Vitamin B6

Vitamin B6 refers to a group of (deep breath) 2-methyl-3-hydroxy-5-hydroxymethyl-pyridines, which the body processes into several important cofactors, the most common being pyridoxal-5'-phosphate, or PLP. PLP itself is essential to over 100 enzymatic reactions, all involving the production or modification of amino acids or proteins. 

The deficiency of B6 leads to lower hemoglobin in the blood as a result of reduced heme production, leading to a condition called microcytic hypochromic anemia. It is possible to overdose on vitamin B6, leading to severe neurological problems. It is exceedingly rare to overdose from food intake, rather it is caused by excessive intake of supplements. Whole grains and tuna are excellent sources of B6.