Correspondence to: William B. Grant, PhD, Sunlight, Nutrition, and Health Research Center (SUNARC), 2107 Van Ness Ave., Ste. 403B, San Francisco, CA 94109-2529 (e-mail: wgrant{at}sunarc.org).
The finding, recently reported in the Journal (1), that total consumption of fruits and vegetables among nurses and other health professionals in the United States is associated with reduced risk of cardiovascular disease but not cancer is very interesting. Many studies have reported that frequent consumption of fruits and vegetables does, indeed, reduce the risk of cancer (2). Compounds found in many fruits and vegetables are well known to have anticancer properties; these include allyl sulfides (in allium-family vegetables), glucosinolates (in cruciferous vegetables), and lycopene (in tomatoes), as well as a large number of antioxidants found in many fruits and vegetables (3). Thus, it is important to seek a reason for the difference in the findings by Hung et al. (1) and those by others.
A likely reason for the discrepancy is that most of the fruits and vegetables consumed by the two cohorts may have been cooked, rather than eaten raw. Green leafy vegetables was the only category for which a statistically significant risk reduction was found for major chronic disease, although a risk reduction was not seen for cancer; it may have represented a very small fraction of total fruits and vegetables consumed by the cohorts. Several researchers have reported that raw vegetables are associated with a statistically significant risk reduction for cancer, whereas cooked vegetables are often not associated with reduced cancer risk and are sometimes associated with increased cancer risk (46). Cooking, processing, and storing all generally reduce the amount of vitamins and minerals available from fruits and vegetables, except for tomatoes, for which cooking increases the bioavailability of lycopene, intake of which is associated with reduced risk of prostate cancer (7). Thus, it would be worthwhile to look further at associations between cancer risk and intake of cooked versus raw fruits and vegetables in this cohort and at which fruits and vegetables were consumed cooked and which were consumed raw. It would also be interesting to know whether eating cooked fruits and vegetables is associated with different dietary patterns than eating raw fruits and vegetables.
The paper by Hung et al. (1) is important because it provides very good data that challenge a common assumption of how dietary factors might affect the risk of cancer. Further investigations should be aimed at identifying the reason for the finding, especially to determine whether intake of cooked fruits and vegetables may actually increase the risk of cancer.
REFERENCES
(1) Hung HC, Joshipura KJ, Jiang R, Hu FB, Hunter D, Smith-Warner SA, et al. Fruit and vegetable intake and risk of major chronic disease. J Natl Cancer Inst 2004;96:157784.
(2) Block G, Patterson B, Subar A. Fruit, vegetables, and cancer prevention: a review of the epidemiological evidence. Nutr Cancer 1992;18:129.[ISI][Medline]
(3) Heber D. Vegetables, fruits and phytoestrogens in the prevention of diseases. J Postgrad Med 2004;50:1459.[Medline]
(4) Adzersen KH, Jess P, Freivogel KW, Gerhard I, Bastert G. Raw and cooked vegetables, fruits, selected micronutrients, and breast cancer risk: a case-control study in Germany. Nutr Cancer 2003;46:1317.[CrossRef][ISI][Medline]
(5) Link LB, Potter JD. Raw versus cooked vegetables and cancer risk. Cancer Epidemiol Biomarkers Prev 2004;13:142235.
(6) Gaudet MM, Olshan AF, Poole C, Weissler MC, Watson M, Bell DA. Diet, GSTM1 and GSTT1 and head and neck cancer. Carcinogenesis 2004;25:73540.
(7) Etminan M, Takkouche B, Caamano-Isorna F. The role of tomato products and lycopene in the prevention of prostate cancer: a meta-analysis of observational studies. Cancer Epidemiol Biomarkers Prev 2004;13:3405.
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