Biotin is involved in the conversion of protein and carbohydrate into fat. Biotin also helps to maintain normal blood glucose levels from fat and protein when carbohydrate is low. Biotin deficiencies are rare but can result in anorexia, nausea and depression. Folic acid is important as a component of tetrahydrofolic acid, which is involved in the transfer of one carbon groups. Folic acid is important for the transfer of methyl groups to help resynthesize methionine from homocysteine and to make choline.
Folic acid is also important in the synthesis of purines and pyrimidines.
A deficiency may also cause neural-tube birth defects. Yeast, spinach and beef liver are good sources of folic acid. It helps in the formation of red blood cells and in the maintenance of the central nervous system. B12 is essential for several enzyme systems. Most reactions using B12 involve the transfer of one carbon units for the methylation of homocysteine.