Low Fat Diet and Ovarian Cancer: Don't jump to conclusions

A new study published in the Journal of the National Cancer Institute has been touted in the media as showing how a low-fat diet can reduce the risk for ovarian cancer. But the results are a little more iffy than you might have been lead to believe.

First, what happened: According to data from the Women's Health Initiative Dietary Modification trial, women who decreased the amount of dietary fat they consumed were 40% less likely to develop ovarian cancer than women who followed "normal dietary patterns". Thinking readers will quickly see some problems with this conclusion. Here are just some of them.

It's not possible to "reduce" one part of the diet (fat, carbohydrate, protein) without affecting the proportion of calories coming from the others. If a diet is low in fat, it's got to be higher in something else. This is true for all dietary studies like this one. It's also possible—and very likely—that when people follow what they think is "healthy" advice (like eat less fat), they are also more likely to do other things that can have health benefits (like stop smoking, or eat more vegetables, or exercise more). There's no way to account for how much the results are due to outside factors (what researchers call "confounding variables") like these.

In addition, more than a little research has shown that even when people are counseled to "eat less fat," and even when they are given a target goal, and even when they actually follow the target goal in the beginning (three very large "ifs") eventually their fat intake starts to look suspiciously like the one they started with. In this particular study the women were counseled to eat 20% fat, but by the end of the first year they were in fact up to 24%, and by the end of the study, 29%.

Most important, the "fat reducing" women also increased their consumption of fruits and vegetables, and there's absolutely no way to know whether their lower rate of ovarian cancer had to do with the reduced fat or the increased anti-cancer compounds, anti-inflammatories and anti-oxidants from the plant foods. Ask me, I'm guessing the latter.

Walter Willet, MD, the chairman of the Department of Nutrition at the Harvard School of Public Health and arguably the most important nutritional epidemiologist of our time, has said of his own monumental "Nurses Health Study" that the percentage of fat in the diet has shown no relationship to any major health outcome. However, the type of fat (trans fats, for example) and the type of carbohydrate (sugar, processed carbs), does.

There's no doubt that obesity puts you at increased risk for a host of diseases, including some cancers. This may turn out to be because the fat cells are mini-endocrine glands that secrete all kinds of hormones that can be problematic for some cancers (including estrogen). But fat on the body isn't the same as fat in the diet. Once we get over the idea that "fat makes you fat" and realize that insulin makes you fat, and carbohydrates—especially processed carbs—drive up insulin, we'll begin to concentrate on the real culprits in our diet: Sugar, processed carbs and junk food.

Fat—not so much.