In vivo methylation capacity is not impaired in healthy men during short-term dietary folate and methyl group restriction.

Ten healthy adult men were fed a diet low in folate and exogenous methyl groups to study the effects on in vivo methylation capability. The men were housed in a metabolic unit for the entire 108 d of the study. After a 9-d baseline period (Period 1), the men were fed a soy-product-amino acid defined diet for 45 d, which provided 25 micrograms/d of folate for 30 d (Period 2) and, with a folate supplement, 99 micrograms/d for 15 d (Period 3). During Period 2 and Period 3, the low methionine and choline diet was supplemented with methionine for half the subjects to vary the dietary methyl group intake. The periods were then repeated over the next 54 d (Periods 4-6), with a crossover of methionine intakes in Period 5 and Period 6. A 1-g oral dose of nicotinamide was given at the end of each period and methylated urine metabolites determined. Other measures related to in vivo methylation capability included urine creatinine, and plasma and urine carnitine. Even with moderate folate depletion, none of these measures was decreased by low methionine and choline intakes. Plasma methionine concentrations were unchanged throughout. Limiting exogenous methyl group intake by restricting dietary methionine and choline did not impair in vivo methylation capabilities for the variables tested, even at low folate intake.