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

A soy-protein isolate diet that contained essentially no bioavailable vitam in B-6 was used to establish the quantitative effect of excess dietary meth ionine on the vitamin B-6 requirement of young chicks. When made adequate i n vitamin B-6, chicks fed the basal diet required 2 g/kg supplemental DL-me thionine to achieve maximal growth, and 10 g/kg additional DL-methionine (t otal = 12 g/kg) was found to be a tolerable excess level that would not dep ress voluntary food intake or growth rate. When chicks were fed seven grade d doses of supplemental pyridoxine (PN) in diets that contained either adeq uate (2 g/kg) or excess (12 g/kg) methionine, the vitamin B-6 requirement f or 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. In deed, this level of excess dietary methionine depressed (P < 0.01) growth a t all PN dose levels less than or equal to1 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) in take should be considered important factors in setting safe and adequate re quirement levels for vitamin B-6.