Intestinal Bacterial Synthesis and Metabolism of Vitamins

The human intestinal bacteria can synthesize vitamin K, a member of the naphtoquinone family. The liver cannot synthesize the prothrombin complex, a blood-clotting factor, unless menaquinone, a substituted naphthoquinone, is present. The peptides that become the glycopeptides of the prothrombin complex require menaquinone for synthesis from the appropriate RNA codon.

Bacteria found in the intestine can also synthesize homologues of menaquinone-7 (vitamin K2). The synthesized homologues range from the 6-isoprene unit side chain containing menaquinone-6 to menaquinone-13 (26,27). The vitamin K bacterial reactions occur, in part, in the ileum, where the menaquinone is absorbed. The importance of bacterial synthesis of vitamin K has been demonstrated in human studies (28). Adult subjects maintained on a low vitamin K diet for several weeks did not develop a deficiency. When these subjects were treated with antibiotics such as neomycin that reduce the bacterial population of the intestine, a significant decrease in plasma prothrombin levels was noted (28,29).

Most of the vitamin B12 (cyanocobalamin) required by humans comes indirectly from the meat and milk of ruminants. The synthesis of B12 in ruminants is exclusively of bacterial origin. The human intestinal microflora also synthesize vitamin B12 as evidenced by the fecal secretion of approximately 5 micrograms per day. However, it appears most of the bacterially formed B12 in humans occurs in the large bowel where absorption most likely does not occur due to lack of B12 mucosal receptors. However, there is a study of healthy subjects from Southern India that reported the synthesis of vitamin B12 in the jejunum and ileum, an area where absorption of the vitamin can occur (30). It was demonstrated that pseudomonas and klebsiella were two of the bacteria that synthesized B12 in the small intestine.

Biotin is synthesized by the human intestinal microflora. The administration of antibiotics can lower human urinary biotin levels. The importance of bacterial involvement in biotin synthesis has been demonstrated in germfree rats. The germfree animals require biotin in their diet; in contrast conventional rats can thrive without dietary biotin (22).

Folic acid and thiamine B complex vitamins are also synthesized by bacteria in the intestinal tract. This synthesis does not solely provide for human requirements, and dietary sources of these vitamins are required to prevent deficiencies (31).

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