That Low-Fat Diet Study

About a week ago, the media eagerly announced the results of a landmark study. From the SF Chronicle on 2/8/2006:

"A landmark study that set out to prove that cutting fat from the American diet could reduce cardiovascular disease and cancer in women over 50 has come up short, showing no significant changes in risk despite years on reduced-fat regimens.

Nearly 50,000 post-menopausal women were enrolled in the $415 million study. Twenty thousand embarked on years of low-fat dieting, while the rest were allowed to eat as they pleased. They participated in the study from 1993 to the end of 2005."

Whenever I see something like this, I like to know more about the study, so I can better understand and interpret the results. Statistical studies are only as good as the underlying experimental design. It turns out that initial frenzy failed to mention two important pieces of information, kudos to Business Week for doing some digging:

"Based on its design, the trial had little chance of making major advances in the science of nutrition. It set out to compare women eating normal diets with women asked to eat less fat and more fruits and vegetables. But the two groups' diets really didn't differ all that much. Those with "healthier" diets cut calories from fat by only 8.2% compared with the normal group and ate only 1.1 additional servings of fruits and vegetables a day. That's too small a difference to expect health benefits.

What's more, scientists now know that the type of fat we eat is generally more important than the amount. Monounsaturated fats, such as those in olive oil, or omega-3 fats found in fish are healthier than saturated fat, as in butter or beef. Worst of all are so-called trans fats, which are created by adding hydrogen to natural fat. Trans fats are widely used in everything from margarine to cookies, and have been linked with increased risks of heart disease. By focusing just on total fat intake, the study missed a chance to tease out the effects of these different types of fat. "It's really important not to go away with the message that diets don't matter," says Willett."

So the test or treatment group (these were the people in the low-fat diet group) ate only 8.2% less fat calories, than the control group. Plus, it's not like they ate way more vegetables. Think about it, this is their low-fat group! We also don't know if those in the so-called low-fat group reduced their intake of trans fat and saturated fat. As they say, the Devil is in the Details.

In a subsequent article, the SF Chronicle notes, that the study was conceived before trans fat and saturated fat emerged as THE types of fat to cut back on:

"Like many other scientists, Anderson noted that the study was conceived before evidence pinpointing the chief dietary evils leading to heart disease. ... A strategy that simply cuts back on all dietary fats -- as the women in the federal study did -- ignores this distinction and other knowledge that experts say they've gained."