Protein Nutrition

MALNUTRITION due to an inadequate supply of protein of good quality is probably the most important deficiency syndrome in the world today other than caloric undernutrition. Protein deficiency may be primary, or occur in association with caloric deficiency, or be secondary to a wide variety of pathologic conditions. Deficiency is common in prolonged febrile illnesses, hyperthyroidism and other diseases associated with hypermetabolism. It is encountered in the many conditions in which protein is lost from the body such as the nephroses, effusions into serious cavities, exudates and weeping wounds. Deficiency is observed, also, in diseases which impair digestion or absorption and in metabolic disturbances which interfere with utilization. Negative nitrogen balance commonly follows trauma such as fractures, burns and operative procedures and may persist for weeks. Nitrogen loss may amount to as much as 0.6 gm. per kilogram of body weight daily, which represents a loss of 3.75 gm. of protein per kilogram.

In approaching the diagnosis of protein deficiency, it is essential to know the requirement of protein and of the eight amino acids which have been found to be dietary essentials for man. The minimal requirement of protein for the adult is in the neighborhood of 0.5 to 0.65 gm. per kilogram of body weight daily. Larger amounts are needed during the period of growth, being greatest when growth is most rapid, and during the latter half of pregnancy and in lactation. The recommended protein allowance for maintenance in adult life is 1.0 gm. per kilo-gram of body weight daily. The minimal requirement of the eight essential amino acids for adult men and women. Part of the requirement of phenylalanine may be supplied by tyrosine and part of the methionine requirement by cystine. Sufficient nitrogen must also be furnished by the diet for manufacture of the nonessential amino acids. Some investigations of the amino acid needs of infants have been reported . No precise information is available for childhood and adolescence.

The quality of dietary protein is dependent on amino acid content. Animal proteins which contain all the essential amino acids have greater biological value than do vegetable proteins which are often deficient in one or more of these amino acids. Numerous methods, direct and indirect, have been suggested for determining the biological value of proteins. These have been reviewed in a recent symposium. The proportionate requirements of essential amino acids for maintenance of nitrogen equilibrium in man appear to be similar to those of growing animals for maximum nitrogen retention. Mitchell has suggested the use of an “essential amino acid index” in assessing the nutritive value of food proteins. This index shows a high correlation with biological value as obtained in animal experiments.

Investigation has shown that not only must all the essential amino acids be included in the diet but they must be present at the same time and in the proper proportions. Thus, the amino acid balance of the diet is a factor requiring consideration in evaluating protein nutrition. Recently an attempt has been made to devise a theoretically optimal protein with proper balance of amino acids with which the intake of dietary protein may be compared (25a). This should prove useful in evaluating the protein supply of population groups or even of individuals. Undoubtedly, revisions of this provisional “optimal” amino acid pattern will be forthcoming as it is tested under varying conditions.

Schoenheimer and others have demonstrated that proteins are constantly being broken down and reconstituted, and that there is a constant interchange between amino acids in tissue protein and free amino acids. Accordingly, a continuous supply of amino acids is needed in the diet. Sprinson and Rittenberg (26) have estimated that the time needed for half replacement of body protein in man is about 80 days. The rate of amino acid exchange varies in different tissues; the turnover in liver and plasma accounts for 41% of the total exchange in man. Likewise, depletion of tissue proteins during the development of deficiency is not uniform, some proteins being depleted more rapidly than others and some tissues more severely than others (17). An adequate supply of calories from non-protein sources is important in proper utilization of protein so that amino acids may be spared for protein synthesis and other vital functions, rather than being utilized for energy.

The functions of protein in the body are many. Amino acids are used in the synthesis of cellular proteins of all tissues, of special proteins such as hemoglobin, albumin, globulin, fibrinogen, etc., and in the formation of enzymes, hormones, and antibodies. Amino acids are important in transmethylation and detoxification reactions; some of them have lipotropic function. In addition, they may be deaminated and the remainder of the molecule trans-formed into glucose or fatty acids which are utilized for energy or stored as fat. Thus, proteins are vital for growth, reproduction, maintenance of health and rehabilitation from injury.