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

Harrisons Internal Medicine Chapter 70. Nutritional Requirements and Dietary AssessmentNutritional 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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Chapter 070. Nutritional Requirements and Dietary Assessment (Part 1) Chapter 070. Nutritional Requirements and Dietary Assessment (Part 1) Harrisons Internal Medicine > Chapter 70. Nutritional Requirements andDietary Assessment Nutritional Requirements and Dietary Assessment: Introduction Nutrients are substances that must be supplied by the diet because they arenot synthesized in the body in sufficient amounts. Nutrient requirements forgroups of healthy persons have been determined experimentally. For good healthwe require energy-providing nutrients (protein, fat, and carbohydrate), vitamins,minerals, and water. Specific nutrient requirements include 9 essential aminoacids, several fatty acids, 4 fat-soluble vitamins, 10 water-soluble vitamins, andcholine. Several inorganic substances, including 4 minerals, 7 trace minerals, 3electrolytes, and the ultratrace elements, also must be supplied in the diet. The required amounts of the essential nutrients differ by age andphysiologic state. Conditionally essential nutrients are not required in the diet butmust be supplied to individuals who do not synthesize them in adequate amounts,such as those with genetic defects, those having pathologic states with nutritionalimplications, and developmentally immature infants. Many organicphytochemicals and zoochemicals present in foods have health effects. Forexample, dietary fiber has beneficial effects on gastrointestinal function. Otherbioactive food constituents or contaminants such as lead may have negative healtheffects. Essential Nutrient Requirements Energy For weight to remain stable, energy intake must match energy output. Themajor components of energy output are resting energy expenditure (REE) andphysical 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 2800kcal/d for American men and about 1800 kcal/d for American women, althoughthese estimates vary with body size and activity level. Formulas for estimatingREE are useful for assessing the energy needs of an individual whose weight isstable. Thus, for males, REE = 900 + 10w, and for females, REE = 700 + 7w,where w is weight in kilograms. The calculated REE is then adjusted for physicalactivity level by multiplying by 1.2 for sedentary, 1.4 for moderately active, or 1.8for very active individuals. The final figure provides a rough estimate of totalcaloric needs in a state of energy balance. Formulas to provide more preciseestimates of energy requirements are provided by the Food and Nutrition Board,Institute of Medicine, National Academy of Sciences in recent reports on dietaryreference 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 arerequired 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 aminoacids (e.g., alanine) can also be used for gluconeogenesis. When energy intake isinadequate, protein intake must be increased, since ingested amino acids arediverted into pathways of glucose synthesis and oxidation. In extreme energydeprivation, protein-calorie malnutrition may ensue (Chap. 72). For adults, the recommended dietary allowance (RDA) for protein is about0.6 g/kg desirable body weight per day, assuming that energy needs are met andthat the protein is of relatively high biologic value. Current recommendations for ahealthy diet call for at least 10–14% of calories from protein. Biologic value tendsto be highest for animal proteins, followed by proteins from legumes (beans),cereals (rice, wheat, corn), and roots. Combinations of plant proteins thatcomplement one another in biologic value or combinations of animal and plantproteins can increase biologic value and lower total protein requirements. Protein needs increase during growth, pregnancy, lactation, andrehabilitation after malnutrition. Tolerance to normal amounts of dietary protein isdecreased in renal insufficiency and liver failure, precipitating encephalopathy inpatients with cirrhosis of the liver. Fat and Carbohydrate Fats are a concentrated source of energy and constitute on average 34% ofcalories in U.S. diets. For optimal health, saturated fat and trans-fat should belimited to proteolysis. Over time, some adaptations in carbohydrate needs are possible inother tissues during hypocaloric states (Chap. 339).