. Loss of calcium from milk can occur when whey is drained during cheese making. Moreover, calcium plays a significant role in cheese making, and hence sometimes calcium chloride is added in milk to make a firmer body of cheese. The presence of calcium, magnesium, phosphate, and citrate in milk is responsible for the heat stability of milk and milk products. Moreover, citrate plays a key role during fermentation of milk in the preparation of dahi and yogurt. Phosphate is a food additive having multifunctions including acidifying (soft drinks), buffering (various beverages), anticaking, leavening, stabilizing, emulsifying, water binding, and protecting against oxidation. Minerals have a tendency to interact with other food components which affect the physical and chemical properties of foods. For example, iron and copper are considered as prooxidants and are responsible for various oxidative deteriorations in high-fat food products. Iron serves as a color modifier in meat and has the ability to form blue, black, or green complexes with polyphenol compounds. For applications in the food industry, nickel is used in the hydrogenation of vegetable oil and copper can be used to produce heat-stable color pigment by replacing magnesium in chlorophyll. Vitamins Although vitamins are only a minor constituent in foods, they play an essential role in human nutrition. Some vitamins function as part of a coenzyme whereas others occur in foods as provitamins. They fall into two groups: water-soluble and fat-soluble vitamins. Sources of vitamins are from both animal and plant products. Milk and dairy products contain riboflavin, pyridoxine, and vitamins B12,A,D,E, and K; fish, poultry, and meats provide riboflavin, niacin, biotin, thiamin, vitamin B12, and pyridoxine; fruits and vegetables are rich in vitamins A, K, and C, folate and riboflavin; bread and cereals contain thiamin, folate, pantothenic acid, niacin, biotin, riboflavin, and pyridoxine, while fats and oils contain vitamins A, D, E, and K. Chemically, many vitamins are unstable during thermal processing and storage. Vitamin A and carotenoids are relatively stable to heat in the absence of oxygen but quite susceptible to oxidation in the presence of light due to their unsaturation. Vitamin D is also susceptible to degradation by light. Certain milk products like UHT milk and milk powders are fortified with vitamins A and D to compensate the losses during heating. Vitamin E can act as antioxidants which provide stability of highly unsaturated vegetable oils. Vitamin K is quite stable to heat treatment but a certain fat substitute has been reported to impair vitamin K absorption. Vitamin C or ascorbic acid is widely distributed in nature, mostly in plant products such as citrus fruits, green vegetables, tomatoes, and berries. Ascorbic acid is commonly used as a food ingredient/additive because of its reducing and antioxidative properties. Ascorbic acid also prevents enzymatic browning, inhibits nitrosamine formation in cured meats, and contributes to the reduction of metal ions. Nevertheless, vitamin C is the least stable of all vitamins and is easily destroyed during thermal processing and storage. Water-soluble vitamins are easily leached out during processing treatments. Enzymes Enzymes are proteins with catalytic properties. Although enzymes are only minor constituents of many foods, they play a major role in foods. Enzymes that are naturally present in foods can cause both desirable and undesirable changes in food composition. Since very often these changes are undesirable, the responsible enzymes must be deactivated. Since enzymes are proteinaceous in nature, various chemical agents and physical factors such as heat, strong acids and bases, organic solvents can denature them and destroy their activity. Some examples of the chemical reactions involved by these enzymes include oxidation catalyzed by lipid peroxides, lipolysis catalyzed by lipases, and enzymatic browning catalyzed by polyphenol oxidases. Lipases and lipoxygenase are responsible for the formation of short-chain fatty acids and other off-flavor products that are responsible for rancidity in high-fat food products. Polyphenol oxidases are responsible for browning in cut fruits and vegetables after exposure to oxygen. On the other hand, food enzymes can have many useful applications. For instance, amylases which are widely found in plants, animals, and some microorganisms are used in the brewing and baking industries. Proteolytic enzymes are used in meat tenderization and cheese production. Peroxidases are responsible for the bleaching of flour during natural aging. Glycolytic enzymes are responsible for the development of rigor mortis in fish and seafood products. Food Additives Many chemical substances are incorporated into foods for functional purposes, and in many cases, these ingredients can also be found occurring naturally in some foods. However, when they are used in processed foods, these chemicals are known as food additives. According