Nutrition and Plasma Proteins

Abstract

Because the plasma proteins are so much easier to sample than other body proteins much of the early work on the effects of protein nutrition centred on changes in plasma protein. This accessability was technically an advantage, but it also tended to distort the true picture of the nutritional role of the circulating and fixed tissue proteins respectively. Thus Whipple and his colleagues1 concluded that during protein depletion there was “raiding” of tissue proteins to maintain plasma protein, and that “the blood proteins in these experiments take priority over the organ and tissue proteins”. This conclusion is hardly justified in view of the techniques used to produce protein depletion: the dogs were fed a protein-free diet and plasma proteins were removed by plasma-pheresis. If thereafter plasma protein was made at the expense of tissue protein this is no more than a demonstration of homeostasis. Had it been feasible to remove progressively portions of liver no doubt this organ too would have been regenerated at the expense of other proteins which were not specifically depleted, and it would have been equally logical to conclude that liver protein had priority over other tissues in time of protein shortage. The true pricture is that protein depletion in an intact animal, however, it is produced, affects all the body proteins since they are in dynamic equilibrium2 although the extent and timing of protein loss varies greatly from one organ to another.3 It is convenient, therefore, to consider the effects of nutrition on plasma proteins from three aspects: first, the interrelationship of plasma and tissue protein stores in times of nutritional stress of various kinds; second, the diagnostic value of plasma protein concentrations in assessing nutritional status; and third, the dynamic changes in plasma protein metabolism which occur in response to changes in diet. Only studies in intact mammals especially man, will be considered, since elegant and ingenious studies on isolated perfused organs are described elsewhere.