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INTRODUCTION

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Nutrients are substances that are not synthesized in sufficient amounts in the body and therefore must be supplied by the diet. Nutrient requirements for groups of healthy persons have been determined experimentally. The absence of essential nutrients leads to growth impairment, organ dysfunction, and failure to maintain nitrogen balance or adequate status of other nutrients. For good health, we require energy-providing nutrients (protein, fat, and carbohydrate), vitamins, minerals, and water. Requirements for organic nutrients include 9 essential amino acids, several fatty acids, glucose, 4 fat-soluble vitamins, 10 water-soluble vitamins, dietary fiber, and choline. Several inorganic substances, including 4 minerals, 7 trace minerals, 3 electrolytes, and the ultratrace elements, must also be supplied by diet.

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The amounts of the essential nutrients that are required by individuals 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 with pathologic conditions such as infection or trauma with nutritional implications, and developmentally immature infants. For example, inositol, taurine, arginine, and glutamine may be needed by premature infants. Many other organic and inorganic compounds that are present in foods, such as pesticides and lead, also have health effects.

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ESSENTIAL NUTRIENT REQUIREMENTS

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Energy

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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 components 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 ~2600 kcal/d for American men and ~1800 kcal/d for American women, though these estimates vary with body size and activity level. Formulas for roughly estimating REE are useful in assessing the energy needs of an individual whose weight is stable. Thus, for males, REE = 900 + 10m, and for females, REE = 700 + 7m, where is m mass 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, the estimated energy requirement (EER), provides an approximation of total caloric needs in a state of energy balance for a person of a certain age, sex, weight, height, and physical activity level. For further discussion of energy balance in health and disease, see Chap. 97.

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Protein

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Dietary protein consists of both essential and nonessential 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. Certain amino acids, such as alanine, can also be used for energy and gluconeogenesis. When energy intake is inadequate, protein intake must be increased, because ingested amino acids are diverted into ...

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