EFFECT OF VARYING INTAKE OF PROTEIN ANDSALTS ONTHE COMPOSITIONANDSPECIFIC GRAVITY OF URINE ' By

It has been shown in a previous paper (1) that the specific gravity of urine is a simple additive function of the concentrations of the individual solutes. By the use of appropriate factors, the specific gravity " pattern," i.e., the fraction of the specific gravity accounted for by each of the constituents of the urine, may be calculated when the composition is known. In urine from subjects on a standard 40 or 50 gram protein diet, urea accounted for 15 to 20 per cent, chloride for 25 to 30 per cent, sulfate and phosphate together for 15 to 25 per cent, bicarbonate for 1 to 5 per cent, and creatinine for 1 to 2 per cent of the observed specific gravity. On a 100 or 110 gram protein diet, urea, sulfate, and phosphate made a slightly greater contribution to the specific gravity. On the lower protein diet, 70 to 80 per cent, and on the higher protein diet, 80 to 90 per cent of the specific gravity could be accounted for by the substances determined (urea, creatinine, inorganic salts, and ammonia). The undetermined fraction of specific gravity made up 10 to 30 per cent of the observed value and, in the same urines, the excretion of undetermined solids amounted to 9 to 15 grams daily, which was 10 to 30 per cent of the total solids. The undetermined solids were organic substances, about one-half being organic acids, and the daily excretion increased only slightly when the protein intake was more than doubled. The specific gravity pattern observed in this previous study was remarkably constant, presumably attributable to the fact that the subjects were on relatively uniform diets, particularly with regard to protein and salt intake. The present study is an extension of this investigation to include the effect of free choice of diet, the influence of specific dietary factors, and the ability of the kidney to vary the composition and specific gravity pattern of urine under ex-