Distribution of Biotin
This vitamin enjoys an ubiquitous distribution in plant and animal tissue. Foods rich in biotin include liver, kidney, molasses, yeast, milk, and egg yolk. The high content of biotin in egg yolk and of avidin in egg white is an interesting point for speculation. Vegetables, nuts, and grains also contain biotin in both the free and the combined form.
For man and for some other animals the biotin produced by the bacteria of the large mtestine may be a more important source (quantitatively) than that of the diet. A study of :he distribution of a near physiological dose of carboxyl labeled (C14) biotin in deficient chicks and rats showed that about 16 per cent of the radio-activity was found in the liver, and around half of this was in the pH 5-2 fraction of the liver cell supernatant. Microsomes contained only a small amount, and very small amounts were oxidized to CO2, Most of the tissue biotin as found in bound form, except in microsomes. Only about one tenth of the normal quantity of the vitamin was found in the liver of the depleted animals.
Requirement of Biotin
The biotin requirement of microorganisms is well established. In animals there is no practical method now of establishing the quantitative need for this vitamin, since large amounts of it are supplied through intestinal bacterial synthesis. In some of the earlier balance studies in man it was found that the urinary excretion of biotin often exceeded the dietary intake, and the fecal excretion was greater than the intake in every case. At any rate, daily needs appear to be met by diets containing 150-300 Ilg of the vitamin.
