Observations on the Renal Threshold for Ascorbic Acid in Man.

Communications by Harris et al. (1, 2), Johnson and Zilva (3), van Eekelen (4), Hawley et al. (5), and Youmans et al. (6) have established the relationship of the output of vitamin C in the urine to the nutritional state of the organism with respect to this vitamin. They have shown that subjects maintained on diets inadequate in their vitamin C content, excrete only traces of ascorbic acid in the urine. Subjects living on diets containing adequate amounts of vitamin C, however, show urinary output of the vitamin roughly proportional to the vitamin C intake. Harris and Ray (2) have also demonstrated that subjects in a state of vitamin C deprivation show no increased output of urinary ascorbic acid when a single dose of 1 gram of the vitamin is fed, while individuals who have been maintained on diets abundant in vitamin C excrete into the urine as much as 80 per cent of a similar test dose. Harris et al. (1, 2) and Johnson and Zilva (3) have designated the former group of individuals as " unsaturated " the latter as " saturated" with respect to vitamin C. The " unsaturated" group included many normal subjects whose diets were otherwise adequate, but not rich in sources of vitamin C. Some of these individuals failed to show an increased urinary output of vitamin C until large doses of the vitamin had been administered. Patients suffering from clinical scurvy required an even longer time to become "saturated," some of them requiring ten to eighteen days before a significant rise in the urinary ascorbic acid occurred. The close dependence of the values for ascorbic acid in the blood serum or plasma upon the nutritional state of the organism has also been well established (1, 3, 7, 8). Low levels of serum ascorbic acid have been demonstrated in subjects suffering from vitamin C undernutrition with or without manifest scurvy and in the presence of infection (9). Furthermore, van Eekelen (10) has demonstrated that "saturation" of the organism coincides with a certain level of ascorbic acid in the blood (about 1.3 mgm. per 100 cc.) and that when this level is exceeded surplus excretion in the urine results. This report deals with observations on the relationship of the level of blood serum ascorbic acid to the urinary output in normal individuals and in patients with scurvy and chronic infection. It deals also with the relationship of the concentration of ascorbic, acid in the blood serum to the so-called saturation point, and with evidence suggesting the existence of a renal threshold for the vitamin.