Effects of serum B vitamins on elevated plasma homocysteine levels associated with the mutation of methylenetetrahydrofolate reductase gene in Japanese.

High plasma homocysteine, a risk factor for atherosclerosis, is frequently caused by a common mutation in the gene for the enzyme, 5,10-methylenetetrahydrofolate reductase (MTHFR), C677T (alanine to valine substitution) or low intake of B vitamins that affect the remethylation or transsulfuration pathways in homocysteine metabolism. However, the interaction of the C677T mutation and B vitamins other than folate has not been well elucidated. We conducted a cross-sectional survey of 324 men and 641 women who participated in a 1996 health examination under a hypothesis that high nutritional status of folate, vitamin B12 and vitamin B6 expressed as high serum levels, may compensate for the hyperhomocysteinemia associated with homozygosity for the C677T mutation, but not for having the mutation per se. Age-adjusted plasma homocysteine levels were higher for both men and women with the homozygous genotype for the mutation than those who were heterozygous or had no mutation. Elevated homocysteine levels in homozygous genotype was attenuated among persons with higher serum levels of vitamin B12 and folate, but not vitamin B6, and among persons with the combination of lower folate and higher vitamin B12 and of higher folate and higher vitamin B12, split by the median. These findings suggest that elevated homocysteine levels among Japanese with the homozygous genotype for the MTHFR gene mutation can be modified efficiently by dietary supplement of vitamin B12 as well as folate.