Three Energy-Yielding Nutrients
There are three energy-yielding nutrients: Carbohydrates, Protein and Fats. These nutrients are written in following lines.
1. Carbohydrates. Carbohydrates usually provide the greater part of the energy in a normal diet, but no individual carbohydrate is an essential nutrient in the sense that the body needs it but cannot make it for itself from other nutrients. If the carbohydrate intake is less than 100 g per day ketosis is likely to occur.
2. Fats. With their high caloric value, fats are useful to people with large energy expenditure; moreover they are helpful in cooking and making food appetizing. Though rats need linoleic or arachidonic acids in their diet, essential fatty acid deficiency is rare in man. It has been demonstrated in patients who have been fed intravenously for long periods without fat emulsions. They develop a scaly dermatitis and eicosatrienoic acid accumulates in plasma lipids. Essential fatty acids are precursors for the synthesis of prostaglandins.
3. Proteins. Protein provides some 20 amino acids, of which eight are essential for normal protein synthesis and for maintaining nitrogen balance in adults. These essential amino acids are methionine, lysine, tryptophan, phenylalanine, leucine, isoleucine, threonine, and valine. Histidine and perhaps arginine are also needed for growth in infants.
The 'biological value' of different proteins depends on the relative proportions of essential amino acids they contain. Proteins of animal origin, particularly from eggs, milk and meat, are generally of higher biological value than the proteins of vegetable origin which are deficient in one or more of the essential amino acids. However it is possible to have a diet of mixed vegetable proteins with high biological value if the principle of supplementation is used. For example cereals, e.g. wheat, contain about 10% protein and are relatively deficient in lysine. Legumes contain around 20% of protein which is relatively deficient in methionine. If two parts of wheat are mixed (or eaten) with one part of legume, a food results which contains 13% of a protein of high biological value. This happens because cereals contain enough methionine and legumes enough lysine to supplement the other component of the mixture.
The usual recommended allowance for an adequate protein intake is 10% of the total calories i.e. about 65 g for the average adult. The minimum requirement is less around 40 g per day of good biological value protein for an adult.