Relevance in population surveys of study place and time to (i) the mean general population serum folate level, and (ii) the excess plasma homocysteine level in the TT versus CC MTHFR C677T genotype.

aMean folate levels average all who were surveyed; SE denotes the standard error due only to within-survey variation. Between-survey variation in folate levels is illustrated in Figure 1.bFrom inverse-variance-weighted averages of within-study differences in log homocysteine; Figure S1, Table S2 in Text S1.cMainly of Japanese, Chinese, or Korean populations; none of South Asians.