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Letter to the Editor |
Federal Research Centre for Nutrition, Institute of Nutritional Physiology, Haid-und-Neu-Strasse 9, 76131 Karlsruhe, Germany, E-mail: bernhard.watzl{at}bfe.uni-karlsruhe.de
Dear Sir:
In light of recently published data from the Nurses' Health Study (NHS) and the Health Professionals Follow-Up Study (HPFS) on fruit and vegetable consumption and cancer risk (1–3), we would like to comment on the validity of the dietary assessment and on the conclusions that were based on these results. Over the past decade, these 2 cohorts have been highly valuable study groups, generating important data on the relation between diet and disease risk. However, the recent analyses of the combined study groups (a total of 
136089 participants) have raised several questions (1–3).
Data on the frequency of fruit and vegetable intake among participants of the HPFS show that 89% of all men consumed 2 servings of vegetables/d and 62% consumed 2 servings of fruit/d (3). Without discussing the problems of overreporting on food-frequency questionnaires for foods preceived as healthy and the potential measurement error of repeated dietary assessments (2, 4), such a low intake of fruit and vegetables should result in low carotenoid intakes (5). However, as reported by Michaud et al (1), men with lung cancer in the lowest quintile of the HPFS had a total carotenoid intake of 7.8 mg/d and men in the highest quintile had an intake of 33.2 mg/d. Because vegetables are the major carotenoid sources in the diet, we wonder how the reported low intake of vegetables could result in the relatively high carotenoid intakes. In comparison, German men consume 
2.5 servings of vegetables daily (6), which in terms of total carotenoids results in a daily average intake of 5.6 mg/d (7). At the time of the dietary assessments in the HPFS, no carotenoid supplements were available except for ß-carotene. Therefore, supplements could not have contributed to the high carotenoid intake. We will not comment on the observations that increased consumption of fruit and vegetables is related to increased energy intake, and on the small variability in the intake of fruit and vegetables (3), because others have already pointed out these issues (8, 9).
A further point is the question of how representative the prevalence of diet-related cancer in nurses and physicians is of the prevalence in the general population. Theoretically, a lower cancer prevalence in health professionals may not correspond to the same reduction in cancer risk in the general population with a similar intake of fruit and vegetables.
Because the media coverage of the results from the NHS and the HPFS is so extensive, it would be helpful to the nutrition community if the discrepancy between the low vegetable consumption and the calculated high carotenoid intake and the other raised questions were resolved before we draw final conclusions from these studies regarding diet and cancer prevention.
REFERENCES
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