to the FDA, food additives are substances added to foods for specific physical or technological effects. They may not be used to disguise poor quality but may aid in preservation and processing or improve the quality factors of appearance, flavor, nutritional value, and texture. Based on their specific functions in the food products, food additives include anticaking/free-flow agents (e.g., calcium silicate), antimicrobials (e.g., benzoic acid), antioxidants (e.g., vitamin E), emulsifiers (e.g., lecithin), stabilizers (e.g., carboxymethylcellulose), humectants (e.g., glycerol monostearate), bleaching/ maturing agents (e.g., benzoyl peroxide), bulking agents (e.g., sorbitol), firming agents (e.g., calcium chloride), flavoring agents (e.g., aldehydes)/flavor enhancers (e.g., monosodium glutamate), coloring agents (e.g., anthocyanins), curing agents (sodium nitrate), dough conditioners/improvers (e.g., ammonium chloride), leavening agents (e.g., ammonium bicarbonate), fat replacers (sucrose polyester), sweeteners (e.g., high-fructose corn syrup), low-calorie sweeteners (e.g., aspartame), and so on. Food additives can be used to maintain the nutritional quality (e.g., use of antioxidants), increase the shelf life and stability (e.g., use of antimicrobial agents), enhance the appearance (use of coloring, flavoring agents, stabilizers, bleaching agents), and act as processing aids (e.g., use of acids, buffers, sequestrants) in food production.  Balanced Diet Nutrient requirements and recommended dietary allowances (RDA) Requirements are the quantities of nutrients that healthy individuals must obtain from food to meet their physiological needs. The recommended dietary allowances (RDAs) are estimates of nutrients to be consumed daily to ensure the requirements of all individuals in a given population. The recommended level depends upon the bioavailability of nutrients from a given diet. The term bioavailability indicates what is absorbed and utilized by the body. In addition, RDA includes a margin of safety, to cover variation between individuals, dietary traditions and practices. The RDAs are suggested for physiological groups such as infants, pre-schoolers, children, adolescents, pregnant women, lactating mothers, and adult men and women, taking into account their physical activity. In fact, RDAs are suggested averages/day. However, in practice, fluctuations in intake may occur depending on the food availability and demands of the body. But, the average requirements need to be satisfied over a period of time. Our diet must provide adequate calories, proteins and micronutrients to achieve maximum growth potential. Therefore, it is important to have appropriate diet during different stages of one‟s life. There may be situations where adequate amounts of nutrients may not be available through diet alone. In such high risk situations where specific nutrients are lacking, foods fortified with the limiting nutrient(s) become necessary. A good example of such fortified foods is the salt fortified with iron and iodine. When it comes to your diet, the most current advice is perhaps the kind that begins with 'eat less' or 'restrict fat'. I've never been convinced and I'm not alone. Most of us may feel overwhelmed with conflicting nutrition and diet opinions but I've learnt that deprivation is not the solution, creating a balance is. It is essential to get the right type and amount of foods to support a healthy lifestyle. Table: Classification of food according to it is function A balanced diet A diet that focuses on providing all the nutrients that your body needs. It comprises of macronutrients like protein, carbohydrates and fat along with micronutrients which include vitamins and minerals. Each of them has a different role to play in maintaining various body functions. These nutrients are derived through a combination of the five major food groups - fruits and vegetables, cereals and pulses, meat and dairy products and fats and oils. The rules seem simple but that's not the whole story - how much do you need daily, when is the best time to eat proteins or carbs and what should the portion size be? The 40/30/30 Diet is balanced between carbohydrates, proteins and fats, thus not causing cravings for any of these three nutrients through deprivation. This balance of nutrients is designed to switch the body into a fatburning mode. In the diet, 40 percent of total calories are derived from carbohydrates in the form of mostly slow- acting or lowglycemic starches, vegetables, grains, beans and fruit; 30 percent from unprocessed and quality fats and oils such as olive oil, nuts, avocado and the natural form of tropical oils; and 30 percent from lean, complete protein in lowfat cottage cheese, lean red meat, poultry, seafood, fish, soy and whey. This diet has been proven over a period of more than five years by various researchers in both research projects and training programs for international- level athletes. Participants typically lose body fat and unhealthy weight, gain