The B vitamins are eight water-soluble vitamins that play important roles in cellmetabolism. Historically, the B vitamins were once thought to be a single vitamin, referred to as Vitamin B (much like how people refer to Vitamin C or Vitamin D). Later research showed that they are chemically distinct vitamins that often coexist in the same foods. Supplements containing all eight B vitamins are generally referred to as a vitamin B complex. Individual B vitamin supplements are referred to by the specific name of each vitamin (e.g. B1, B2, B3).
Deficiency, along with a deficiency of tryptophan causes Pellagra. Symptoms include aggression, dermatitis, insomnia, weakness, mental confusion, and diarrhea. In advanced cases, pellagra may lead to dementia and death.
Deficiency does not typically cause symptoms in adults but may lead to impaired growth and neurological disorders in infants.
Deficiency results in a macrocytic anemia, and elevated levels of homocysteine. Deficiency in pregnant women can lead to birth defects. Supplementation is often recommended during pregnancy. Researchers have shown that folic acid might also slow the insidious effects of age on the brain.
Deficiency causes macrocytic anemia, elevated homocysteine, peripheral neuropathy, memory loss and other cognitive deficits. It is most likely to occur among elderly people as absorption through the gut declines with age; the autoimmune disease pernicious anemia is another common cause. In rare extreme cases, paralysis can result.
Many of the following substances have been referred to as vitamins because they were believed to be vitamins at one time, and they are relevant to vitamin nomenclature in that the numbers that were assigned to them form "gaps" in the series of B-vitamin names. Some of them, though not essential to humans, are essential in the diets of other organisms; others have no known nutritional value. While they are non-"essential" in that they may be synthesized by the body from other starting materials, they have dietary significance. See also Orthomolecular medicine.
Vitamin B7: "Vitamin I" of Centanni E. (1935) — also called 'Enteral factor' — is a water and alcohol soluble rice-bran factor which prevents digestive disturbance in pigeons. It governs the anatomical and functional integrity of the intestinal tract. Later found in yeast. Possible candidates for this substance are inositol, niacin (nicotinic acid), and biotin. Carnitine was also claimed to be a candidate but is not soluble in alcohol.
Vitamin B11: Pteryl-hepta-glutamic acid – Chick growth factor, which is a form of Folic acid. Later found to be one of five folates necessary for humans; (L-carnitine) is called Vitamin B11 in France.
Vitamin B13: Orotic acid.
Vitamin B14: cell proliferant, anti-anemia, rat growth, and antitumor pterin phosphate named by Earl R. Norris (biochemist of folic acid fame). Isolated from human urine at 0.33ppm (later in blood), but later abandoned by him as further evidence did not confirm this. He also claimed this was not Xanthopterin.
Vitamin B16 (dimethylglycine) – also known as DMG. (However Lipoic acid was discovered and named a B-Vitamin after B15 and before B17)
Vitamin B17 (Amygdalin, Nitrilosides, or laetrile) – A substance found in a number of seeds, sprouts, beans, tubers and grains. While toxic in large quantities, proponents claim that it is effective in cancer treatment and prevention. 
All B vitamins are water soluble, and are dispersed throughout the body. Most of the B vitamins must be replenished daily, since any excess is excreted in the urine. A six year cobalamin store can be found in the liver, despite its water soluble nature.
Vitamin B sources
Vitamin B comes from a number of natural sources, including potatoes, bananas, lentils, chilli peppers, tempeh, liver oil, liver, turkey, and tuna. Nutritional yeast (or brewer's yeast) and molasses are especially good sources of Vitamin B. Marmite and the iconic Australian spread Vegemite bills itself as "one of the world's richest known sources of vitamin B". As might be expected, due to its high content of brewer's yeast, beer is a good source of B vitamins--in fact, beer is sometimes referred to as "liquid bread"--although this may be less true for filtered beersand the alcohol in beer impairs the body's ability to activate vitamins.
Another popular means of increasing one's Vitamin B intake is through the use of dietary supplements purchased at supermarkets, health centers, or natural food stores.
B vitamins are also commonly added to energy drinks.
One mnemonic to remember the most commonly referenced B vitamins is "The (B1, Thiamine) Rhythm (B2, Riboflavin) Nearly (B3, Niacin) Proved (B6, Pyridoxin) Contagious (B12, Cobalamin)".
^ Vera Reader (1930). "The assay of vitamin B4". Biochem J. 24 (6): 1827–31.