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OBJECTIVE To determine the effect of habitual omnivorous and vegetarian diets on folate and vitamin B12 status and the subsequent effect on homocysteine concentration. METHODS Cross-sectional comparison of free-living habitual meat-eaters and habitual vegetarians. METHODS The study was conducted at RMIT University, Melbourne. METHODS 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. METHODS Fasting plasma or serum from each subject was analysed for folate, vitamin B12 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 B12 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 B12 intake from animal foods leads to depleted plasma vitamin B12 concentration with a concomitant increase in homocysteine concentration. The suggested mechanism is the failure to transfer a methyl group from methyl tetrahydrofolate by vitamin B12 in the remethylation of homocysteine to methionine.
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