A large US government study has found A that a diet low in fat and high in vegetables, grains and fruits does not reduce the risk of cardiovascular disease, breast cancer or colorectal cancer in postmenopausal women. The results, from the Women's Health Initiative (WHI), are not likely to be the last word on the subject, however.
Previous research has suggested that low-fat diets might protect against cancer and heart disease, but no other studies have been as large or as well-designed as this one.
The WHI is a large, 15-year study that is designed to identify the most common causes of death, disability and poor quality of life in post menopausal women.
The study looked at 50,000 postmenopausal women, who were randomly assigned to one of two groups. The first group followed a reduced-fat diet (20% of total calories) and ate large quantities of vegetables and fruits (five or more servings per day), as well as grains (six or more servings per day). The second group served as a control group in which the women did not change their diets. The participants were followed-up for approximately eight years.
The women in the low-fat group had a 9% lower risk of breast cancer than the women in the control group, but this difference is not statistically significant, the researchers say.
However, women in the low-fat group who had eaten a higher fat diet before the study began did have a significant reduction in their risk of breast cancer, indicating that such a diet might indeed confer a benefit.
The low-fat diet did not reduce the risk of colorectal cancer, but there was a reduction in the incidence of polyps, a precursor to this type of cancer allowing for the possibility that some benefit may appear in the future.
"It's possible that with longer-term adherence to such a diet, a benefit might emerge," says Dr. Jacques E. Rossouw, WHI project officer at the National Heart, Lung, and Blood Institute (NHLBI).
The low-fat diet did not appreciably reduce the risk of coronary heart disease, stroke or cardiovascular disease, achieving only modest effects on triglyceride levels.
However, LDL ('bad') cholesterol levels and diastolic blood pressure were significantly reduced.
The researchers also note that a high intake of carbohydrates did not increase body weight, but rather, tended to maintain it.
Dr. Michael Thun, vice president of epidemiology and surveillance research at the American Cancer Society says, "This is by far the most definitive study showing that a concerted effort to reduce fat intake to 20% of total [calories] over an eight-year period did not reduce the incidence of breast or colorectal cancer in these women. But it's unlikely to end the debate completely."
It is possible that a diet that is lower in particular types of fat might be beneficial. "Reducing total fat didn't make any difference to heart disease, but women who chose to reduce their saturated fat or trans fat had a significant reduction" in heart disease, Rossouw says.
"With heart disease, it's very clear that total fat isn't enough. We've got to focus on specific types of fat." he adds.
Dr. Elizabeth G. Nabel, director of the NHLBI, says, "We really think these findings are good news. This study was the most comprehensive study of this kind, and the findings are very consistent with current US dietary recommendations about following a diet low in saturated and trans fats and cholesterol and keeping fat calories to approximately 20% to 35% of total calories."
It's also not clear if starting a low-fat diet earlier in life can confer a greater reduction in heart disease risk. All of the women in the study were postmenopausal when they changed their eating habits.
Despite the somewhat surprising findings, experts still believe women should follow a healthy lifestyle.
"We have to be very careful. The last thing I want is someone to go out eating a high-fat, high-calorie diet," says Dr. Nieca Goldberg, former chief of women's cardiac care at Lenox Hill Hospital in New York City and a spokeswoman for the American Heart Association (AHA).
"It would be easy to misinterpret the results of this study," adds Dr. Robert H. Eckel, president of the AHA.
"Reducing the risk of cardiovascular disease is about following an integrated lifestyle program, rather than concentrating solely on dietary composition," he says.
"It's a combination of things that lowers cardiovascular risk. It's no one diet, no one exercise, no one pill," Goldberg adds. "We have to avoid extremes. Really, it's about a balance-how we exercise and how we eat-and balancing the stress level as well. Nobody wants to hear it. We all want the quick fix."
The Study Continues
"The issue isn't over," Rossouw says. "We plan to follow these women for another five years because it's quite possible that a benefit for both breast cancer and colorectal cancer will emerge over time."
More examination will be done on the existing data to determine if specific dietary components have a beneficial effect on health, experts say.