Overturning three decades of conventional wisdom, a new study of low-fat diets shows that eating less fat does not significantly reduce the risk of heart disease, stroke, breast cancer or colorectal cancer, researchers report today.
Results on weight loss from the same study, published last month, also show that reducing fats without reducing calories does not lead to significant weight loss.
The $415-million study, which tracked about 50,000 women for as many as 13 years, is by far the largest ever to address the role of fats in health and, though it hints at some potential benefits, largely closes the book on a highly publicized chapter of dietary history.
"Basically, the low-fat, high-starch diets completely struck out," said Dr. Walter Willett of the Harvard School of Public Health, whose own smaller studies have shown similar results. "This is ... the end of the low-fat era."
But, he cautioned, "the one really important point here is that it would be a serious mistake to interpret this study as 'Go and load up on sausage, butter and fast food.' "
Experts said that the results on cardiovascular disease and colorectal cancer, obtained in older women, probably applied to men as well because the disease mechanisms are the same. It is not clear, however, if changing the diet earlier in life would produce a different outcome.
The results, published today in three papers in the Journal of the American Medical Assn., might be surprising to a public that has grown accustomed to seeing more than 15,000 low-fat products on the shelves of supermarkets.
But to nutritionists and physicians working in the field over the last decade, they are consistent with new ideas about the roles of so-called good and bad fats.
"Nutrition knowledge has progressed dramatically since the study began," said Mara Vitolins, a professor of public health sciences at Wake Forest University Baptist Medical Center in Winston-Salem, N.C., and a coauthor of the study. "Today we know that reducing total fat may not be enough -- we need to focus on the types of fat we eat."
That means reducing consumption of saturated fats like those found in meat and butter and increasing the intake of vegetable and fish oils, which studies have shown have a protective effect.
Although the study found no significant benefits from a low-fat diet, it also found no harm from the accompanying increased consumption of carbohydrates -- grains, starches and sugars.
The finding contradicts the claims of proponents of low-carbohydrate diets, such as the Atkins Diet, that high carbs increase the risk of diabetes.
The women in the study "did not show any signs of diabetes, their triglycerides were normal, and their blood glucose was normal," said Dr. Elizabeth G. Nabel, director of the National Heart, Lung and Blood Institute, which sponsored the study.
When the study was conceived in the 1980s, fat was perceived as a universal villain.
For weight control, it was a problem because of its high energy density -- 9 calories per gram, compared with 4 calories per gram for carbohydrates.
For cardiovascular disease, animal fats were known to play a role in the buildup of artery-clogging plaque, whereas the role of protective vegetable fats had not yet been recognized.
Studies of women in countries with low-fat diets showed a lower incidence of breast cancer, which rose when the women migrated here and began consuming an American diet. And red meats, which have a high fat content, had been linked to colorectal cancer.
Nabel said the study was initiated to explore these issues, and was conducted in women to broaden the range of clinical trials.
"We realized most clinical trial results came from men, and that we had very little information about health as it related to women."
The new results indicate that the concerns about fat were overblown.
"People just think fat is the devil incarnate, and that's an incorrect message," said Abby Bloch, vice president for programs and research for the Dr. Robert C. Atkins Foundation in New York. Atkins proponents have long argued that fats are harmless.
"Americans unfortunately tend to go to extremes," Bloch said. "So if you say fat may be bad, then they'll try to eliminate all fats."
The trial was conducted as part of the Women's Health Initiative, a $750-million program that has studied about 161,000 women. Another leg of that study overturned the long-held dogma that estrogen replacement therapy after menopause reduced the risk of heart disease.
For the study, 48,835 healthy women with an average age of 62 were enrolled. Forty percent of them were given intensive counseling -- 16 sessions in the first year and four per year thereafter -- to help them reduce their fat intake, while the remainder continued their normal diet.
The women filled out forms indicating what they ate and were tracked for an average of eight years.
Despite the size and scope of the study, there were problems.