Biochemistry Nutrition PDF

Biochemistry Nutrition Lecturer: Dr. Rithab Ibrahim Al-Samawi Nutrition Nutrients are the constituents of food necessary to sustain the normal functions of the body. All energy is provided by three classes of nutrients: fats, carbohydrates, protein The intake of these energy-rich mol...

Biochemistry Nutrition Lecturer: Dr. Rithab Ibrahim Al-Samawi Nutrition Nutrients are the constituents of food necessary to sustain the normal functions of the body. All energy is provided by three classes of nutrients: fats, carbohydrates, protein The intake of these energy-rich molecules is larger than that of the other dietary nutrients. Therefore, they are called macronutrients Energy requirement in humans The Estimated Energy Requirement is the average dietary energy intake predicted to maintain an energy balance (that is, when the calories consumed are equal to the energy expended) in a healthy adult of a defined age, gender, and height whose weight and level of physical activity are consistent with good health. Energy content of food The energy content of food is calculated from the heat released by the total combustion of food in a calorimeter. It is expressed in kilocalories (kcal, or Cal). Dietary fats The incidence of a number of chronic diseases is significantly influenced by the kinds and amounts of nutrients consumed Dietary fats most strongly influence the incidence of coronary heart disease (CHD); evidence linking dietary fat and the risk for cancer or obesity is much weaker. Biomedical importance In addition to water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins, as well as a source of metabolic fuel), fiber (for bulk in the intestinal lumen), minerals (containing elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in smaller amounts for other metabolic and physiologic functions). The polysaccharides, triacylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before it can be absorbed and utilized. Vitamins & Minerals BIOMEDICAL IMPORTANCE Vitamins are a group of organic nutrients, required in small quantities for a variety of biochemical functions that, generally, cannot be synthesized by the body and must therefore be supplied in the diet. lipid-soluble vitamins The lipid-soluble vitamins are hydrophobic compounds that can be absorbed efficiently only when there is normal fat absorption. Like other lipids, they are transported in the blood in lipoproteins or attached to specific binding proteins. They have diverse functions for example, vitamin A, vision and cell differentiation; vitamin D, calcium and phosphate metabolism, and cell differentiation; vitamin E, antioxidant; and vitamin K, blood clotting. lipid-soluble vitamins Conditions affecting the digestion and absorption of the lipidsoluble vitamins, such as a very low fat diet, steatorrhea and disorders of the biliary system, can all lead to deficiency syndromes, including night blindness and xerophthalmia(vitamin A); rickets in young children and osteomalacia in adults (vitamin D); neurological disorders and hemolytic anemia of the newborn (vitamin E); and hemorrhagic disease of the newborn (vitamin K). Water-soluble vitamins The water-soluble vitamins are vitamins B and C, folic acid, biotin and pantothenic acid; they function mainly as enzyme cofactors. Folic acid acts as a carrier of one-carbon units. Deficiency of a single vitamin of the B complex is rare since poor diets are most often associated with multiple deficiency states. Water-soluble vitamins specific syndromes are characteristic of deficiencies of individual vitamins beriberi (thiamin) cheilosis, glossitis, (riboflavin) pellagra (niacin) megaloblastic anemia, and pernicious anemia (vitamin B12) megaloblastic anemia (folic acid) scurvy (vitamin C). Digestion & absorption of vitamins & minerals Vitamins and minerals are released from food during digestion, although this is not complete, and the availability of vitamins and minerals depends on the type of food and, especially for minerals, the presence of chelating compounds. Digestion & absorption of vitamins & minerals The fat-soluble vitamins are absorbed in the lipid micelles that are the result of fat digestion; water-soluble vitamins and most mineral salts are absorbed from the small intestine either by active transport or by carrier-mediated diffusion followed by binding to intracellular proteins to achieve concentrative uptake. Digestion & absorption of vitamins & minerals Vitamin B12 absorption requires a specific transport protein, intrinsic factor. calcium absorption is dependent on vitamin D. zinc absorption probably