muscle mass, and raise their HDL (“good” cholesterol) levels. Our bodies consume blood sugar as fuel for energy, but store excess fuel as body fat due to that remarkable storage hormone produced by the pancreas known as insulin. How much simple sugar (white sugar, for instance), then, should our diet contain to deliver that correct level of blood sugar? The answer is: none at all. At a stable blood sugar level our bloodstream contains about two teaspoons of glucose. Since all carbohydrates -- including such unlikely candidates as lemons and spinach -- once digested, are absorbed into our bloodstream as glucose, our bodies‟ sugar requirements are easily met. And, unfortunately, easily exceeded. Our bodies do not need any pure sugar at all, nor do they need processed carbohydrates like white rice, white-flour pasta, bagels, breads and cereals, which raise blood sugar levels more than sugar itself. The digestion of slow- acting or low-glycemic complex carbohydrates, proteins, and fats provides an adequate supply of blood sugar. In fact, the right amounts of protein and dietary fat are crucial because they slow down the entry of carbohydrates into the system and allow for extended hunger satisfaction. Dietary fat is actually the best blood sugar stabilizer, while protein is considered neutral in this respect. The longings for particular foods that we so often feel when following some new diet or nutritional recommendations are indicative that what we are eating is unbalanced in some way. A balanced diet should not leave us with cravings for some food type not included. The optimal diet is one in which you do not have to use will power to succeed because balancing the body‟s chemistry is the key. The bottom line is that too much or too little of the wrong kinds of carbohydrates, proteins, or fats can cause health problems. The following is a simple overview of our bodies‟ nutritional requirements. Carbohydrates: The truth about carbs may be hard to digest but nutritionists say they're an important part of a healthy diet. Carbohydrates are your body's main source of energy. In India, 70-80% of total dietary calories are derived from carbohydrates present in plant foods such as cereals, millets and pulses. "Half of your total calories of the day should come from carbs. The problem is that we emphasize more on refined carbs in the form of breads, biscuits, white rice and wheat flour. We forget that carbs come from other healthier sources like whole grains which include brown rice, millets and oats that have a higher nutritive value. These are also great sources of fiber. Your meal would be incomplete without fiber - both soluble and insoluble. It helps with digestion but few people are getting enough. Eat, don't drink your fruits and vegetables. Most fruits and vegetables (besides potatoes and corn) and whole grains are also foods with a low glycemic index which means that they don't cause sudden spikes in blood sugar levels and help maintain them. The National Institute of Nutrition (NIN) suggests 30 grams of cereals and millets along with 100 grams of starchy vegetables. (5 Fiber-Rich Foods You Should be Eating Everyday) "Your breakfast should definitely have cereal or bananas or some form of good carbs that keeps you fuelled until lunch," she suggests. Don't curfew carbs, it's all about quality and quantity. Simple carbohydrates like glucose and fructose are found in fruits, vegetables and honey, sucrose in sugar and lactose in milk, while the complex polysaccharides are starches in cereals, millets, pulses and root vegetables and glycogen in animal foods. Recommended dietary allowance- Men: 2320 Kcal/day Female: 1900 Kcal/day Proteins About 30 to 35% of your diet should consist of protein. This could be in the form of pulses, milk, leafy greens, eggs, white meat or sprouts." I'd agree since protein is the main component of all of your body's cells, as well as your hair, skin and soft tissues. Moreover, we burn more calories in digesting proteins than carbs. Since men tend to be muscular and usually weigh more than women, they require more protein. Protein deficiency in our country and recommends that we should have one helping of protein with every meal, be it in any form like whole dals, cottage cheese or gram flour or 30 grams of pulses as per NIN. A recent survey conducted by the Indian Market Research Bureau revealed that 9 out of 10 people of the sample consumed inadequate amount of protein. This could be due to the increasing consumption of convenience foods that are high in carbs and sugars and low in protein. Recommended dietary allowance - Men: 60 grams/day Female: 55 grams/day Fats: Fats provide energy, store vitamins and synthesize hormones. According to NIN, about 1/5th of your diet or 20% should be devoted to fats all three kinds -polyunsaturated, monosaturated and omega-3 fatty acids. Vegetable oil used in day to day cooking is a major source of visible fat in our diet. To ensure optimal fat quality the use of a combination of vegetable oils is