Human Nutrition PDF

INTRODUCTION TO HUMAN NUTRITION  Total Fibers can be the sum of Dietary fibers and Functional fibers. Dietary fibers (all the previous). Functional fibers : fibers usually occurs naturally in plants when these fibers have been extracted from plants or manufactured and then added to foods or us...

INTRODUCTION TO HUMAN NUTRITION  Total Fibers can be the sum of Dietary fibers and Functional fibers. Dietary fibers (all the previous). Functional fibers : fibers usually occurs naturally in plants when these fibers have been extracted from plants or manufactured and then added to foods or used in supplements.  Resistant starches:  A few starches are classified as dietary fibers. known as resistant starches, these starches escape digestion and absorption in the small intestine.  Starch may resist digestion for several reasons, including the individual efficiency in digesting starches and the food physical properties.  Resistant starch is common in whole legumes, raw potatoes, and unripe banana.  High fiber diet: Is the dietary fibers exceed 40 gm/day. Food sources: fruits, vegetables, bread & cereals. Disadvantage of increasing fiber in the diet are: 1-Abdominal fullness 2-Increasing flatulence 3-Nausea and vomiting. 4-Gases 5-May interfere with mineral absorption. 6-More seriously can obstruct the GI tract. FUNCTIONS OF CARBOHYDRATES 1- Basic energy supply  The main function of carbohydrate is to provide the primary energy.  Carbohydrates burn in the body at a rate of 4 kcal/gm.  Carbohydrates furnish readily available energy that needed not only for physical activities but also for all the work of the body cells. 2- Reserve energy supply  The human body reserves carbohydrate as glycogen (in liver an muscles).  Maintain a normal blood glucose level and to prevent a breakdown of fat and protein in tissue.  People must eat carbohydrate foods regularly and fairly frequent intervals to meet energy demand. 3- Special tissue functions  Liver: glycogen reserves in the liver and muscle provide constant exchange with the body's overall energy balance system. Reserves glycogen especially in the liver protect cell from depressed metabolic function and resulting injury. 4- Central nervous system The brain has no stored supply of glucose, therefore, it is dependent on a minute-to-minute supply of glucose from the blood. Sustained and profound shock from blood sugar may cause brain damage. 5- Fibers A. Cellulose Cellulose it remains undigested in the gastrointestinal tract. And provide important bulk to the diet. This bulk helps 1 move the food mass along. 2 stimulate normal muscle action in the intestine. 3and forms the feces for elimination of waste products. B. Noncellulose polysaccharides  Absorb water and swell to large bulk.  Slow the emptying of the food from the stomach.  Bind bile acids including cholesterol in the intestine.  Provide fermentation material for colon bacteria to work on. DIGESTION , ABSORPTION AND METABOLISM OF CARBOHYDRATE  The goal is to break sugar and starches into small molecules-mainly glucose-that the body can absorb and use.  A large starch molecule require extensive breakdown. The disaccharide needs only to be broken once. And the monosaccharide not at all.  The initial splitting begins in the mouth, and the final splitting and absorption occur in small intestine, and the conversion to common energy (glucose) takes place in the liver. CARBOHYDRATE DIGESTION In the mouth: Mastication: the process by which food is crushed and ground by teeth.The chewing of high-fiber food slows eating and stimulate the flow of saliva. Starch: salivary glands secretes saliva into the mouth to moisten the food. The salivary enzyme amylase begins digestion. Starch Amylase small polyshcarrides, maltose(disacharride) Fibers: the mechanical action of the mouth crushes and tears fiber in food and mixes it with saliva to moisten it for swallowing. In the stomach: Peristalsis: wave like muscular contraction. The swallowed bolus(a portion of food swallowed at one time) mixes with stomach acids and protein digesting enzymes, which inactivate salivary amylase. Starch: stomach acid inactivates salivary enzymes, halting starch digestion. To small extent ,the stomach acid continue breaking the starch, but it juices contain no enzymes to digest CHO. Fibers: is not digested in stomach and delays gastric emptying thereby provide feeling of fullness and satiety. In the small intestine : In it most of the work of CHO digestion. Starch: the pancreas produces an amylase that is released through pancreatic duct into the small intestine. The major CHO-digesting enzyme is the pancreatic amylase which will continue breaking polysaccharides. Starch pancreatic amylase small polysaccharides, maltoses the pancreatic amylase in the duodenum breaks down all small polysaccharides into disaccharides. The final step takes place in the outer membrane of the intestinal cells. The disaccharide digestion begins