<P> Animals store vitamin B in liver and muscle and some pass the vitamin into their eggs and milk; meat, liver, eggs and milk are therefore sources of the vitamin for other animals, including people . For humans, the bioavailability from eggs is less than 9%, compared to 40% to 60% from fish, fowl and meat . Insects are a source of B for animals (including other insects and humans). </P> <P> Animal sources with a significant content of vitamin B (range among top 20 sources of 50 to 99 μg per 100 grams) include clams, organ meats (especially liver) from lamb, veal, beef, and turkey, fish eggs, mackerel, and crab meat . </P> <P> B is produced in nature only by some prokaryotes (certain bacteria and archaea); it is not made by any multicellular or single - celled eukaryotes . It is synthesized by some gut bacteria in humans and other animals, but humans cannot absorb the B made in their guts, as it is made in the colon which is too far from the small intestine, where absorption of B occurs . Ruminants, such as cows and sheep, absorb B produced by bacteria in their guts . For gut bacteria of ruminants to produce B the animal must consume sufficient amounts of cobalt . </P> <P> Grazing animals pick up B and bacteria that produce it from the soil at the roots of the plants they eat . </P>

Which vitamin is produced by bacteria in the colon
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