important. The thumb rule - don't fear trying different oils. It is suggested to have a good blend of various types of oils in your diet. You could juggle between butter, ghee, olive oil, mustard oil, soyabean, sesame or even groundnut oil for different meals, suggests Dr. Shikha Sharma. Depend more on unrefined (Kachi Ghani) or cold pressed oils versus refined oils, goes without saying but that always seems to be a struggle. Vitamins and Minerals: These micronutrients support metabolism, nerve and muscle function, bone maintenance and cell production. Minerals are inorganic and so minerals from plants, meat and fish easily find their way into body. Vitamins are fragile compounds and it's difficult to shuttle them as they may be destroyed during cooking or storing. They can be derived from nuts, oilseeds, fruits and green leafy vegetables. Vitamin A, E, B12and D are vital and so is calcium and iron. The National Institute of Nutrition recommends the consumption of 100 grams of greens and 100 grams of fruits each day. In India, iron deficiency or anaemia affects about 50% of the population, more women than men. "Since women go through several hormonal changes from pregnancy to menstrual and menopause, they need to maintain a steady dose of calcium, Vitamin D, folic acid, iron and biotin," says Dr. Shikha Sharma. Another crucial aspect that Dr. Shikha throws the spotlight on is the need to drink adequate water. Lack of it can lead to acidity and water retention. Anywhere between six to eight glasses of water is needed to keep your body hydrated. Recommended Dietary Allowance of Calcium - (100 grams milk and milk products) Men: 600 mg/day Female: 600 mg/day Recommended Dietary Allowance of Iron - Men: 17 mg/day Female: 21mg/day Choose wisely To keep your body running smoothly, you require three main meals coupled with healthy snacking to curb cravings. Ideally, breakfast should be the heaviest meal of the day but with our busy schedules all we manage to do is chug a glass of milk and grab a toast. When your day starts on a light note followed by a hurried lunch, you end up eating much more for dinner than needed. While dinner should be the lightest, in a common Indian household, it is an elaborate family meal. Time to change. The components of the balanced diet remain the same, the difference lies in how they're served at every meal. Dr. Gargi Sharma guides us to create an ideal routine. Breakfast: A good morning meal should comprise of three things. These are dietary fiber or carbohydrates (whole-grain bread, oatmeal, white oats, wheat flakes), proteins (eggs and egg whites, yoghurt, milk and sprouts) and nuts (almonds, walnuts, apricots and figs). This way you'll eat fewer calories the rest of the day. Lunch: Make it a mix of high-fibre whole grains like brown rice, barley or jowar, starchy carbs and some good source of proteins like cottage cheese, pulses, chicken or fish. Include some probiotics like yoghurt or buttermilk and fibre from fresh salads to complete your meal. Dinner: Pick foods with a high satiety value that keep you full for longer and curb midnight binging. Fill your plate with greens to load up on vitamins and minerals. Limit carbs but don't cut them off. Combine them with some healthy fats like fish, nuts and seed oils. Your body can use these for regeneration and repair overnight. Don't give up on snacking. It supplies the quick 'pick-me-up' you need. Trade the junk for fresh fruits, crudités with hung curd dip, nuts or a salad. Eating small yet frequent meals is the ideal way. This doesn't mean you eat more but spread your daily requirements throughout the day. Why additional diet is required during pregnancy and lactation? Pregnancy is a demanding physiological state. In India, it is observed that diets of women from the low socioeconomic groups are essentially similar during prepregnant, pregnant and lactating periods. Consequently, there is widespread maternal malnutrition leading to high prevalence of low birth weight infants and very high maternal mortality. Additional foods are required to improve weight gain in pregnancy (10-12 Kg) and birth weight of infants (about 3 Kg). What are the nutrients that require special attention? The daily diet of a woman should contain an additional 350 calories, 0.5 g of protein during first trimester and 6.9 g during second trimester and 22.7 g during third trimester of pregnancy. Some micronutrients are specially required in extra amounts during these physiological periods. Folic acid, taken throughout the pregnancy, reduces the risk of congenital malformations and increases the birth weight. The mother as well as the growing fetus needs iron to meet the high demands of erythropoiesis (RBC formation). Calcium is essential, both during pregnancy and lactation, for proper formation of bones and teeth of the offspring, for secretion of breast- milk rich in calcium and to prevent osteoporosis in the mother. Similarly, iodine intake ensures proper mental health of the growing fetus and infant. Vitamin