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, 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.
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.
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 diets. For optimal health, saturated fat and trans-fat should be limited to <10% of calories, and polyunsaturated fats to <10% of calories, with monounsaturated fats constituting the remainder of fat intake. At least 55% of total calories should be derived from carbohydrates. The brain requires about 100 g/d of glucose for fuel; other tissues use about 50 g/d. Some tissues (e.g., brain and red blood cells) rely on glucose supplied either exogenously or from muscle proteolysis. Over time, some adaptations in carbohydrate needs are possible in other tissues during hypocaloric states.
Water
For adults, 1.0–1.5 mL water per kcal of energy expenditure is sufficient under usual conditions to allow for normal variations in physical activity, sweating, and solute load of the diet. Water losses include 50–100 mL/d in the feces, 500–1000 mL/d by evaporation or exhalation, and, depending on the renal solute load, 1000 mL/d in the urine. If external losses increase, intakes must increase accordingly to avoid underhydration. Fever increases water losses by approximately 200 mL/d per °C; diarrheal losses vary but may be as great as 5 L/d with severe diarrhea. Heavy sweating and vomiting also increase water losses. When renal function is normal and solute intakes are adequate, the kidneys can adjust to increased water intake by excreting up to 18 L/d of excess water. However, obligatory urine outputs can compromise hydration status when there is inadequate intake or when losses increase in disease or kidney damage.
Infants have high requirements for water because of their large ratio of surface area to volume, the limited capacity of the immature kidney to handle high renal solute loads, and their inability to communicate their thirst. During pregnancy, 30 mL/d additional water is needed. During lactation, milk production increases water requirements by approximately 1000 mL/d, or 1 mL for each mL of milk produced. Special attention must be paid to the water needs of the elderly, who have reduced total body water and blunted thirst sensation, and may be taking diuretics.
