Low-Fat Diet's Benefits Rejected
Wednesday, February 8, 2006
Low-fat diets do not protect women against heart attacks, strokes, breast cancer or colon cancer, a major study has found, contradicting what had once been promoted as one of the cornerstones of a healthy lifestyle.
The eight-year study of nearly 50,000 middle-age and elderly women -- by far the largest, most definitive test of cutting fat from the diet -- did not find any clear evidence that doing so reduced their risks, undermining more than a decade of advice from many doctors.
The findings run contrary to the belief that eating less fat would have myriad health benefits, which had prompted health authorities to begin prominent campaigns to get people to eat less fat and the food industry to line grocery shelves with low-fat cookies, chips and other products.
"Based on our findings, we cannot recommend that most women should follow a low-fat diet," said Jacques Rossouw of the National Heart, Lung and Blood Institute, which funded the $415 million study.
Although the study involved only women, the findings probably apply to men as well, he said.
Several experts cautioned, however, that the study hints that there still may be some benefits to reducing the total amount of fat in the diet, especially for breast cancer. In addition, there is clear evidence from this and other studies that particular fats -- saturated fats from meat and trans fats from processed foods -- are unhealthful and should be avoided.
But the findings, being published today in three papers in the Journal of the American Medical Association, deflate the notion that a simple, easily communicated message of reducing overall fat intake would stave off a host of ills.
"We set out to test a promising but unproven hypothesis that has proven to be less promising than we anticipated," Rossouw said. "This is the nature of science: to have incremental gains and setbacks. We have a duty as scientists to put the best information out there at any given time, even if it can become confusing at times."
Skeptics said the findings confirm their long objections to the message that all fat is bad. That strategy may have diverted attention from much more effective approaches that differentiate between healthful and detrimental fats and may have contributed to the obesity epidemic because people worried more about how much fat they ate than how many calories they consumed, they said.
"It was a mistake, and this study really confirms that it was the wrong direction to go for nutritional advice," said Walter Willett of the Harvard School of Public Health. "It did do harm. It was a lost opportunity. People were given the idea that it was only fat calories that counted. This should be the nail in the coffin for low-fat diets."
Willett and other researchers fear that the findings will leave the public skeptical about all health advice, or will be misinterpreted to mean that diet and lifestyle are unimportant. A large and convincing body of evidence shows that eating a diet rich in fruits, vegetables and whole grains and low in saturated and trans fats; avoiding smoking; exercising regularly; and maintaining an appropriate weight have a powerful effect on health, they said.
"There's a danger people will throw up their hands and say, 'Why should I believe anything else?' " Willett said. "But there is strong evidence that diet and lifestyle do make a big difference."
The findings stem from the Women's Health Initiative, which also shocked the medical establishment in 2002 when it showed that taking hormones not only did not protect the hearts of postmenopausal women but also was dangerous.
For the new findings, researchers analyzed data from 48,835 women age 50 to 79 who joined the study between 1993 and 1998. About 40 percent were counseled to eat more fruits and vegetables and to cut their overall fat intake, with the goal of reducing their total fat consumption to no more than 20 percent of their daily calories.
After about eight years, those women had cut their total fat from 35 to 38 percent to 24 to 29 percent on average, while the rest continued to consume about the same amount.
The women on the low-fat diet had slightly lower levels of "bad" cholesterol -- low-density lipoprotein -- and blood pressure, but their risk of heart attack, stroke and heart disease was unaffected, one paper showed. There were indications, however, that women who cut down on saturated fat, or who ate more fruits and vegetables, did lower their risk.
Similarly, when the researchers looked at colorectal cancer, the women who cut their fat intake had no decrease in risk, according to the second paper. But they were less likely to develop polyps that increase the risk, suggesting that a benefit may emerge later on, the researchers said.
The third paper found that the low-fat diet also did not significantly decrease the risk of breast cancer. Women on the low-fat diet did have 9 percent fewer breast cancers, but researchers could not be sure that difference was not the result of chance. There were other encouraging hints, however, including signs that women who were consuming the most fat when the study began, or those prone to certain types of tumors, may benefit, especially if they were followed longer.
"I think women who are currently following a low-fat diet should be encouraged to do so. We didn't see any unfavorable effects," said Ross Prentice of the Fred Hutchinson Cancer Research Center in Seattle, who noted that the women on the diet also avoided gaining weight. "For women who are at high risk for breast cancer, they should talk it over with their physicians whether adopting a low-fat diet might be warranted."
But overall, the findings fell far short of warranting a broad recommendation for low-fat diets, several experts said.
"We had hoped that this approach would prove to be beneficial," said Barbara Howard of the MedStar Research Institute, who helped conduct the study. "I think we've learned that nutrition is never simple and there are no simple solutions."