A is required during lactation to improve child and C need to be taken by the lactating mother. survival. Besides these, nutrients like vitamins B How can the pregnant and lactating women meet these nutritional demands? The pregnant/lactating woman should eat a wide variety of foods to make sure that her own nutritional needs as well as those of her growing foetus are met. There is no particular need to modify the usual dietary pattern. However, the quantity and frequency of usage of the different foods should be increased. She can derive maximum amount of energy (about 60%) from rice, wheat and millets. Cooking oil is a concentrated source of both energy and polyunsaturated fatty acids. Good quality protein is derived from milk, fish, meat, poultry and eggs. However, a proper combination of cereals, pulses and nuts also provides adequate proteins. Mineral and vitamin requirements are met by consuming a variety of seasonal vegetables particularly green leafy vegetables, milk and fresh fruits. Bioavailability of iron can be improved by using fermented and sprouted grams and foods rich in vitamin C such as citrus fruits. Milk is the best source of biologically available calcium. Though it is possible to meet the requirements for most of the nutrients through a balanced diet, pregnant/lactating women are advised to take daily supplements of iron, folic acid, vitamin B and calcium (Annexure 3). What additional care is required? Adequate intake of a nutritious diet is reflected in optimal weight gain during pregnancy (10 kg) by the expectant woman. She should choose foods rich in fibre (around 25 g/1000 kcal) like whole grain cereals, pulses and vegetables, to avoid constipation. She should take plenty of fluids including 8-12 glasses of water per day. Salt intake should not be restricted even to prevent pregnancy-induced hypertension and pre-eclampsia. Excess intake of beverages containing caffeine like coffee and tea adversely affect fetal growth and hence, should be avoided. In addition to satisfying these dietary requisites, a pregnant woman should undergo periodic health check-up for weight gain, blood pressure, anaemia and receive tetanus toxoid immunization. She requires enough physical exercise with adequate rest for 2-3 hrs during the day. Pregnant and lactating women should not indiscriminately take any drugs without medical advice, as some of them could be harmful to the fetus/baby. Smoking and tobacco chewing and consumption of alcohol should be avoided. Wrong food beliefs and taboos should be discouraged. The most important food safety problem is microbial food borne illness and its prevention during pregnancy is one of the important public health measure. Avoiding contaminated foods is important protective measure against food borne illness.  Vegetarian diets Vegetarian diets can be quite varied. They can contain low amounts of or no animal products. They are personally chosen, culturally determined, or mandated by scarcity. Vegans  They are strict vegetarians.  They consume no animal sources of food.  They are at the highest risk for nutritional problems. Lacto-vegetarians, ovo-vegetarians, or lactoovo-vegetarians will consume milk, eggs, or milk and eggs, respectively. Some vegetarians will not eat meat or poultry but will eat fish. Other variations of vegetarian diets occur. Macronutrients Carbohydrates and fats (oils) are plentiful. Complete proteins may require using:  Soy protein (e.g., soy milk and tofu).  Simultaneous consumption of combinations of legumes/lentils, nuts/seeds, grains, and/or selected vegetables such as potatoes. Intake of several minerals may be limited. Vegetarians not eating dairy products should:  Eat enough vegetables containing significant calcium, e.g., spinach, turnip and collard greens, kale, and broccoli.  Eat calcium fortified foods such as tofu, soy milk, and fortified fruit juices. Iron and zinc  High phytate content of whole grains and legumes may decrease absorption.   Good vegetable sources of iron include: cooked legumes (beans, peas, lentils), enriched cereals, dark leafy green vegetables, whole grain products, and dried fruit.  Eating fruits and vegetables with high vitamin C content helps use iron.  Good vegetable sources of zinc include whole grains, soy products, nuts, and wheat germ. Intake of several vitamins may be limited. B vitamins  Vitamin B12 is not present in foods from plants.  Riboflavin may be deficient if little milk is consumed.  Vegetarians can obtain these vitamins from enriched cereals, fortified soy products, or dietary supplements. Adequate intake of vitamin D may be a challenge.  Food sources include fortified milk (if consumed), enriched cereals, fortified juice, or supplements.  Unblocked sun exposure between the 40- degree latitude parallels provides vitamin D. This exposure is enough year-round for young to middle-aged persons. This exposure may not be enough in older people due to decreased synthesis in the sun. They must rely on supplements.  