at this point. There specific enzymes secreted from the intestinal glands breaks down specific disaccharides. The disaccharide enzymes on the surface of the small intestinal cells hydrolyze the disaccharides into monosaccharaides Maltose maltase Glucose +Glucose Sucrose sucrase Fructose+Glucose Lactose lactase Galactose+Glucose ==>intestinal cells absorb these monosaccharaides. Fibers: is not digested ,and delays the absorption of other nutrients. Large intestine: Within 1-4 hours after a meal, all of the sugars and most of the starches have been digested. Only fibers remain in the digestive tract. Fibers in large I. attracts water, which softens the stool for passage without straining. Also, bacteria in the GI tract ferment some fibers. Most fiber passes intact through digestive tract to the large intestine. Here, bacterial enzyme digest fiber. Some fibers Bacterial Enzymes short-chain fatty acids, gas & water Colon uses these small fat molecules for energy. Metabolism of short chain fatty acids occurs in the cells of liver. fiber therefore can contribute some energy. Depending on the extent to which they are broken down by bacteria and the fatty acids are absorbed. Fiber holds water ,regulate intestine activity, and bind substances such as bile ,cholesterol, and some minerals, carrying them out of the body. CARBOHYDRATE ABSORPTION  Peristaltic movement moves the monosaccharaides into the jejunum where digestion is completed and absorption begins.  Absorption is increases as a result of the intestinal villi(small mucus projections lining the small intestine). Each villi contain blood capillary into which the monosaccharaides passes via diffusion or active transport.  Glucose is unique that it can be absorbed to some extent through the lining of mouth, but for most part nutrient absorption takes place in the small intestine.  Glucose and galactose traverse the cell lining the small I. by active transport.  Fructose is absorbed by facilitated diffusion, which slows its entry and produces a smaller rise in blood glucose.  Likewise, unbranced chains of starch are digested slowly and produce a smaller rise in blood glucose than branched chains, which have many more places for enzymes to attack and release glucose rapidly. o As the blood from the intestine circulates through the liver, cells there take up fructose and galactose and convert them to other compounds ,most often to glucose. o Thus all disaccharides provide at least one glucose molecule directly ,and they can provide another one indirectly –through the conversion of fructose and galactose to glucose. LACTOSE INTOLERANCE  A condition that results from inability to digest the milk sugar lactose, characterized by bloating, gas, abdominal discomfort, and diarrhea.  Lactose intolerance differs than milk allergy, which is caused by an immune reaction to the protein in milk.  It is a common condition that occurs when there is insuffient lactase to digest the disaccharide lactose found in milk and milk products.  Because treatment requires limiting milk intake, other sources of riboflavin, vitamin D , and calcium must ne included in diet. GALACTOSEMIA  Galactosemia (is a rare genetic metabolic disorder that affects an individual's ability to metabolize the sugar galactose properly. Although the sugar lactose can metabolize to galactose, galactosemia is not related to and should not be confused with lactose intolerance.  Lactose in food (such as dairy products) is broken down by the enzyme lactase into glucose and galactose.  In individuals with galactosemia, the enzymes needed for further metabolism of galactose are severely diminished or missing entirely, leading to toxic levels of galactose 1- phosphate in various tissues as in the case of classic galactosemia, resulting in hepatomegaly (an enlarged liver), cirrhosis, renal failure, cataracts, brain damage, and ovarian failure. CARBOHYDRATE METABOLISM  Glucose plays a central role in CHO metabolism.  Metabolism: is the sum of various chemical processes in a living organism by which energy is made available for the functioning of the whole organism.  Also includes building and breaking tissues.  Products of metabolism is called metabolite.  The cell is the functional unit of life in the humans.  Energy metabolism occurs in all cells to sustain life process.  With the help of enzymes, glucose is chemically broken down to produce energy.  Excess glucose maybe converted to fat and held in reserve in adipose tissue.  Storing glucose as glycogen The liver stores about 1/3 of the body’s total glycogen and release glucose into the blood stream as needed. After a meal ,blood glucose rises, and liver cells link the excess glucose molecules by condensation reaction into long, branching chains of glycogen. When blood Glucose falls liver cells breaks glycogen by hydrolysis reactions into single molecules of glucose and release them into blood stream.  