The effect of diet on plasma homocysteine concentrations in healthy male subjects

Objective: To determine the effect of habitual omnivorous and vegetarian diets on folate and Vitamin Bit status and the subsequent effect on homocysteine concentration. Design: Cross-sectional comparison of free-living habitual meat-eaters and habitual vegetarians. Setting: The study was conducted at RMIT University, Melbourne. Subjects: One hundred and thirty-nine healthy male subjects (vegans n = 18, ovolacto vegetarians n = 43, moderate meat-eaters n = 60 and high meat-eaters n = 18) aged 20-55 y who were recruited in Melbourne. Outcome measures: Fasting plasma or serum from each subject was analysed for folate, vitamin B-12 and homocysteine concentration. A semi-quantitative Food Frequency Questionnaire was completed by a subset of subjects from each group to determine methionine intake. Results: The two meat eating groups consumed significantly greater levels of methionine (P < 0.001), There was no clear trend in plasma folate status between groups, however the plasma vitamin B-12 concentration decreased progressively from the high-meat-eating group to vegans (P < 0.05). An inverse trend was observed with plasma homocysteine concentration, with vegans showing the highest levels and high meat eaters the lowest (P < 0.05). Conclusions: Dietary methionine intake has no observable effect on plasma homocysteine concentration. In habitual diets, where folate intake is adequate, lowered vitamin B-12 intake from animal foods leads to depleted plasma vitamin B-12 concentration with a concomitant increase in homocysteine concentration. The suggested mechanism is the failure to transfer a methyl group from methyl tetrahydrofolate by vitamin B-12 in the remethylation of homocysteine of homocysteine to methionine. Sponsorship: The study was funded by the Meat Research Corporation of Australia. Descriptors: homocysteine; folate; vitamin B-12; methionine; cysteine; trans-sulphuration; remethylation; ovolacto-vegetarian; vegan; vitamin B-6 hyperhomocyseinemia; omnivore; vascular disease.