Dietary guidelines: a role in cancer risk reduction?

A study has found that compliance with the Dietary Guidelines established in 1980 advising Americans on nutrition to promote health may reduce the overall risk for cancer.

In an article published in the American Journal of Clinical Nutrition​, Harnack et al. evaluated data from the Iowa Women's Health Study regarding nutrition-related behaviour in a large group of postmenopausal women. They found that better compliance with the guidelines resulted in an overall reduced risk for cancer, although measures of physical activity and weight status may have influenced this outcome.

The female study population were rural residents of Iowa and were 99 per cent white. A total of 34,708 postmenopausal women completed a series of questionnaires on diet and lifestyle factors and were followed for cancer incidence and mortality. During a 13-year period, 5,038 cancers were diagnosed in the group.

Each woman was assigned an index score based on compliance with dietary recommendations contained in the Dietary Guidelines, as well as non-dietary recommendations relating to lifestyle. The women were divided into five quintiles according to their index scores; for instance, those in quintile five tended to be more physically active, eat more wholegrains and less fat, eat more fruits and vegetables, and to be non-smokers.

Compared to the lowest quintile of compliance, overall cancer incidence for the second lowest quintile was reduced by a non-significant 5 per cent, by a significant 12 per cent for quintiles three and four, and by 15 per cent for quintile five.

The decline in risk was quite dramatic for some forms of cancer: women in quintile four had less than half the risk of uterine cancer of those in the lowest quintile. Risk reduction for breast and colon cancer declined an average of 25 per cent from quintile one to quintile four. The reduction in cancer risk was associated not just with healthy eating, but also with regular exercise and avoidance of overweight and obesity.

In an accompanying editorial, McCullough and Stampfer point out that the relative risk reductions should be interpreted cautiously. When guidelines relating to weight and physical activity were removed from the analysis, the apparent benefits of adherence to the Guidelines were much more modest.

In preventing cancer, lifestyle modifications that include smoking cessation and avoidance of overweight may be just as important as observing a dietary pattern that is low in red meats and high in a variety of vegetables, fruits, and wholegrains, said the authors.

The Dietary Guidelines are reviewed and updated every five years to reflect current nutritional research.