To evaluate the validity of fruit and vegetable intakes as it relates to plasma carotenoid and vitamin C concentrations in Chinese women, using three classification schemes.

Intakes were calculated using an interviewer-administered FFQ. Fruits and vegetables, botanical groups and high-nutrient groups were evaluated. These three classification schemes were compared with plasma carotenoid and vitamin C concentrations from blood samples collected within 1 week of questionnaire completion.

Shanghai, China.

Participants (n 2031) comprised women who had participated in a case–control study of diet and breast-related diseases nested within a randomized trial of breast self-examination among textile workers (n 266 064)

Fruit intake was significantly (P < 0·05) and positively associated with plasma concentrations of α-tocopherol, β-cryptoxanthin, lycopene, α-carotene, β-carotene, retinyl palmitate and vitamin C. Fruit intake was inversely associated with γ-tocopherol and lutein + zeaxanthin concentrations. Vegetable consumption was significantly and positively associated with γ-tocopherol and β-cryptoxanthin concentrations. Each botanical and high-nutrient group was also significantly associated with particular plasma nutrient concentrations. Fruit and vegetable intakes and most plasma nutrient concentrations were significantly associated with season of interview.

These results suggest that the manner in which fruits and vegetables are grouped leads to different plasma nutrient exposure information, which may be an important consideration when testing and generating hypotheses regarding disease risk in relation to diet. Interview season should be considered when evaluating the associations of reported intake and plasma nutrients with disease outcomes.

The food-frequency questionnaire (FFQ) can be an efficient tool to evaluate dietary intake in large, population-based studies, especially for specific foods. The objective of this study was to validate the assessment of soy and isoflavone (daidzein and genistein) intakes, measured by an FFQ, by comparing intakes with serum isoflavone concentrations.

Soy and isoflavone intakes and serum isoflavone concentrations were determined as part of a case–control study of dietary factors and risks of benign breast disease and breast cancer. The FFQ, administered during an in-person interview, included six soy-specific line items. Blood was drawn within one week of FFQ completion.

In total, 1823 women living in Shanghai, People's Republic of China.

In this population, soybean milk, fresh bean curd and other bean foods were eaten once per week, and fermented bean curd, fried bean curd puff and soybeans were eaten less than once per week. A significant linear trend (P > 0.01) in serum isoflavone concentrations across increasing categories of soy and isoflavone intakes was observed, indicating that soy and isoflavone intakes, measured by the FFQ, well distinguished serum isoflavone concentrations. Linear trends were also observed in both case and control groups in stratified analyses, suggesting little differential bias by case–control status.

The results suggest that the FFQ provides a useful marker of soy food consumption and isoflavone exposure in this population.