Vitamins are essential molecules that must be taken in from the diet because the human body is incapable of synthesizing them. Water-soluble vitamins are absorbed in the upper small intestine. Bacteria in the large intestine synthesize many water-soluble vitamins; however lower intestinal bacterial synthesis of vitamins is not a major source of vitamins for humans. In contrast, cattle have a digestive system that is structurally different and bacterial synthesis of vitamins contributes much more to the total absorbed vitamins in these animals.
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Thiamine, also called B1, is important for glucose and protein metabolism. Thiamine deficiency primarily occurs in people lacking sufficient dietary sources and can cause beriberi, which is characterized by neuropathy in the extremities, other neurological disorders and heart abnormalities. Thiamine is found in many plant and animal products, but excessive food processing, such as what occurs in the production of white flour and white rice, strips most of the thiamine from the food. Thiamine is produced by some bacteria in the colon.
Riboflavin, also called B2, is important for energy production. Riboflavin is found in many plant and animals sources and some bacteria in the colon can synthesize riboflavin. The Linus Pauling Institute reports that riboflavin deficiency is rare.
Pantothenic acid, also called B5, is essential for the synthesis and metabolism of proteins, carbohydrates and fatty acids. The Linus Pauling Institute reports that experiments in cells that line the colon do suggest they have a specific process of pantothenic acid uptake suggesting it can be absorbed from the lower intestine.
Pyroxidine, also known as B6, plays a predominant role in protein and steroid hormone action. Some very strict vegetarians may have to pay particular attention to finding sources of vitamin B6 as it is predominant in chicken, fish and meat. Because strict vegetarians are sometimes deficient in vitamin B6 it is unlikely that bacterial synthesis in the gut is a reliable source for this water-soluble vitamin.
Biotin, also called B7, functions in several biochemical reactions and is important for glucose and fatty acid synthesis. The Linus Pauling Institute reports that it is unknown if biotin is absorbed in humans, but experiments in cells that line the colon do show they have a specific process for biotin uptake suggesting it can be absorbed from the lower intestine.
Folic acid, also called B9, is important for DNA synthesis, growth and development. Folate deficiency is common especially in pregnant women. The Office of Dietary Supplements reports that in order to prevent widespread deficiency folate has been added to certain foods including breads, cereals, flours since 1996. Because folate deficiency is common it is unlikely that bacterial synthesis in the gut contributes much to total absorbed folate.
Cobalamin, also called B12, is essential for nervous system function and in the production of red blood cells. It is also involved in DNA synthesis and energy, fat and protein metabolism. Food sources of cobalamin include seafood, beef, chicken mild and cheese. Strict vegetarians can avoid deficiencies either by taking supplements or eating vegan foods that are fortified with vitamin B12. Because strict vegetarians are sometimes deficient in vitamin B12 it is unlikely that bacterial synthesis in the gut is a reliable source for this water-soluble vitamin.