Validity of the Self-administered Food Frequency Questionnaire Used in the 5-year Follow-up Survey for the JPHC Study to Assess Folate, Vitamin B6 and B12 Intake: Comparison with Dietary Records and Blood Level

To validate a self-administered food frequency questionnaire (FFQ) for the estimation of dietary intake of folate, vitamins B6 and B12, we compared measures of these vitamins by the FFQ and plasma concentrations among 87 male volunteers aged 40-69 years in the Japan Public Health Center-based prospective study (JPHC Study). No men used folate, vitamins B6 or B12 as supplements. There was a moderate association between energy-adjusted dietary intake and plasma concentration for folate and vitamin B6. The Spearman correlation coefficient was 0.26 and 0.23, respectively, in the total samples. These correlation coefficients were slightly higher when men in the Ishikawa PHC area were excluded from the analysis; the correlation coefficient was 0.29 for folate and 0.26 for vitamin B6. No significant correlation was found between dietary vitamin B12 intake and serum B12 concentration; the correlation coefficient was 0.06 in the total sample and 0.15 when the Ishikawa men were excluded from the analysis. In conclusion, we found that the FFQ used for the 5-year follow-up survey of JPHC Study could reasonably rank individuals by dietary intake of folate and vitamin B6, but not vitamin B12, among Japanese community-dwelling population samples.

Dietary intake of folate, vitamins B6 and B12 has been associated with blood concentrations of homocysteine, which was found to be a risk factor for coronary heart disease and ischemic stroke.'Some studies also suggested that the intake of these vitamins reduces the risk of cardiovascular disease, through an independent effect of blood homocysteine concentrations.2,3I However, these data were mostly from case-control studies, and the information from prospective studies was limited to Caucasian populations.A prospective study is warranted to examine the relation of such vitamin intake and the risk of cardiovascular disease among Japanese, whose dietary habits are markedly different from Caucasians.The Japan Public Health Center-based prospective Study on cancer and cardiovascular diseases (JPHC Study)' is an attempt to examine the relations between various health behaviors and the risk of cancer and cardiovascular disease.In the present study, we examined the validation of a self-administered food frequency questionnaire (FFQ) by comparing measures of folate, vitamins B6 and Bit intake by FFQ and plasma concentrations in a community-dwelling population of Japanese men.

METHODS
The subjects of the present validation study were a subsample of the participants in the JPHC Study Cohort I, aged 40 to 69 years old in four Public Health Center (PHC) areas: Ninohe, Yokote, Saku and Ishikawa.trations were shown for each PHC area.Energy-adjusted dietary intake according to the residual model8 was used to calculate the Spearman correlation coefficients between these vitamin intakes and average plasma concentrations in winter and summer.To examine the validity of quintile classification according to the FFQ, mean plasma concentrations were calculated in each quintile subgroup and ratio of concentrations in the lowest vs. higher quintiles.Furthermore, the quintiles of dietary intakes and plasma concentrations were cross-tabulated to compare these two measures.

RESULTS
Dietary intakes of folate, vitamin B6 and B12 in the four surveyed areas were shown in Table 1.Mean crude folate intake was lowest in Ishikawa, intermediate in Ninohe and Yokote areas, and highest in the Saku area.Mean folate intake per 1,000 kcal of energy, however, was lower in Ninohe than in the other three areas.Both mean values of crude vitamin B6 intake and vitamin B6 intake per 1,000 kcal of energy were lower in Ishikawa than in Table 3 presents the correlation coefficients between dietary folate, vitamin B6 and B12 and the plasma concentrations of these vitamins.The correlation coefficients between crude dietary intake and average plasma concentrations of folate and vitamin Bs in total samples were 0.05 and 0.17, respectively.The corresponding correlation coefficients after the dietary intakes were adjusted for energy were 0.26 and 0.23, respectively.These correlation coefficients were slightly higher when the Ishikawa samples were excluded from the analysis, i.e., correlation coefficients of 0.29 and 0.26, respectively (data not shown).The correlation coefficients between dietary intake and plasma concentration of vitamin B12 were 0.001 for crude intake and 0.06 for energyadjusted intake.A steady increase in mean intake from the lowest to the highest quintile was observed only for folate (Table 4).Table 3.Crude values of dietary folate (* g/day), vitamins B6 (mg/day) and B12 (* g/day) assessed with FFQ, and plasma concentrations of folate (nmollL), vitaminB6 (nmollL), and B12 (pmol/L) and their correlation coefficients in men (n=87) 1 FFQ, food frequency questionnaire. 2 Energy was adjusted by residual model for intake .
One-third of the men had the same category of the quintiles of dietary intake plasma concentrations for folate (Table 5).The respective proportions were 20% for vitamin Bs, against only 11% for vitamin B12.

