Low-fat diets may not reduce risk of breast, colorectal cancers or CVD for older women

Large, randomized trial casts doubts on the preventive benefits of low-fat diets.

Issue: March 1, 2006

Reducing the amount of total dietary fat will not lower the risk of breast cancer, colorectal cancer or cardiovascular disease (CVD) in postmenopausal women, according to a recent study.

The latest clinical results from the Women’s Health Initiative (WHI), a 15-year, broad-based study of the causes of disease and preventive measures for older women, are drawn from the experiences of about 49,000 postmenopausal women (from 50 to 79 years old) who were tracked for about eight years.

Forty percent (19,541) were randomly assigned to the dietary modification (low-fat diet) group. They were asked to reduce their fat intake to 20% of their total calories while eating five or more daily servings of vegetables and fruits and six servings of grains. The remaining 60% (29,294) were placed in a comparison group. They were not asked to change their diet.

At the end of the first year, the low-fat diet group was consuming 24% of their calories from fat, compared with 35% in the comparison group. Six years later, the low-fat diet group consumed approximately 29% of calories from fat, compared with 37% in the comparison group.

The two groups had no significant statistical difference in risk of breast cancer, colon cancer, heart disease or stroke, according to three studies published in the Journal of the American Medical Association.

“Just switching to low-fat foods is not likely to yield much health benefit in most women,” said Marcia Stefanick, PhD, professor of medicine at the Stanford Prevention Research Center in California and chair of the WHI steering committee. “Rather than trying to eat ‘low-fat,’ women should focus on reducing saturated fats and trans fats.”

She also recommended that women eat more vegetables, in particular dark, leafy greens and cruciferous vegetables, though the trial did not specifically study these foods.

Breast cancer risk

The diet and breast cancer trial began in 1992 with 48,835 postmenopausal women without prior breast cancer. Average follow-up time was 8.1 years. Overall, 655 women (3.35%) in the intervention group and 1,072 women (3.66%) in the comparison group developed invasive breast cancer during follow-up.

“Among postmenopausal women, a low-fat dietary pattern did not result in a statistically significant reduction in invasive breast cancer risk over an 8.1-year-average follow-up period,” researchers wrote. “However, the nonsignificant trends observed, suggesting reduced risk associated with a low-fat dietary pattern, indicate that longer, planned, nonintervention follow-up may yield a more definitive comparison.”

Additionally, the low-fat diet was associated with a statistically significant 15% reduction in estradiol, a form of blood estrogen that increases the risk of breast cancer. Women in this group also experienced a 30% risk reduction for progesterone-receptor—negative breast cancer.

“Women in the low-fat diet group reduced their overall rate of breast cancer by about 9% as compared to the women who didn’t change their eating patterns, but that difference was not statistically significant,” said Ross L. Prentice, PhD, lead researcher and former director of the Fred Hutchinson Cancer Center’s public health sciences division in Seattle. “So at this point we’re not able to say with certainty that a low-fat diet reduces the risk of breast cancer.”

Colorectal cancer risk

Over 8.1 years of follow-up, there were 201 cases of invasive colorectal cancer (0.13% per year) found in the low-fat diet group and 279 (0.12%) in the comparison group. A low-fat diet did not have a statistically significant reduction in the risk of invasive colorectal cancers.

“Evidence from this study, along with that from polyp prevention trials, strongly suggests that lowering dietary fat intake and increasing fruit, vegetable and fiber intake in mid to late life cannot be expected to reduce the risk of colorectal cancer in this length of time,” researchers wrote.

However, a low-fat diet did factor into a 9% decrease in self-reported colon polyps, said Shirley A.A. Beresford, PhD, lead researcher of the colorectal cancer study.

“It is important to remember that cancers often take decades to develop, and we may only be seeing the early stages of the impact of a low-fat diet intervention on the risk of colorectal cancer and other diseases,” said Beresford, a member of the Hutchinson Center’s public health sciences division and a professor of epidemiology at the University of Washington School of Public Health and Community Medicine in Seattle.

Cardiovascular disease risk

Following patients in the cardiovascular arm for eight years showed that a low-fat diet significantly reduced levels of low-density lipoprotein cholesterol and diastolic blood pressure. However, there was no significant difference in levels of high-density lipoprotein cholesterol, triglycerides, glucose or insulin. The diet had no significant effects on incidence of coronary heart disease (CHD), stroke, CVD or heart attack.

Promisingly, researchers observed trends toward greater reductions in CHD risk in those with lower intakes of saturated fat or trans fat or higher intakes of vegetables and fruits.

“To achieve a significant public health impact on CVD events, a greater magnitude of change in multiple macronutrients and micronutrients and other behaviors that influence CVD risk factors may be necessary,” the researchers wrote.

“Despite null findings from the WHI Dietary Modification Trial, dietary changes can have powerful, beneficial effects on CVD risk factors and outcomes,” wrote Cheryl A. M. Anderson, PhD, MPH, and Lawrence J. Appel, MD, MPH, of Johns Hopkins University Bloomberg School of Public Health and School of Medicine in Baltimore. “To reduce the risk of CVD, individuals should maintain a desirable body weight, be physically active, avoid tobacco exposure and eat a diet consistent with national guidelines. Additional results from the WHI Dietary Modification Trial, likely forthcoming, should provide valuable evidence that will refine these recommendations and further enhance CVD prevention efforts in women.”

Fat content

“Nutrition knowledge has progressed dramatically since the study began,” said Mara Vitolins, PhD, study researcher and associate professor of public health sciences at Wake Forest University Baptist Medical Center, Durham, N.C. “Today, we know that reducing total fat may not be enough – we need to focus on the types of fat we eat.”

In a press statement, Vitolins said that the study focused on a diet that reduced total fat. It did not distinguish good fats (nuts, fish and vegetables oils) from bad fats (saturated fat in meats and the trans fat in baked goods and potato chips.) Additionally, women in the study began their diets when they were between 50 and 79 years. “Our diets start when we are born and it makes sense that what you eat over a lifetime will make a difference,” she said.

“The results of this study do not change established recommendations on disease prevention,” said National Heart, Lung and Blood Institute director Elizabeth G. Nabel, MD, in an NIH press statement. “Women should continue to get regular mammograms and screenings for colorectal cancer, and work with their doctors to reduce their risks for heart disease including following a diet low in saturated fat, trans fat and cholesterol.” – by Mark Palacio

For more information:
Anderson CAM, Appel LJ. Dietary modification and CVD prevention: a matter of fat. JAMA. 2006;295:693-695.
Beresford SAA; Johnson KC; Ritenbaugh C, et al. Low-fat dietary pattern and risk of colorectal cancer: the Women’s Health Initiative randomized controlled dietary modification trial. JAMA. 2006;295:643-654.
Howard BV, Van Horn L, Hsia J, et al. Low-fat dietary pattern and risk of cardiovascular disease: the Women’s Health Initiative randomized controlled dietary modification trial. JAMA. 2006;295:655-666.
Prentice RL, Caan B, Chlebowski RT, et al. Low-fat dietary pattern and risk of invasive breast cancer: the Women’s Health Initiative randomized controlled dietary modification trial. JAMA. 2006;295:629-642.