For years, Americans have been bombarded with dietary advice -- much of it conflicting -- that asserts that diet composition per se has a major impact on health. Claims that high-fiber diets protect against colon cancer haven't been supported by scientific research, for example. Nor have low-carbohydrate diets been shown to be better for weight loss than low-fat diets.
Now, the latest health headlines claim that diets low in fat won't prevent breast cancer, heart disease, or colon cancer in women -- but the results are more nuanced than such claims suggest. Today's Journal of the American Medical Association (JAMA) presents results from the dietary-modification arm of the Women's Health Initiative studies -- probably the largest such study of its kind (we've discussed other results from this study recently on FactsAndFears).
In this study, over 48,000 post-menopausal women were randomly assigned to either an intervention group (19,500 subjects) or control group (29,300 subjects). The intervention group received intensive instructions and behavior modification education to help them limit their fat intake to 20% of calories. The control group was not advised to decrease fat intake. On average, the women were followed for slightly more than eight years.
While both groups actually decreased their fat intake -- more in the intervention group, by about 10% -- neither achieved the goal of consuming only 20% of calories from fat. The intervention group did significantly increase their intake of fruit and vegetables and decreased their consumption of saturated fat compared to the control group, however. Both groups lost small amounts of weight (though they weren't advised to do so); the intervention group's loss was slightly greater.
In spite of the positive dietary changes, the intervention group did not experience a significant drop in their risk of cardiovascular disease or stroke, nor did they reduce the risk of colorectal cancer. The results with respect to invasive breast cancer were similarly disappointing, with no significant decrease in risk over the period studied.
So do these results mean that dietary fat plays no role? Perhaps, but like most such studies, this one raises as many questions as it answers. For example:
¢The activity levels of the women in the study weren't described -- if they had increased their exercise, would that have increased their weight loss and thus decreased at least their risk of heart disease?
¢Would the results have been different with a longer follow-up period?
¢With respect to breast cancer, there were non-significant trends towards a decreased risk -- longer observation might find more impressive results.
One take-home lesson from these reports is that making significant changes in presumably health-affecting aspects of lifestyle is a difficult proposition. Even with instruction and support, the women in the intervention group only reduced their fat intake from about 38 to 24% of calories after one year in the study, and they didn't maintain even this decrease for the whole period. Perhaps a greater degree of supervision and/or education would have made a difference.
ACSH has long taken the position that simply tweaking diet composition is not the solution to the chronic health issues that are major sources of morbidity and mortality in modern America -- a healthful lifestyle requires more change than that. As far as they go, the current studies support rather than refute this view.
Beresford SAA et al. Low-fat dietary pattern and risk of colorectal cancer. JAMA 2006; 295:643-654.
Howard BV et al. Low-fat dietary pattern and risk of cardiovascular disease. JAMA 2006; 295:655-666.
Prentice RL et al. Low-fat dietary pattern and risk of invasive breast cancer. JAMA 2006; 295:629-642.