Health Significance And Biological Functions Of Food: Qualitative Relevance Of Proteins To Man

Proteins together with water, form the basic building units of tissues. In addition, they function as enzymes and hormones and also take part in transport mechanisms. Of the twenty amino acids present in proteins, eight have to be supplied to man through dietary sources almost continually. For adults, the essential amino acids are lysine, methionine, valine, leucine, isoleucine, rtryptophan, phenylalanine and threonine. For infants, histidine is also essential. Animal proteins are rich in essential amino acids, whereas vegetable proteins are deficient in one or more of them.

An ideal protein is one which contains all essential amino acids in optimal amounts and is completely digestable and utilizable. For the human infant, its mother’s milk fulfils these requirements. Whole egg proteins are considered the next best and this is given a rating of 100 arbitrarily. Other proteins are compared with whole egg protein. Biological value (BV) and net protein utilization (NPU) are most popular indices used for comparison. In addition, chemical method, ie. Chemical score (CS), is also widely employed to assess dietary protein quality.

Vegetable proteins have lower quality ratings than animal proteins. In the ordinary African diet, cereals, pulses and other vegetable sources also contribute proteins. The biological efficiency of such a mixed protein source is far higher than the average to be expected. The enhancement in protein quality resulting from a mixture of vegetable sources is known as the supplementary value of proteins. Well balanced European diets are quite adequate to supply the protein requirements. An important reason for protein deficiency in places like India is the availability of foods in amounts far too short to the amount needed.

Energy requirements are determined by physical activity, body size and composition, age and sex, physiological state, climate and environment. Generally, the energy requirement of people living in tropical countries is lower than those in the colder climates.

Pregnancy, lactation and periodic infections and infestations widely prevalent in the developing regions increase the need for proteins. Since the nutritive value for proteins vary widely, it is convenient ro express the requirements of protein in terms of a reference protein with an assumed NPU of 100, which is fully digested and utilized. The reference protein can be converted into proteins of customary diets using the formula:

Ref. protein requirement/NPU of dietary protein X 100.

The NPU of Chinese diets is approximately 71.