For the body to make use of protein, you need B6 and, together with zinc, it is needed to make the enzymes that digest food - the process essential for turning food into energy. B6 also boosts brainpower as well as immunity. It helps make the brain chemical serotonin, a deficiency in which can cause depression. One study showed that about a fifth of depressed people are deficient in pyridoxine. It can become toxic above l,000mg a day. The ideal amount to supplement is 50 to lOOmg a day. Good multivitamins provide around 50mg.