Health: The Real Story About Low Fat

You may have noticed the big headlines last week suggesting that low-fat diets--long recommended as the path to better health--don't do any good. Before you rush off to order a cheeseburger with an ice-cream chaser, however, you should take a closer look at the studies on which those headlines were based. You'll probably end up concluding, as I did, that paying attention to how much and what kind of fat you consume is pretty important after all.  First, some background. There were three studies, all published in the Journal of the American Medical Association and all part of a much larger project called the Women's Health Initiative (WHI), which started in the early 1990s. The low-fat diet section of the investigation was designed to answer two related questions: 1) Can you get a lot of middle-aged women to adopt a diet that contains no more than 20% of its calories from fat? and 2) Will that low-fat diet protect them against breast or colon cancer? (As an afterthought, the investigators added a question about the diet's effect on heart disease.)

Nearly 49,000 women were divided into two groups: one received intensive training to reduce dietary fat; the other was given literature on healthy eating. Fat consumption in the intervention group fell from 38% to 24% in the first year, then slid back to 29% by the sixth year. The control group started at 38% fat and finished at 37%.

So, the answer to the first question is that it's really, really hard to get a lot of women to cut their fat intake to 20%--basically no butter or nuts and very little meat. As for the question about whether low-fat diets prevent cancer, the WHI study simply may not have gone on long enough. True, there was no statistically significant benefit when you compared the two large groups. But the women who had the highest fat consumption at the start of the trial and who managed to cut it back the closest to 20% for the longest period developed 22% fewer breast cancers than the women in the control group. That's a statistically significant reduction.

Furthermore, the women in the intervention group had 9% fewer polyps and other precancerous growths in their lower gastrointestinal tract. Given that it takes a decade or more for colorectal cancer to develop, it may be too soon to see if there's a corresponding drop in cancer rates. Also, the women increased their average consumption of fruits and vegetables only slightly, from four to five servings a day. "Maybe if we had gotten up to seven or so, on average, we would have been in a better place," says Shirley Beresford, a study leader and an epidemiologist at the University of Washington.

In hindsight, it's easy to say that the investigators made a mistake in tracking only the total amount of fat consumed and not the saturated and trans fats now known to damage arteries. But all the earlier research on preventing cancer suggested that total fat was the culprit, so investigators decided to concentrate on that.