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Nutritional Epidemiology

Validation of a Food Frequency Questionnaire for Assessing Dietary Soy Isoflavone Intake among Midlife Chinese Women in Hong Kong^1,2

³ Department of Community and Family Medicine, Chinese University of Hong Kong, Hong Kong SAR; ⁴ Department of Public Health Sciences, University of Toronto, M5G 2L7 Toronto, Canada; and ⁵ Department of Mathematics and Statistics, University of Guelph, N1G 2W1 Guelph, Canada

^* To whom correspondence should be addressed. E-mail: sieugaen2003{at}yahoo.com.

Substantial evidence suggests that soy isoflavones may protect against certain chronic diseases. This study aims to assess the reproducibility and validity of a 47-item semiquantitative soy FFQ (SFFQ) designed to measure the usual intake of soy isoflavones among 145 Chinese mid-life women in Hong Kong. Reproducibility of the SFFQ was assessed by the stability of dietary intake obtained at baseline (SFFQ₀) and at 13-mo follow-up (SFFQ₁). Relative validity was measured by comparing data collected from SFFQ₁ with those derived from 23-d, 24-h dietary recalls (DR) collected during the same 1-y validation period. Isoflavone intake was calculated using analytical values from the Chinese University of Hong Kong Soy Isoflavone Database. A total of 3,217 DR were successfully obtained. The median (interquartile range) absolute intake estimated by the SFFQ₁ was 0.91 (–2.7 to 6.8) mg aglucon equivalents/d higher than the 6.3 (3.7–10.7) mg aglucon equivalents/d measured by the DR (P < 0.0057; Wilcoxon's Signed Rank test). Bland-Altman analysis further demonstrated the presence of significant proportional bias between methods among Cantonese women with above-median intake (Spearman correlation coefficient; r = 0.44; P = 0.0005). Nonetheless, the intraclass and Pearson correlation coefficients, respectively, were 0.84 and 0.72 for non-Cantonese and 0.65 and 0.50 for Cantonese, showing moderate to good levels of reproducibility and validity of the SFFQ (difference between 2 intraclass correlation coefficients, P = 0.09; difference between 2 Pearson r, P = 0.16). The unadjusted and BMI-adjusted correlations were of similar magnitude. The SFFQ is a reasonably valid instrument for assessing dietary soy isoflavone exposure in Hong Kong Chinese mid-life women.