Body Composition Methods Introduction The recent rise in prevalence of type 2 diabetes is concomitant with the sharp rise in obesity in the United States and other developed countries. Changes in body composition that accompany the onset and progression of obesity have a dramatic impact on metabolism and insulin sensitivity. Adipose tissue is postulated to be a key factor in regulating whole body lipid flux, thus modulating lipid and glucose homeostasis. Given the role of fat and lean tissue in lipid metabolism and insulin resistance, it is clear that assessing the body‟s tissue composition is an important part of the management of the diabetic patient. We provide here the most common methods for assessing body composition, including anthropometry, body density, and dual-energy X-ray absorptiometry (DXA). The human body can be quantified at several levels, depending on clinical concerns. Body composition can be assessed at the atomic level with the basic elements of carbon, calcium, potassium, and hydrogen; at the molecular level by amounts of water, protein, and fat; at the cellular level with extracellular fluid and body cell mass; and at the tissue level for amounts and distributions of adipose, skeletal, and muscle tissues. Indirect Methods Anthropometry Anthropometric measurements are the most basic method of assessing body composition. Anthropometric measurements describe body mass, size, shape, and level of fatness. Because body size changes with weight gain, anthropometry gives the researcher or clinician an adequate assessment of the overall adiposity of an individual. However, the associative power among anthropometric measures and indices is altered as weight is gained or lost. Standardized anthropometric techniques are necessary for comparisons between clinical and research studies, and video and text media describing these techniques are available. Those interested in using anthropometric equipment and methods should consult these resources. Weight, Stature, and Body Mass Index (BMI). Body weight is the most frequently used measure of obesity. In general, persons with high body weights typically have higher amounts of body fat. A variety of scales are available for measuring weight, and these should be calibrated regularly for accurate assessments of weight. Changes in weight correspond to changes in body water, fat, and/or lean tissue. Weight also changes with age in children as they grow and in adults as they accumulate fat. However, body weight taken without other measures of body size is misleading because a person‟s weight is highly related to stature (i.e., tall people are generally heavier than short people). Stature is measured easily with a variety of wall-mounted equipment. Additional methods have been developed for predicting stature when it cannot be measured directly, e.g., for the handicapped or mobility impaired. One way to overcome the lack of specificity in body weight is to use the body mass index. BMI is a descriptive index of body habitus that encompasses both the lean and the obese16 and is expressed as weight divided by stature squared (kg/m2). A significant advantage of BMI is the availability of extensive national reference data and its established relationships with levels of body fatness, morbidity, and mortality in adults.16 BMI is particularly useful in monitoring the treatment of obesity, with a weight change of about 3.5 kg needed to produce a unit change in BMI. In adults, BMI levels above 25 are associated with an increased risk of morbidity and mortality,17 with BMI levels of 30 and greater indicating obesity.18 In children, BMI is not a straightforward index because of growth. However, high BMI percentile levels based on Centers for Disease Control and Prevention (CDC) BMI growth charts and changes in parameters of BMI curves in children are linked to significant levels of risk for adult obesity at corresponding high percentile levels. The use of BMI alone is also cautioned in athletes and persons with certain medical conditions (e.g., sarcopenia) where body weight may be altered significantly by changing proportions of muscle and fat masses. Abdominal Circumference. Obesity is commonly associated with increased amounts of intra-abdominal fat. A centralizedfat pattern is associated with the deposition of both intra-abdominal and subcutaneous abdominal adipose tissue. It should be noted that abdominal circumference is an imperfect indicator of intra-abdominal adipose tissue, as it also includes subcutaneous fat deposition, as well as visceral adipose tissue. This does not preclude its usefulness, as it is associated with specific health risks. Persons in the upper percentiles for abdominal circumference are considered obese and at increased risk for morbidity, specifically type 2 diabetes and the metabolic syndrome, and mortality. There has been a steady increase in the prevalence of high abdominal circumference in the general population from 10 to 20% in the 1960s to between 40 and 60% in the year 2000. Circumferences of other body segments