Health news can be confusing especially when it comes to nutrition. It often seems as though what's touted as good to eat on Tuesday is out of favor by Friday. And it seems to be happening again with the most favored food groups fruits and vegetables. Although numerous epidemiological studies support the concept that consumption of ample amounts of these foods can help protect against various types of cancer, a study in the February 14 issue of the Journal of the American Medical Association (JAMA) finds no such benefit, at least with respect to breast cancer.
Stephanie Smith-Warner, Ph.D. from the Harvard School of Public Health and colleagues from several other institutions combined and reevaluated data from reports about the influence of fruits and vegetables on risk of breast cancer. They note that most of the earlier studies have been case-control studies, which are studies that compared the dietary habits of women who had breast cancer with those of healthy women.
The problem with such studies, the authors noted, is that "recall and selection bias may influence the results." In other words, knowledge that one has a disease may influence both the way a person eats, and how they recall previous dietary habits.
To avoid such problems, the authors used only prospective cohort studies studies in which the health and habits of groups of people are followed over time to see who does and does not develop the disease. The subjects initially are free of the disease or condition of interest. At the end of the study, the risk of getting the disease is compared in people who do have a particular habit or eat in a certain way, with those who do not.
In this new JAMA report, the researchers pooled data from eight studies that reported at least 200 cases of breast cancer. The studies had compiled information on the women's usual dietary intake, typically when the investigation began. A total of over 350,000 women had participated in the eight studies, and there had been 7,377 new cases of breast cancer diagnosed. Investigations ranged from six to eleven years in length. The women in the various studies were adults between the ages of 20 to 93 years.
When Dr. Smith-Warner and colleagues reanalyzed the combined data from the eight studies, they did not find any significant differences in risk of breast cancer between women who reported consuming large amounts of fruits and vegetable and those who did not. The authors looked at consumption in several ways as total fruit and vegetable consumption, as total vegetables, or as total fruits. In addition, they found no associations between risk of breast cancer and consumption of green leafy vegetables, eight botanical groups of fruits and vegetables, or 17 specific fruits and vegetables.
Summarizing their results, the authors noted, "our results suggest that fruit and vegetable consumption during adulthood is not significantly associated with breast cancer risk." "Other types of interventions are needed to reduce the risk of breast cancer," they continued.
In an accompanying editorial, Martha L. Slattery, Ph.D., from the University of Utah Medical School, was more positive. She emphasized that the public health message to Americans to consume more fruits and vegetables is "probably not" harmful, as the benefits are likely to be "a reduction in risk for heart disease, other cancers, diabetes and obesity."
Despite the findings of this latest study, one group that promotes the link between diet and cancer is not impressed. The American Institute of Cancer Research (AICR) published a report in 1997 stating that "Diets high in vegetables and fruits probably decrease the risk of breast cancer." Ritva Butrum, Ph.D., AICR vice-president for research, continues to stand by her organization's position. Dr. Butrum contends that since AICR's report was a "more comprehensive review" of several different types of studies, AICR will not alter their emphasis of the value of diets high in fruits and vegetables for decreasing the risk of breast cancer.
Gilbert Ross, MD, medical director of ACSH, noted that "while there are abundant reasons to consume five fruits and vegetables a day, if you are concerned about reducing your risk of breast cancer there is a better way. Recent studies of selective estrogen receptor modulators, tamoxifen and raloxifene, show that these pharmaceutical approaches are far and away the most effective way we know to reduce the risk of breast cancer."