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Chapter 070. Nutritional Requirements and Dietary Assessment (Part 1)

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Chapter 070. Nutritional Requirements and Dietary Assessment (Part 1)

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Harrison's Internal Medicine Chapter 70. Nutritional Requirements and Dietary Assessment Nutritional Requirements and Dietary Assessment: Introduction Nutrients are substances that must be supplied by the diet because they are not synthesized in the body in sufficient amounts. Nutrient requirements for groups of healthy persons have been determined experimentally. For good health we require energy-providing nutrients (protein, fat, and carbohydrate), vitamins, minerals, and water. Specific nutrient requirements include 9 essential amino acids, several fatty acids, 4 fat-soluble vitamins, 10 water-soluble vitamins, and choline. Several inorganic substances, including 4 minerals, 7 trace minerals, 3 electrolytes, and the ultratrace elements, also must...

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  1. Chapter 070. Nutritional Requirements and Dietary Assessment (Part 1) Harrison's Internal Medicine > Chapter 70. Nutritional Requirements and Dietary Assessment Nutritional Requirements and Dietary Assessment: Introduction Nutrients are substances that must be supplied by the diet because they are not synthesized in the body in sufficient amounts. Nutrient requirements for groups of healthy persons have been determined experimentally. For good health we require energy-providing nutrients (protein, fat, and carbohydrate), vitamins, minerals, and water. Specific nutrient requirements include 9 essential amino acids, several fatty acids, 4 fat-soluble vitamins, 10 water-soluble vitamins, and choline. Several inorganic substances, including 4 minerals, 7 trace minerals, 3 electrolytes, and the ultratrace elements, also must be supplied in the diet.
  2. The required amounts of the essential nutrients differ by age and physiologic state. Conditionally essential nutrients are not required in the diet but must be supplied to individuals who do not synthesize them in adequate amounts, such as those with genetic defects, those having pathologic states with nutritional implications, and developmentally immature infants. Many organic phytochemicals and zoochemicals present in foods have health effects. For example, dietary fiber has beneficial effects on gastrointestinal function. Other bioactive food constituents or contaminants such as lead may have negative health effects. Essential Nutrient Requirements Energy For weight to remain stable, energy intake must match energy output. The major components of energy output are resting energy expenditure (REE) and physical activity; minor sources include the energy cost of metabolizing food (thermic effect of food or specific dynamic action) and shivering thermogenesis (e.g., cold-induced thermogenesis). The average energy intake is about 2800 kcal/d for American men and about 1800 kcal/d for American women, although these estimates vary with body size and activity level. Formulas for estimating REE are useful for assessing the energy needs of an individual whose weight is stable. Thus, for males, REE = 900 + 10w, and for females, REE = 700 + 7w,
  3. where w is weight in kilograms. The calculated REE is then adjusted for physical activity level by multiplying by 1.2 for sedentary, 1.4 for moderately active, or 1.8 for very active individuals. The final figure provides a rough estimate of total caloric needs in a state of energy balance. Formulas to provide more precise estimates of energy requirements are provided by the Food and Nutrition Board, Institute of Medicine, National Academy of Sciences in recent reports on dietary reference intakes. For further discussion of energy balance in health and disease, see Chap. 72. Protein Dietary protein consists of both essential and other amino acids that are required for protein synthesis. The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine/cystine, phenylalanine/tyrosine, threonine, tryptophan, and valine. All amino acids can be used for energy, and certain amino acids (e.g., alanine) can also be used for gluconeogenesis. When energy intake is inadequate, protein intake must be increased, since ingested amino acids are diverted into pathways of glucose synthesis and oxidation. In extreme energy deprivation, protein-calorie malnutrition may ensue (Chap. 72).
  4. For adults, the recommended dietary allowance (RDA) for protein is about 0.6 g/kg desirable body weight per day, assuming that energy needs are met and that the protein is of relatively high biologic value. Current recommendations for a healthy diet call for at least 10–14% of calories from protein. Biologic value tends to be highest for animal proteins, followed by proteins from legumes (beans), cereals (rice, wheat, corn), and roots. Combinations of plant proteins that complement one another in biologic value or combinations of animal and plant proteins can increase biologic value and lower total protein requirements. Protein needs increase during growth, pregnancy, lactation, and rehabilitation after malnutrition. Tolerance to normal amounts of dietary protein is decreased in renal insufficiency and liver failure, precipitating encephalopathy in patients with cirrhosis of the liver. Fat and Carbohydrate Fats are a concentrated source of energy and constitute on average 34% of calories in U.S. diets. For optimal health, saturated fat and trans-fat should be limited to
  5. proteolysis. Over time, some adaptations in carbohydrate needs are possible in other tissues during hypocaloric states (Chap. 339).
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