such as the arm and leg are possible, but there are few reference data available for comparative purposes. Furthermore, the calculation of fat and muscle areas of the arm is not accurate or valid in the obese. The ratio of abdominal circumference (often referred to incorrectly as “waist” circumference) to hip circumference is a rudimentary index for describing adipose tissue distribution or fat patterning. Abdomen-to-hip ratios greater than 0.85 represent a centralized distribution of fat. Most men with a ratio greater than 1.0 and women with a ratio greater than 0.85 are at increased risk for cardiovascular disease, diabetes, and cancers. Skinfolds. Skinfold measurements are used to characterize subcutaneous fat thickness at various regions of the body, but it should be noted that they have limited utility in the overweight or obese adult. The primary limitation is that most skinfold calipers have an upper measurement limit of 45 to 55 mm, which restricts their use to subjects who are moderately overweight or thinner. A few skinfold calipers take large measurements, but this is not a significant improvement because of the difficulty of grasping and holding a large skinfold while reading the caliper dial. The majority of national reference data available are for skinfolds at the triceps and subscapular locations. The triceps skinfold varies considerably by sex and can reflect changes in the underlying triceps muscle rather than an actual change in body fatness. Skinfolds are particularly useful in monitoring changes in fatness in children because of their small body size, and the majority of fat is subcutaneous even in obese children. However, the statistical relationships between skinfolds and percent or total body fat in children and adults are often not as strong as that of BMI. Also, the true upper distribution of subcutaneous fat measurements remains unknown because most obese children and adults have not had their skinfolds measured. Bioelectric Impedance Analysis The analysis of body composition by bioelectrical impedance produces estimates of total body water (TBW), fat-free mass (FFM), and fat mass by measuring the resistance of the body as a conductor to a very small alternating electrical current. Bioelectrical impedance analyzers do not measure any biological quantity or describe any biophysical model related to obesity. Rather, the impedance index [stature squared divided by resistance (S2/R) at a frequency, most often 50 kHz] is proportional to the volume of total water and is an independent variable in regression equations to predict body composition. Bioelectrical impedance analyzers use such equations to describe statistical associations based on biological relationships for a specific population, and as such the equations are useful only for subjects that closely match the reference population in body size and shape. BIA has been applied to overweight or obese samples in only a few studies; thus, the available BIA prediction equations are not necessarily applicable to overweight or obese children or adults. The ability of BIA to predict fatness in the obese is difficult because they have a greater proportion of body mass and body water accounted for by the trunk, the hydration of FFM is lower in the obese, and the ratio of extracellular water to intracellular water is increased in the obese. Bioelectrical impedance analysis validity and its estimates of body composition are significant issues even for normal weight individuals. BIA is useful in describing mean body composition for groups of individuals, but large errors for an individual limit its clinical application, especially among the obese. The large predictive errors inherent in BIA render it insensitive to small improvements in response to treatment. Commercial bioelectrical impedance analyzers are popular and widely available to the public, but it is important to remember that these units contain all of the problems associated with this methodology. Recent BIA prediction equations have been published43 along with body composition mean estimates for non-Hispanic whites, non-Hispanic blacks, and Mexican-American males and females from 12 to 90 years of age. However, these equations are not recommended for obese individuals or groups. Direct Methods Total Body Water Total body water is easy to measure because it does not require undressing or any real physical participation. Water is the most abundant molecule in the body, and TBW volume is measured by isotope dilution. Water maintains a relatively stable relationship to FFM; therefore, measured water/isotope-dilution volumes allow prediction of FFM and fat (i.e., body weight minus FFM) in normal weight individuals. As with the other methods mentioned earlier, the TBW method is limited in the obese. The major assumption is that FFM is estimated from TBW based on an assumed average proportion of TBW in FFM of 73%, but this proportion ranges from 67 to 80%.44,45 In addition, about 15 to 30% of TBW is present in adipose tissue as