Medical experts have spent years encouraging people to eat a diet low in fat to prevent certain illnesses. Now, the largest study ever to look at low-fat diets suggests the link between diet and disease isn't so clear.
Three related studies in older women found that lowering total fat in their diet didn't significantly reduce the incidence of breast cancer, heart disease or stroke. Nor did it reduce their risk of colorectal cancer.
The research from the Women's Health Initiative followed the eating habits of 48,835 postmenopausal women for more than eight years, including 1,138 from Western New York.
Overall, the findings, published in today's Journal of the American Medical Association, indicate that a diet low in fat and higher in fruits, vegetables and whole grains may be helpful in some cases, but not as helpful as previously believed.
"These interventions have beneficial effects, but they may not make a huge impact," said Jean Wactawski-Wende, a University at Buffalo researcher and co-author with Dr. Maurizio Trevisan of the breast and colorectal cancer papers.
The Women's Health Initiative is a 15-year effort by the U.S. government to look into the most common causes of death, disability and poor quality of life in postmenopausal women aged 50 to 79.
Among the conclusions of the latest studies:
*Women who reduced total fat had a 9 percent lower risk of breast cancer than women who made no dietary changes. The difference was small enough to have been due to chance, yet big enough to suggest that a low-fat diet offers modest protection against breast cancer.
*A high-carbohydrate, low-fat diet proved to be safe. It didn't increase body weight, triglycerides or indicators of increased diabetes risk, such as blood glucose level.
*A low-fat diet didn't change the risk of colorectal cancer but did reduce by 9 percent the occurrence of polyps and adenomas, which are thought to be precursors of cancer. A low-fat diet also showed possible benefit in women who were taking aspirin or combined hormone therapy.
"There are little nuggets in the data. One of them is the reduction in indicators of later colon cancer risk," said Dr. Shirley A.A. Beresford, professor of epidemiology at the University of Washington in Seattle and lead author of the colorectal cancer study.
The results for breast and colon cancer are considered the most definitive evidence to date of the impact of a low-fat diet. Despite little evidence of a link between total fat and these diseases, changes are unlikely in dietary guidelines.
Currently, health authorities recommend that adults keep total fat intake between 20 and 35 percent of calories, and saturated fats less than 10 percent of calories.
The results also are unlikely to stand as the last word on the matter because of the shortcomings of the studies, experts said.
For instance, the women may not have lowered their consumption of fat enough to see a significant effect on disease. Participants on a low-fat diet reduced the total amount of fat they ate from 38 percent to 29 percent of calories from fat. But the researchers had hoped to achieve a reduction of 20 percent.
"We learned that it is very hard to change behavior," said Wactawski-Wende, a professor of social and preventive medicine.
The study, which enrolled women between 1993 and 1998, was likely too short to find breast and colon cancers because those cancers develop over many years.
"It's possible that had we designed the study differently, there would have been different results. But it would be an extraordinarily expensive undertaking to follow people for a longer period," said Beresford.
The researchers also focused on total fat, reflecting knowledge at the time they started their work. They did not differentiate between healthy fats found in fish, nuts and vegetable oils, and unhealthy saturated and trans fats found in meats, some dairy products and processed foods. In heart disease, specific fats may be more important than total fat.
"These results will not change the American Cancer Society dietary guidelines, which pertain to broad dietary patterns rather than specific macro- or micro-nutrients. They will further dampen confidence in the benefits of a low-fat diet for preventing breast cancer in postmenopausal women," said Dr. Michael Thun, the society's vice president of epidemiology and surveillance research.
Thun said much of the interest in low-fat diets as an approach to prevent breast cancer has been eclipsed by the emergence of the obesity epidemic, and recognition of its importance for cancer.
At the end of the study, the average difference in weight between the women on a low-fat diet and those on their usual diet was less than two pounds, suggesting that something other than fat, namely obesity, increased one's breast cancer risk, he said.
If there's a key message from the research, it's that a diet low in fat and high in fruits, vegetables and grains remains very healthy, according to Wactawski-Wende.
"This trial tested the diet's effects on specific conditions," she said. "The fact that it showed little effect on those specific conditions does not mean that anyone should abandon a proven healthy diet."
The 40 research centers nationwide involved in the study plan to follow most of the women for five more years.