Dietary strategies for preventing cancer

After analyzing more than 7,000 studies, an international team of cancer experts says, in effect, that in many cases cancer could be prevented by staying lean through diet and exercise. Though oncologists have known for some time that certain health habits can help prevent cancer, the report represents perhaps the strongest scientific statement yet on the topic and is expected to have widespread impact.

“We need to think about cancer as the product of many long-term influences, not as something that just happens,” says Walter C. Willett, MD, DrPH. Dr. Willett, of the Harvard School of Public Health in Boston, is one of 21 experts who wrote the report, entitled Food, Nutrition, Physical Activity and the Prevention of Cancer: A Global Perspective. (See additional comments from Dr. Willett and other panel members in “Putting the prevention report into practice”. The 517-page document, issued jointly by the World Cancer Research Fund (WCRF) and the American Institute for Cancer Research (AICR), is based on evidence evaluated by nine teams of scientists worldwide.

The report amends an edition of the same name issued in 1997. At that time, findings regarding weight, food, nutrition, physical activity, and cancer prevention were emerging so quickly that the edition was soon outdated. A new round of evaluation was launched in 2001, culminating in the current publication.

As its title suggests, the latest report ties together how food, nutrition, physical activity, and body composition affect cancer risk. The most surprising conclusion was how strongly being overweight or obese contributes to cancer risk. As the report notes, scientists have learned that the adipocyte produces pro-inflammatory factors. Compared with lean people, obese individuals have elevated concentrations of circulating tumor necrosis factor-α, interleukin-6, C-reactive protein, and leptin. Such chronic inflammation can set the stage for cancer. “Maintenance of a healthy weight throughout life may be one of the most important ways to protect against cancer,” asserts the expert panel. “This will also protect against a number of other common chronic diseases.”

Aditionally, the panel members believe cancer prevention should start early—in infancy. A special recommendation on the importance of breastfeeding underscores their declaration that “some of the most persuasive evidence in the whole field of food, nutrition, and physical activity indicates that the basis for prevention of cancer should be a whole-life-course approach, starting at the beginning of life or even in maternal preparation for pregnancy.”

The recommendations for cancer prevention

The report makes eight recommendations regarding body mass, food, alcohol, and supplements and adds two “special recommendations”: one for women who are breastfeeding and one for cancer survivors. All recommendations were formulated based on “convincing” or “probable” judgments made collectively by the panel members based on their extensive review of the evidence. The recommendations are summarized below:

1. Be as lean as possible within the normal range of body weight. The panel uses the term “fatness” in this recommendation and throughout the report, but neither clinicians nor patients should be put off by the seemingly blunt terminology. The word “fatness” is used in a purely scientific sense to denote adiposity and to distinguish between a heavy but muscular person and a heavy person with a lot of adipose tissue. Ideally, individuals should maintain a body weight at the low end of the BMI range until age 21 years, and then stay within the normal range thereafter.

2. Be physically active as part of everyday life. Individuals should engage in moderate physical activity, such as brisk walking, for at least 30 minutes every day. They should eventually build up their daily duration to 60 minutes or more of moderate activity or 30 minutes of vigorous activity. Television viewing and other sedentary habits should be limited.

3. Limit consumption of energy-dense foods and avoid sugary drinks. Energy-dense foods—foods that are high in calories per unit of weight—are to be consumed sparingly. The report defines these products as those that contain more than 225-275 cal per 100 g, but the quantity in which these foods are typically eaten should also be considered. For example, fast foods, which tend to be high in fat and sugar but low in the fiber that makes a person feel full, are usually consumed in large portions, causing the person to take in more calories than he may realize. Not surprisingly, the report recommends that fast foods be consumed sparingly, if at all.

Almonds, on the other hand, are high in calories, but because they are usually not eaten in large quantities and because they have great nutritional value (i.e., they are nutrient-dense), they don't fit the concept of the energy-dense foods that are cause for concern. And some foods that are relatively low in calories should not necessarily be included in a person's daily menu, depending largely on the item's nutritional value. Sugary drinks should be avoided altogether. While their water content makes them less energy-dense than foods, these beverages don't appear to satisfy hunger, so the person may drink more, increasing calorie intake and promoting weight gain.

Foods that are high in water and fiber, such as fruits, vegetables, and beans, can generally be counted on as being favorably low in energy density. Even though many whole grains, nuts, and seeds are more energy-dense than fruits and vegetables, in most cases it is better to have more whole grains, nuts, and seeds on the plate.

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