DISCUSSION
We found a moderate association between dietary intake and plasma concentrations of folate and vitamin B6 when the intakes were estimated by the FFQ used for the 5-year follow-up survey.However, the estimation of dietary intake of vitamin B12 was not associated with plasma concentrations of vitamin B 12.This lack of correlation may be in part due to the larger inter-individual variability in the absorption of vitamin B12 compared with that of folate and vitamin B6.9 The validity of the food frequency questionnaire used in the Nurses' Health Study was tested among Caucasian samples of 57 men and 82 women for the estimation of dietary intake of folate, vitamins B6 and B12.10The correlation coefficients between dietary intake and plasma concentration were 0.62 for folate, -0.16 for vitamin B6, and 0.34 for vitamin B12.When persons using these vitamin supplements were excluded from the analysis, the respective correlation coefficients were 0.51, 0.04, and 0.20.Compared with these correlation estimates, ours were somewhat lower for folate and vitamin B12 and higher for vitamin B6.In the Framingham Heart Study in which the same questionnaire as in the Nurses' Health Study was used, the correlation between dietary intake and plasma concentration was found to be moderate for folate and vitamin B6, but low for vitamin B12.9 Therefore, the dietary measure for folate was consistently valid in two Caucasian studies.In the present study, the correlation was higher for folate and vitamins B6 than for B12.The population level of dietary folate intake was correlated with the population level of plasma folate level; the Ishikawa men showed a lower dietary intake and plasma folate level than men in the other three PHC areas.
The validity of the dietary measures for vitamin B6 and B12 has not been consistent among various studies.The dietary and biochemical measures of vitamin B6, for example, were moderately associated in the Framingham Heart Study and our study, but not in the Nurses' Health Study.The measures for vitamin B12 were moderately associated in the samples of the Nurses' Health Study, but not in the Framingham Heart Study or the present study.
Low crude folate intake was observed in Ishikawa.Since a low dietary folate intake is a strong determinant for high plasma homocysteine concentrations which may increase the risk of cardiovascular disease,12 we will examine the adverse health effect in Ishikawa men in the PHC cohort analyses.
In conclusion, we found that the FFQ used for the 5-year follow-up survey of JPHC Study could reasonably rank individuals by dietary intake of folate and vitamin B6 for epidemiological use among Japanese community-dwelling population samples .The estimate of vitamin B12 from the FFQ, however, might not be reliable.
5An FFQ with 138 food items was filled out by subjects from the Ninohe, Yokote and Saku PHC areas in February of 1995, and by subjects from the Ishikawa PHC area in February of 1996.Blood samples were collected in February (winter) and August (summer) of 1994 from the subjects of the Ninohe, Yokote and Saku PHC areas, and in February

Table 1 .
Dietary intake of folate, vitamin B6, and B12 assessed with FFQ by area in men

Table 2 .
Plasma concentrations of folate, vitamins B6 and B12 by area and season in men S-100 Validity for Folate, Vitamin Be and B12 the other three areas.Both mean values of crude vitamin B12intake and vitamin B12 intake per 1,000 kcal of energy were low-