Just the Fats, Ma'am

In a recent widely publicized Women's Health Initiative study, the NIH found no significant differences in the incidence of breast cancer, heart disease, or stroke for women following a low-fat diet. This pronouncement was met with surprise by many clinicians (and one suspects with glee by the National Restaurant Association). But should we really conclude that years of warnings about reducing fat intake were mistaken?

Too little?
The study was meant to compare one group of women who had reduced their total fat intake to 20% of dietary calories with a control group who got 35-37% of their calories from fat. Investigators expected that such a substantial difference in fat consumption would reveal any impact of fats on women's health. And it might have... except that the low-fat group never met the study's goal. The women managed to reduce their fat consumption to 24% of total calories in the first year, but by year six were up to 29%, a difference of only 8% of calories from the control group*. This might not have been enough to affect their chances of developing cancer, heart disease, or stroke, even if fat intake were an important factor. In fact, the subset of women who reduced their fat intake the most did seem to reduce their risk of breast cancer.

Too late?
The participating women were aged 50-79 years at the start of the study and followed for about 8 years. This means that they were already old enough to be at fairly high risk for various cancers as well as heart disease before the changes in diet. These illnesses develop over time, and it is still quite possible that a low-fat diet earlier in life could help to prevent them.

Good fat, bad fat
Most troubling was the failure of this study to distinguish between the various types of fat that make up total fat intake. Saturated fats (mainly from animal products like red meat and dairy), and especially trans fats (from hydrogenated oils), have long been linked to increased risk for heart disease. But unsaturated fats, from fish, nuts, and vegetable oils, are considered "good fats" that lower risk for heart disease. So instructing the participants specifically to reduce their trans and saturated fat intake (and maybe even increase unsaturated fat) could have produced very different results.

All in all, just the kind of situation that confuses everyone and makes me crazy. No wonder people lose faith in the ever-fluctuating recommendations of scientists and clinicians. Even studies like this - huge, well-funded, controlled studies that should be setting the standards - have built-in design flaws that finally allow us to conclude... absolutely nothing.


*This made me grateful that I study the effects of diet in fruit flies and not human beings. If I want to look at brain development on a protein-free diet, my experimental subjects will stick to sucrose, and dammit, they'll like it.