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GUEST EDITORIAL |
Dr. Willett is Professor, Departments of Epidemiology and Nutrition, Harvard School of Public Health, Boston, MA.
For anyone desiring to reduce his or her personal or community's cancer risk, the 2001 American Cancer Society guidelines on nutrition and physical activity for cancer prevention1 are well worth half an hour of careful reading. The need for periodic updating and refinement of nutritional guidelines is apparent to anyone subjected to the deluge of new and often seemingly conflicting studies reported in the general media. Accepting the optimistic assumption that study results are reported accurately in the general media, even the most diligent person could be confused by this information because the findings are often not placed in the context of other knowledge. Further, subsequent studies either confirming or refuting earlier findings may never reach the general media. Yet such updates are critical for informed decisions.
Though it may not be apparent to the general public, the quality of epidemiologic studies on diet and cancer has improved enormously during the last decade. Studies are now much more powerful due to greater sample size, and the results of prospective studies started many years ago are now available; until recently almost all studies assessed diets retrospectively from people with and without cancer, and were therefore more prone to bias. Thus, periodic reviews of all sources of information on nutrition and cancer by a group of experts who take into account the strengths and weaknesses of various pieces of evidence and succinctly summarize the findings, can be an invaluable way to provide the public with the best available scientific evidence.
The committee members who developed these new guidelines are all leading investigators working in the field of diet and cancer, and they have done an outstanding job of distilling the dauntingly large and complex literature into a highly readable and accurate summary.
Because the body of data relating nutrition to cancer risk is growing rapidly in both quantity and quality, some shifts in guidelines are inevitable if the goal is to provide the public with the best available evidence rather than to repeat dogma. The committee has been careful to incorporate new evidence, but has also required confirmation from multiple studies before suggesting action. In these new guidelines, increased emphasis is appropriately given to the closely related goals of maintaining a healthy weight and engaging in regular physical activity. Only in the last several years has evidence become reasonably clear that regular physical activity will reduce risk of colon and breast cancer. This represents an important step forward.
Evidence is now firm that overweight and obesity are related to higher risks of cancers of the colon, endometrium, kidney, breast (after menopause), and lower esophagus. After avoidance of tobacco, staying lean and active provides the greatest potential for minimizing cancer risk. Because efforts to control weight and increase physical activity will also have great benefits for prevention of cardiovascular disease and many other conditions, they deserve major support.
In earlier guidelines of the ACS and other organizations,2 a reduction in total fat consumption was emphasized. However, hypothesized associations with major cancers have not been supported in more recent studies, and evidence is now conclusive that reductions in intake of some forms of unsaturated fat will actually increase risks of cardiovascular disease.3 Thus, the guidelines committee has appropriately focused on limiting consumption of red meat, which contains unhealthy forms of fat and has been associated with cancer risk in many studies. And while excessive dietary fat can contribute to obesity, overconsumption of carbohydrates will have the same effect. Thus limiting calories from all sources and increasing expenditure of calories by physical activity is appropriately emphasized.
Although earlier studies suggested that increasing intake of fruits and vegetables might have a large benefit in reducing risks of many cancers, much weaker associations have been seen in more recent investigations that were less subject to methodological biases. Advice to eat five servings per day of fruits and vegetables nevertheless remains sound because a modest reduction in cancer risk is likely, and benefits for cardiovascular disease have become even better established. However, no one should expect substantial reductions in cancer incidence from eating more fruits and vegetables without attention to nonsmoking, weight control, and regular physical activity.
Future shifts and refinements in nutritional guidelines for cancer prevention are inevitable because many studies, some funded by ACS, are ongoing. The guidelines committee refrained from specific recommendations on folic acid, calcium, dairy products, selenium, soy estrogens, omega-3 fatty acids, and lycopene. All of these are under active investigation and some appear highly promising. Hopefully, at the time of the next guidelines review, the state of evidence will be sufficient to provide clear guidance on at least some of these topics.
The new recommendation for community action is a welcome and major step forward for cancer prevention. This statement recognizes that improvements in nutrition and physical activity by individuals acting in isolation will often be difficult and ineffective without a supportive community environment. This is particularly clear in efforts to control weight and increase physical activity; safe and attractive places to walk, bicycle, and engage in other activities are essential for achieving an optimally healthy lifestyle.
Sadly, physical education programs in schools and funding for bike paths have been given low priority and are among the first programs being cut here in Massachusetts and elsewhere as government revenues shrink. These programs should no longer be regarded as optional frills because their elimination will have direct adverse effects on the long-term cancer risks for both children and adults.
ACS volunteers and staff have provided tremendous and effective leadership in mobilizing support for legislation aimed at the reduction of smoking. Hopefully, and without reducing attention to smoking, some of this experience and energy can be directed toward control of the obesity epidemic.
The new ACS guidelines on nutrition and physical activity will play a key role in guiding the actions of individuals, organizations, and governments aimed at reducing the burden of cancer by making these efforts optimally effective. The ACS and committee members who developed this report deserve our thanks for their important contribution. These guidelines represent the best of a process that brings the fruits of scientific research to a public that has admirably supported the many years of scientific endeavor.
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This article has been cited by other articles:
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  A. M. Lindsey, N. Waltman, G. Gross, C. D. Ott, and J. Twiss Cancer Risk-Reduction Behaviors of Breast Cancer Survivors West J Nurs Res, December 1, 2004; 26(8): 872 - 890. [Abstract] [PDF]
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