Vitamin C Requirements of Man Re-examined

Abstract

A new excretory pathway for ascorbic acid catabolism in man, the respiratory tract, is here first experimentally reported. We have established that in man the major portion of ascorbic acid is excreted within the first few hours following ingestion, through the expired air and the urine, in a manner similar to our reported findings in the guinea pig and the monkey. On a diet high in ascorbic acid the half-life of ascorbic acid is short, conversely on a diet lower in ascorbic acid the half-life of ascorbic acid in the human being is longer. For man the half-life varied from thirteen to thirty days depending on the daily dietary intake of ascorbic acid. This variation also was found for the guinea pig and the monkey. Each molecule of C¹⁴O₂ in the expired air represents a molecule of the radioactive ascorbic acid administered. The excretion of ascorbic acid in the urine of man represents intact ascorbic acid as well as degradation products. According to older calculations the total body store of ascorbic acid is about 5 gm. From our calculations we find that the total miscible ascorbic acid space is in the neighborhood of 1 gm. for the normal adult. Definitions for minimum requirement, recommended daily dietary allowance and optimum requirement are restated. The minimum daily requirements of 1.0 to 3.0 mg. ascorbic acid for man, estimated from our data, are approximately one twenty-fifth to one seventieth of the recommended American daily requirement. The recommended daily requirement of vitamin C presented here is for man under normal conditions of environment and activity. However, our previous studies have indicated an increased requirement for vitamin C in wound healing, infections and other special conditions. By obtaining the half-life of ascorbic acid for the normal adult on a reasonable adequate daily dietary allowance, 30 mg. daily of vitamin C, and utilizing the data obtained from the summation of exhalatory and urinary excretion pathways for calculation, new values are presented for the body pool and minimal requirements of ascorbic acid. It is hoped that these values, based on sound experimental procedures and evidence, will help to clarify and formulate the daily minimum requirements and dietary allowances of vitamin C for the normal adult.