Assessment of vegetable, fruit, and antioxidant vitamin intake in cancer epidemiology

Inverse associations are consistently observed in epidemiological studies on the relations between the consumption of vegetables and fruits and different types of cancer. The strength of these associations is, however, unknown amongst others because of measurement error in data on vegetable and fruit intake. The antioxidant (pro)vitamins β-carotene, vitamin C, and vitamin E, are three of many substances in vegetables and fruits which may be responsible for the anticarcinogenic effect. This thesis is focused on the problem of intake assessment of vegetables, fruits, and antioxidant (pro)vitamins.In the first part of the thesis, two studies on the relationships between the consumption of vegetables, fruits, and antioxidant (pro)vitamins and the occurrence of cancer are described. In the Seven Countries Study intake of vitamin C was inversely related to stomach cancer mortality at ecological level. Subjects with low intakes of vegetables, fruits, and β-carotene that were stable over time experienced more than two-fold increased risks of lung cancer in the Zutphen Study than subjects with high stable intakes. A lack of information on the extent of measurement error in the dietary data in both studies hampered the correct interpretation of the results.The second part of the thesis includes several studies on the estimation of measurement error in data on vegetable, fruit, and antioxidant (pro)vitamin intake and biochemical markers. In a study on the effects of frozen storage at -20°C it was shown that vitamin E concentrations in EDTA-plasma decreased dramatically between 6 and 12 months, whereas for β-carotene this took place after 1 year. The use of such plasma in nested case-control or case-cohort studies would result in highly attenuated odds ratios for β-carotene and vitamin E.Reproducibility and relative validity for food group and nutrient intake assessed with an extensive semi-quantitative food frequency questionnaire was also investigated. The questionnaire seemed adequate for ranking subjects according to intake of most nutrients and food groups including fruits, but it did not yield such good results for vegetables, β-carotene, vitamin C for men, and vitamin E for women. The observed correlation coefficients between questionnaire and repeated 24-h recall data may be either over- or underestimates of the true validity coefficients, because of unknown error structure in both types of data. Validity coefficients estimated by a triangular comparison between questionnaire, 24-h recall, and biomarker measurements will probably be overestimates of true validity coefficients.From these studies it is concluded that measurement error in assessing vegetable, fruit, and antioxidant (pro)vitamin intake may be large, which is a handicap for epidemiological studies. Further progress lies in improvement of dietary assessment methods, and probably even more in understanding error structures and the development of analytical methods to recognize and cope with those structures

Variety in fruit and vegetable consumption and 10-year incidence of CHD and stroke

Objective: Consuming a variety of fruit and vegetables provides many different micronutrients and bioactive compounds. Whether this contributes to the beneficial association between fruit and vegetables and incident CHD and stroke is unknown. Design: Prospective population-based cohort study. Setting: The Netherlands. Subjects: Men and women (n 20 069) aged 20-65 years. Participants completed a validated 178-item FFQ, including nine fruit and thirteen vegetable items. Variety in fruit and vegetables was defined as the sum of different items consumed at least once per 2 weeks over the previous year. Hazard ratios (HR) for variety in relation to incident CHD and stroke were calculated using multivariable Cox proportional hazards models additionally adjusted for quantity of fruit and vegetables. Results: Variety and quantity in fruit and vegetables were highly correlated (r=0.81). Variety was not associated with total energy intake (r=-0.01) and positively associated with nutrient intakes, particularly vitamin C (r=0.70). During 10 years of follow-up, 245 cases of CHD and 233 cases of stroke occurred. Variety in vegetables (HR per 2 items = 1.05; 95% CI 0.94, 1.17) and in fruit (HR per 2 items = 1.00; 95% CI 0.87, 1.15) were not related to incident CHD. Variety in vegetables (HR per 2 items = 0.93; 95% CI 0.83, 1.04) and in fruit (HR per 2 items = 1.03; 95% CI 0.89, 1.18) were also not related to incident stroke. Conclusions: More variety in fruit and vegetable consumption was associated with higher intakes of fruit and vegetables and micronutrients. Independently of quantity, variety in fruit and vegetables was related neither to incident CHD nor to incident stroke

Vergelijking inname van nutriënten en contaminanten uit voedselconsumptiepeilingen & duplicaatvoedingen

Voedselconsumptiepeilingen (VCP) en onderzoek van duplicaatvoedingen zijn twee verschillende methoden om de inname van nutriënten en blootstelling aan chemische stoffen te kunnen schatten. Doel van dit onderzoek is het vergelijken van de resultaten van duplicaatvoedingenonderzoek en voedselconsumptiepeilingen die in dezelfde periode zijn uitgevoerd. Het betreft onderzoeken bij (jong) volwassenen en kinderen