Protein Malnutrition
Reporter and Curator: Dr. Sudipta Saha, Ph.D.
A large part of the world’s population is undernourished by the standards of Western Europe and North America. Scientists and nonscientists alike recognize as one of the major challenges of our time the problem of how to ensure that the production and distribution of food keep pace with the increasing number of mouths to be fed. In the world as a whole the most widespread and serious dietary deficiency is that of protein. This fact emerges clearly from the reports of the expert committees of WHO and FAO (World Health Organization, 1951, 1953). Nevertheless, many protein chemists, even those associated with medical research, may not realize the extent and severity of protein malnutrition, because it occurs chiefly in the technically underdeveloped countries far from where they work.
Dietary histories and response to treatment point to deficiency of total protein as the primary cause of the clinical syndrome kwashiorkor. The level of calorie intake has an important influence on the pattern of the disease. Deficiency of one or more specific amino acids, or amino acid imbalances in the diet, may perhaps be responsible for some of the symptoms and signs, particularly those whose incidence varies from one part of the world to another. All these variations on a theme are covered by the general term protein malnutrition. The onset is often precipitated by the added burden of diarrhea, infection, and parasitic infestation. The nutritional state influences the resistance to infection, and conversely the presence of an infection affects the state of nutrition. A further contributory factor may be the psychological upheaval in the child when the next baby in the family is born. At the root of all these causes lie poverty, ignorance, and disruption of the family life.
The planning of preventive measures cannot be effective unless it is based on some knowledge of the magnitude of the problem to be tackled. At a very rough estimate, in some countries perhaps 10% of the children suffer from severe protein malnutrition at some age between birth and 4 years. The marginal deficiency states must be much more common, Clinical signs and biochemical changes are of little value in diagnosing the early case; a deficit in body weight still seems to be the best criterion. Prevention ideally would be by greater production and consumption of animal protein, and by the increased use of skim milk and of surplus fish at present often wasted. However, animal protein is likely to remain scarce and expensive. Plant sources are being investigated with a view to encouraging not only domestic production, but also the production on an industrial scale of cheap foodstuffs rich in protein. A preventive program that is nutritionally sound may fail if account is not taken of local food habits, traditions, and customs. Protein requirements are affected by the quality of protein, the intake of calories, and by the state of the body (growth, the presence of disease, etc.). The maintenance requirement and the amount required for growth in children can be estimated, but the requirement for health is still unknown. For the time being, the allowances of protein recommended for people in the world as a whole are based empirically on the known physiological requirement with an arbitrarily added wide margin of safety.
The absorption of nitrogen is remarkably efficient even in severely malnourished infants. In general the nitrogen of plant proteins is less well absorbed than that of milk. When a baby receives a diet in which the protein is derived entirely from vegetabIe sources, incomplete absorption of nitrogen may play a significant part in the production of protein malnutrition. The malnourished baby who responds to treatment is able to retain and utilize nitrogen very efficiently; there is no evidence of any impairment in the mechanisms of protein synthesis. It is possible, however, that these mechanisms may be irreversibly damaged in babies who die, and that this may be the cause of death. The level of calorie intake has an important influence on the efficiency of utilization of nitrogen. An adequate calorie intake promotes conservation of nitrogen in the body as a whole when supplies of protein are short, but this protective effect may not be exerted equally in all organs. In this way the level of calorie intake may modify the pattern of protein depletion. A greater than normal calorie intake is needed for the restoration of depleted protein stores.
The discussion of protein metabolism in protein malnutrition has been purposely limited to a narrow field-to studies made on man, and to the few animal experiments that have a direct bearing on those studies. For technical reasons most of the work discussed relates to plasma proteins. There is a conflict of evidence between results obtained in man and animals about the effect of protein depletion or a low protein diet on the rate of catabolism of plasma albumin. It is of great importance to settle this point. A priori there seems no reason why the rate of protein catabolism should be affected by nutritional state. Preliminary studies with radioactive methionine in infants suggest, as working hypotheses, that in protein malnutrition there may be an increase in the reutilization of amino acids liberated by tissue catabolism, and an apparent concentration of protein synthesis in the more essential organs at the expense of the less essential. There is some experimental support for both these ideas, but further work is badly needed. The concept of protein stores or reserve protein is based entirely on dynamic and not on chemical considerations. It is suggested that the essential difference between a “labile” and a “fixed” protein is a difference in turnover rate. An attempt is made to show that the changes produced by protein depletion in the protein content of organs such as liver and muscle are a necessary consequence of the metabolic characteristics of proteins in those organs. There may be no need to invoke the help of homeostatic or compensatory regulations to explain the changes found in protein depletion.
Aging and growth are processes during which some metabolic adjustments must take place. It is believed that it may be better to regard the changes which are found in protein malnutrition in a similar light: as evidence of an alteration in functional pattern, rather than of damage or disease. Protein malnutrition in man has two aspects-a practical and a theoretical one. From the practical point of view it is an extremely common disease with a high mortality, and there is every reason to believe that it will become more common unless urgent preventive measures are taken. Theoretically it raises many questions that are of interest in relation to other branches of medicine and biochemistry. It is believed that the two aspects are linked, and that progress towards prevention is still impeded by our lack of basic knowledge as well as by our failure to apply what is already known. In protein malnutrition there is no sharp line between health and disease. The simple concept of specific deficiency diseases that grew from the discovery of vitamins is not applicable. We have to go back instead to the ideas of an earlier era, when nutrition was regarded as a branch of physiology, concerned with the functions, fate, and metabolic interrelationships of the major nutrients.
It is a characteristic of protein metabolism that nitrogen balance can be maintained at many different levels of protein intake. These different steady states are achieved by adjustments of the amount and distribution of proteins in the body as a whole, in organs, and in cells. It is believed that these changes in amount and distribution of proteins must result in alterations of metabolic pattern, with a gradation of change from an optimum, which cannot be defined, to a state of irreversible breakdown incompatible with life. In the intermediate stages function is modified and efficiency perhaps impaired. It seems possible that variations in diet, and particularly in the amount and quality of the protein, may underlie many of the differences in incidence and symptomatology of disease which are gradually being uncovered in different parts of the world.
Source References:
http://www.sciencedirect.com/science/article/pii/S0065323308603095#
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