requires a zinc-binding ligand secreted by the exocrine pancreas and the absorption of iron is limited. Calcium Absorption In addition to its role in regulating calcium homeostasis, vitamin D is required for the intestinal absorption of calcium. Synthesis of the intracellular calcium-binding protein, calbindin, required for calcium absorption, is induced by vitamin D. Calcium Absorption Phytic acid in cereals binds calcium in the intestinal lumen, preventing its absorption. Other minerals, including zinc, are also chelated by phytate. This is mainly a problem among people who consume large amounts of unleavened whole-wheat products. Calcium Absorption High concentrations of fatty acids in the intestinal lumen, as a result of impaired fat absorption, can also reduce calcium absorption by forming insoluble calcium salts High intake of oxalate can sometimes cause deficiency since calcium oxalate is insoluble. Iron Absorption Although iron deficiency is a common problem in both developed and developing countries, about 10% of the population are genetically at risk of iron overload (hemochromatosis), and in order to reduce the risk of adverse effects of nonenzymic generation of free radicals by iron salts, absorption is strictly regulated. Iron Absorption Inorganic iron is absorbed in the Fe2+ (reduced) state, and hence, the presence of reducing agents enhances absorption. The most effective compound is vitamin C, and while intakes of 40 to 80 mg of vitamin C per day are more than adequate to meet requirements, an intake of 25 to 50 mg per meal enhances iron absorption, especially when iron salts are used to treat iron deficiency anemia. Energy balance: over- &undernutrition Food intake above energy expenditure leads to obesity, while intake less than expenditure leads to weakness and wasting, marasmus, and kwashiorkor. Both obesity and severe undernutrition are associated with increased mortality. The body mass index = weight (in kg)/height2 (in m) is commonly used as a way of expressing relative obesity; a desirable range is between 20 and 25. Protein & amino acid requirements Protein requirements can be determined by measuring nitrogen balance The state of protein nutrition can be determined by measuring the dietary intake and output of nitrogenous compounds from the body. Although nucleic acids also contain nitrogen, protein is the major dietary source of nitrogen and measurement of total nitrogen intake gives a good estimate of protein intake (as N is 16% of most proteins). The output of N from the body is mainly in urea. Nitrogen balance The difference between intake and output of nitrogenous compounds is known as nitrogen balance. Three states can be defined. In a healthy adult, nitrogen balance is in equilibrium, when intake equals output, and there is no change in the total body content of protein. Nitrogen balance In a growing child. a pregnant woman. a person in recovery from protein loss. the excretion of nitrogenous compounds is less than the dietary intake and there is net retention of nitrogen in the body as protein positive nitrogen balance. Nitrogen balance In response to trauma or infection. If the intake of protein is inadequate to meet requirements. there is net loss of protein nitrogen from the body negative nitrogen balance. Except when replacing protein losses, nitrogen equilibrium can be maintained at any level of protein intake above requirements. The Determination of Micronutrient Requirements For any nutrient, there is a range of intakes between that which is clearly inadequate, leading to clinical deficiency disease, and that which is so much in excess of the body’s metabolic capacity that there may be signs of toxicity. Between these two extremes is a level of intake that is adequate for normal health and the maintenance of metabolic integrity. The Determination of Micronutrient Requirements Inorganic mineral elements that have a function in the body must be provided in the diet. When the intake is insufficient, deficiency signs may arise, eg, anemia (iron), and goiter (iodine). Excessive intakes may be toxic. The Determination of Micronutrient Requirements Individuals do not all have the same requirement for nutrients, even when calculated on the basis of body size or energy expenditure. There is a range of individual requirements of up to 25% around the mean. Therefore, in order to assess the adequacy of diets, it is necessary to set a reference level of intake high enough to ensure that no one either suffers from deficiency or is at risk of toxicity Reference Nutrient Intakes of Vitamins and Minerals, UK 1991