Distribution of the caloriesCarbohydrate, protein, and fats are determined partly by physiologic factors and partly by taste and economic considerations.
The aim of the science of nutrition is the determination of the kinds and amounts of foods that promote health and well-being. This includes not only the problems of undernutrition but those of overnutrition, taste, and availability. However, certain substances are essential constituents of any human diet. Many of these compounds have been mentioned in the previous sections, and a brief summary of the essential and desirable dietary components is presented below.
Essential dietary components: An optimal diet includes, in addition to sufficient water, adequate calories, protein, fat, minerals, and vitamins.
Caloric intake and distribution: The caloric value of the dietary intake must be approximately equal to the energy expended if body weight is to be maintained. In addition to the 2000 kcal/d necessary to meet basal needs, 500 to 2500 kcal/d (or more) are required to meet the energy demands of daily activities.
The distribution of the calories among carbohydrate, protein, and fat is determined partly by physiologic factors and partly by taste and economic considerations. A daily protein intake of 1 g/kg body weight to supply the eight nutritionally essential amino acids and other amino acids is desirable. The source of the protein is also important. Grade 1 proteins , the animal proteins of meat, fish, dairy products, and eggs, contain amino acids in approximately the proportions required for protein synthesis and other uses. Some of the plant proteins are also grade 1, but most are grade 2 because they supply different proportions of amino acid and some lack one or more of the essential amino acids.