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Nutritional Requirements for Healthy Individuals

	For good health we require energy-providing nutrients (protein, fat, and carbohydrate), vitamins, minerals, and water.
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Nutrients are substances that are not synthesized in the body in sufficient amounts and therefore must be supplied by the diet. 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 four minerals, seven trace minerals, three electrolytes, and the ultratrace elements, must also be supplied in the diet. The required amounts of the essential nutrients differ by age and physiologic state.

Energy

For weight to remain stable, energy intake must match energy output.

The average energy intake is about 2800 kcal/d for American men and about 1800 kcal/d for American women, though these estimates vary with body size and activity level.

The major categories of energy output are resting energy expenditure (REE) and physical activity; minor sources include the energy cost of metabolizing food (thermic effect of food) and shivering thermogenesis (e.g., cold-induced thermogenesis).

There are several formulas for estimating REE:

• For males, REE= 900 + 10w

• and for females, REE = 700 +7w

‘w’ is weight in kg.

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 an estimate of total caloric needs in a state of energy balance.

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Protein

Dietary protein consists of both essential and nonessential amino acids that are required for protein synthesis, whereas certain amino acids can also be used for energy and gluconeogenesis.

The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine/cystine, phenylalanine/tyrosine, threonine, tryptophan, and valine.

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 to 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.

Fat and Carbohydrate Fats

Fats are a concentrated source of energy and constitute on average 34% of calories in U.S. diets. However, for optimal health, fat intake should total no more than 30% of calories. Saturated fat and trans-fat should be limited to <10% of calories, and polyunsaturated fats to <10% of calories, with monounsaturated fats comprising 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.

Water

For adults, 1 to 1.5 mL water per kcal of energy expenditure is sufficient under usual conditions to allow for normal variations in physical activity levels, sweating, and solute load of the diet.  

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. Increased water needs during pregnancy are low, perhaps an additional 30 mL/day. 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.

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