= Full article availableDespite the headlines, the latest findings on dietary fat don’t mean you should give up on watching the fat in your food. True, the widely reported $415 million government study, the Women’s Health Initiative (WHI) Dietary Modification Trial, generally failed to find benefits from a low-fat diet against breast and colon cancer or cardiovascular disease. But that doesn’t mean, as a New York Times editorial opined, “The more we learn about nutrition, the less we seem to know.”
“This was a carefully done and well carried out study that was badly served by the press,” says Healthletter editor Irwin H. Rosenberg, MD, University Professor at Tufts’ Friedman School of Nutrition Science and Policy. “The damage done to efforts to persuade people to take responsibility in picking their diets is huge. If the subtext is that diet has nothing to do with health and disease, that’s a dangerous and irresponsible message.”
The real lesson here, according to Dr. Rosenberg, is the limitation of any one study—even a controlled clinical trial, considered the “gold standard” of nutrition research. And this was certainly a large-scale study: Researchers at 40 US clinical centers involved 48,835 postmenopausal women, ages 50 to 79, in three trials of the effects of reducing total dietary fat. Those assigned to the intervention group—19,541 women—were given behavior modification with a goal of reducing fat intake to 20% of calories, while increasing fruits and vegetables to at least five daily servings and grains to six servings.
The results were published in three articles in the Journal of the American Medical Association (JAMA). Researchers found no significant difference between women on the low-fat diet and a control group in risk of breast or colon cancer or cardiovascular disease. “The results, of course, are somewhat disappointing,” study co-authors JoAnn Manson, MD, DrPH, chief of preventive medicine at Harvard’s Brigham and Women’s Hospital, told the Associated Press. “We would have liked this dietary intervention to have a major impact on health.”
But Dr. Manson cautioned, “These results do not suggest that people have carte blanche to eat fatty foods without health problems.”
Robert H. Eckel, MD, president of the American Heart Association (AHA), agrees: “It would be easy to misinterpret the results of this study, and it is important that we get it right. Reducing the risk of cardiovascular disease is about following an integrated lifestyle program, rather than concentrating solely on dietary composition.”
To understand why not all experts view this study as the final or even definitive word on fat and health, it helps to know a little about the genesis of the research. A number of animal tests and observational studies on humans had suggested a link between total fat intake and cancer risk, especially breast cancer. Although many people assume cutting fat can fight cancer, the idea has actually been very controversial in the medical community, says Michael Thun, MD, vice president of epidemiology for the American Cancer Society. Since this WHI study began, he adds, there’s been greater evidence that being overweight or obese is much more important to cancer risk than total fat intake.
Although researchers also looked at heart disease, “by the time the study was initiated, we already knew that for heart disease at least the evidence was much more strongly related to saturated fat than to total fat,” Dr. Rosenberg points out. The study did not specifically target saturated fat—which the AHA recommends limiting to 10% of total calories for healthy people—or differentiate between total fat and heart-healthier choices such as monounsaturated or polyunsaturated fat. The federal dietary guidelines released since the study’s inception don’t focus on total fat, but instead advise limiting both saturated fat and trans fat.
The women in the study also failed to achieve their fat-cutting goals: They got down to 24% in the first year, then slid to 29% as the study went on.
And although participants were studied for an average of 8.1 years, even that may not be long enough, Dr. Rosenberg says: “You’re looking at conditions that may take decades in some cases to develop.”
Finally, the research did discover some positive results: On breast cancer, women consuming the most fat prior to the study and those at high risk—testing positive for the estrogen receptor and negative for the progesterone receptor—showed significant benefits. Researchers found a small but statistically significant reduction in levels of LDL (“bad” cholesterol) and diastolic blood pressure. And, as current thinking would suggest, women with lower consumption of saturated and trans fats and more fruits and vegetables did see a trend toward reduced heart disease risk.
Nonetheless, the results may help burst the low-fat bubble that led food marketers to label 12.8% of new products released last year as low-fat or fat-free. Oversimplifying nutritional advice to merely “avoid fat” may have been a mistake in the first place, Dr. Rosenberg observes. “It underestimated the ability of the American public to hear a more complex nutritional message.”
To learn more: Journal of the American Medical Association, Feb. 8, 2006. American Heart Association. American Cancer Society.
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