extracellular fluid, and this proportion increases with the degree of adiposity.46 These proportions tend to be higher in women than in men, higher in the obese, and therefore produce underestimates of FFM and overestimates of fatness.41 Importantly, variation in the distribution of TBW as a result of disease associated with obesity, such as diabetes and renal failure, affects estimates of FFM and TBF further. Total body water is a potentially useful method applicable to the obese but there are details that need to be considered. The several analytical chemical methods used to quantify the concentration of TBW (and extracellular fluid) have errors of almost a liter. Equilibration times for isotope dilution in relation to levels of body fatness are unknown because, theoretically, it might (and should) take longer for the dilution dose to equilibrate in an obese person as compared with a normal weight individual. Also, a measure of extracellular space is necessary to correct the amount of FFM in an obese person. Such data could also be very useful in the treatment of end-stage renal disease. Total Body Counting and Neutron Activation In addition to total body water, two other direct methods of body composition assessment are available to the researcher/clinician: total body counting and neutron activation. Total body counting (also called whole body counting) measures the amount of naturally radioactive potassium 40 (40K) in the body. Because potassium is found almost entirely within cell bodies, measuring potassium can provide an estimate of body cell mass. Fat- free mass can then be estimated once total body potassium is known, assuming a constant concentration of potassium in FFM. There are only a few of the detectors required for this technique currently in use in the United States, which precludes its use in most research. For further details regarding total body counting, readers are encouraged to consult Ellis. Neutron activation techniques have been reported to be highly accurate for tissue-specific body composition, with a typical body scan occupying up to 1 hour. After subject exposure to a neutron field, gamma output can be measured as the cell nucleus relaxes and goes back to its pre-exposed state. Gamma output can be measured immediately upon activation (“prompt gamma neutron activation”) or at a somewhat delayed period (“delayed gamma neutron activation”). Using this technique, many elements in the body can be measured, including carbon, nitrogen, sodium, and calcium.48 Body nitrogen quantified by this method has been used to predict the amount of protein in the body to further analyze components of FFM. A significant concern with this technique is that it involves high levels of neutron radiation exposure and therefore has not been used in large-scale population research. Criterion Methods Body Density Hydrodensitometry (commonly called “underwater weighing”) is a technique that estimates body composition using measures of body weight, body volume, and residual lung volume. Historically, body density was converted to the percentage of body weight as fat using the two- compartment models of Siri45 or Brozek et al.,but more recently, a multicompartment model is used to calculate body fatness.52 The multicompartment models combine body density with measures of bone density and total body water to calculate body fatness43 and are more accurate than two-compartment models. Hydrodensitometry is highly reliant upon subject performance. This is particularly problematic in children or obese subjects because it is difficult, if not impossible, for them to submerge completely under water. Weight belts reduce buoyancy, but cannot compensate for all aspects of performance problems.Air displacement plethysmography53–55 works under many of the same assumptions as hydrodensitometry and affords some advantages over it (e.g., subject compliance does not involve breath holding or aversions to being under water). Air displacement devices do make assumptions regarding tissue density, much like other methods of body composition assessment.8 Thus, caution should be taken when applying these methods to persons suspected to have alterations in the density of fat-free mass tissues, such as the elderly and children.8 Unfortunately, body density methodologies (hydodensitometry and air displacement plethysmography) are rarely applied to obese subjects, as most overweight and obese persons are reluctant to put on a bathing suit and participate in body density measurements. Dual-Energy X-ray Absorptiometry Dual energy X-ray absorptiometry is the most popular method for quantifying fat, lean, and bone tissues. The two low-energy levels used in DXA and their differential attenuation through the body allow the discrimination of total body adipose and soft tissue, in addition to bone mineral content and bone mineral density. DXA is fast and user-friendly for the subject and the operator. A typical whole body scan takes approximately 10 to 20 minutes and exposes the subject to