Excess dietary methionine markedly increases the vitamin B-6 requirement of young chicks

A soy-protein isolate diet that contained essentially no bioavailable vitamin B-6 was used to establish the quantitative effect of excess dietary methionine on the vitamin B-6 requirement of young chicks. When made adequate in vitamin B-6, chicks fed the basal diet required 2 g/kg supplemental DL-methionine to achieve maximal growth, and 10 g/kg additional DL-methionine (total = 12 g/kg) was found to be a tolerable excess level that would not depress voluntary food intake or growth rate. When chicks were fed seven graded doses of supplemental pyridoxine (PN) in diets that contained either adequate (2 g/kg) or excess (12 g/kg) methionine, the vitamin B-6 requirement for maximal growth was found to increase (P: < 0.01) from 0.73 to 1.05 mg/kg, a 44% increase, when 10 g/kg excess methionine was present in the diet. Indeed, this level of excess dietary methionine depressed (P: < 0.01) growth at all PN dose levels < or =1 mg/kg, but not at PN doses of 1.2 or 1.4 mg/kg. Because dietary intakes of both vitamin B-6 and methionine can affect plasma homocysteine levels, dietary methionine (and protein) intake should be considered important factors in setting safe and adequate requirement levels for vitamin B-6.