Thus glucose available to supply energy for brain and other tissues regard less if person eat recently. Muscle cells can also store glucose as glycogen (the two other thirds),but they hoard most of their supply, using it just for themselves during exercise. The brain maintain a small amount of glycogen, which is thought to provide an emergency energy reserve during times of severe glucose deprivation. The body can store glycogen for only enough for short period of time(less than a day for rest and few hours for most during exercise).  Using glucose for energy  Glucose fuels the work of most of the body’s cells.  The liver glycogen stores last only 4 hrs, not for days.  To keep glucose meeting energy needs, person has to eat CHO frequently.  The conversion of protein to glucose is called gluconeogensis (making of new glucose).  Only adequate dietary CHO can prevent this use of protein for energy ,and this role of CHO is known as its protein sparing action.  Metabolism of CHO Catabolism Anabolism -Glycogenlysis: the Glycogenesis: synthesis catabolism of glycogen of glycogen in liver and muscle. -Oxidation of glucose and synthesis of ATP. GLUCOSE IN THE BODY Every body cell depends on glucose for its energy to some extent, but the cells of the brain and the rest of the nervous system depends almost exclusively on glucose for energy. The activities of these cells never stop, and they have limited ability to store glucose. Day and night, they continually draw on the supply of glucose in the fluid supplying them. To maintain the supply, a steady stream of blood moves past these cells bringing more glucose from either the intestine (food) or the liver(via glycogen breakdown or gluconeogensis). Maintaining glucose homeostasis: To function optimally, the body must maintain blood glucose within limits that permits the cells to nourish themselves. If blood glucose falls below normal, a person may become dizzy and weak If it rises above normal, a person become fatigue. Left untreated fluctuation to the extremes-either high or low-can be fatal.  The regulating hormones :  Blood glucose homeostasis is regulated primarily by two hormones: Insulin: which move glucose from blood into cells and Glucagon: which brings glucose out of storage when necessary.  After a meal, as blood glucose rises, special cells of pancreas respond by creating insulin into blood.  In general the amount of insulin secreted corresponds with the rise of glucose.  As the circulating insulin contacts the receptors on the body's other cells, the receptors respond by ushering glucose from the blood into the cells.  Most of the cells takes only the glucose they can use for energy right away, but the liver and muscle cells can assemble the small glucose units into glycogen for storage. The liver cells can also convert glucose to fat for export to other cells.  Thus elevated blood glucose returns to normal as excess glucose is stored as glycogen and fat.  When blood glucose falls(as occurs between meals),other special cells of pancreas respond by secreting glucagon into blood.  Glucagon raises blood glucose by signaling the liver to break down its glycogen stores and release glucose into blood for use by all other body cells.  It elicits release of glucose from liver glycogen.  Another hormone that signals liver cells to release glucose is epinephrine.  Epinephrine :Is a hormone of the adrenal gland that modulate the stress response, formally called adrenaline.  Balance within normal range: (blood glucose- fasting): Normal: 70-120 mg/dl Prediabetes: 100- 125 mg/dl Diabetes: more or equal 126 mg/dl Falling outside the normal range: In some people, blood glucose regulation fails, which results in either: diabetes or hypoglycemia. Diabetes: is a chronic disorder of CHO metabolism, usually resulting from insuffient or ineffective insulin. Type 1 DM: the less common type of DM in which pancreas fails to produce insulin. The exact cause is unclear(genetic or virus..) Type 2 DM: the more common type of DM in which the cells fail to respond to insulin. Usually occurs as a result of obesity.  Hypoglycemia: an abnormally low blood glucose concentration. Symptoms: weakness, rapid heartbeat ,sweating, anxiety, hunger and trembling. Usually it is a consequence of poorly managed DM. It is caused by too much insulin, strenuous physical activity, inadequate food intake, or illness that causes blood glucose to plummet.  The glycemic response: the extent to which a food raises the blood glucose concentration and elicit insulin response. Refers to how quickly glucose is absorbed after a person eats, how high blood glucose rises, and how quickly it returns to normal. Low glycemic response is desirable and high glycemic response is less desirable.  Glycemic index: a method classifying food according to their potential for raising blood glucose.  What are source of blood glucose? Either from: 1CHO sources 1-dietary CHO(high %) 2-glycogen 3-lactic acid and pyruvic acid 2Non-CHO sources 1-protein 2-fats