Low-Fat Diet Under Fire but Questions Remain
March 2, 2006—Results from the Women’s Health Initiative (WHI) Dietary Modification Trial suggest that a low-fat diet may not protect against breast cancer, colon cancer, or cardiovascular disease (CVD), reports the Journal of the American Medical Association (2006;295:629–66). These findings may influence future recommendations regarding fat in the diet for disease prevention, though there is still good reason to believe that the type of fat a person eats may have more impact on health than overall amount.
In an effort to explore interactions between fat intake and the risk of developing cancer and heart disease, the WHI Dietary Modification Trial was initiated in 1991. The study aimed to compare the incidences of breast cancer, colon cancer, and CVD among a group of almost 50,000 postmenopausal women. Forty percent were instructed to eat a low-fat diet in which 20% of total calories came from fat; the other women served as a comparison group.
Women in the low-fat group were told to eat more fruits and vegetables (five or more servings per day) and grains (six or more servings per day). Women in the intervention group were given intensive instruction throughout the study period to help them meet these goals while the comparison group received no dietary instructions.
The women filled out questionnaires at the beginning of the study and at regular intervals in order to assess the composition of their diets. Women in the intervention group consumed significantly less total fat and saturated fat than did women in the comparison group. By the sixth year of the study, women in the intervention group were getting about 29% of their total calories from fat, 9.5% of which was saturated fat, compared with 37% of total calories from fat and 12% from saturated fat in the comparison group. Women in the low-fat group also ate significantly more fruits, vegetables, and grains than did the comparison group women.
The incidence of disease was similar between the two groups. Over the course of the study, 3.4% in the intervention group developed breast cancer, versus 3.7% in the comparison group. Although this difference was not significant, some trends in the results did emerge. Women who had higher initial fat intakes experienced a slight, but nonsignificant, reduction in breast cancer risk. Similarly, the incidence of colon cancer did not differ significantly between the two groups.
The incidence of CVD was also not affected by dietary intervention; rates of heart attack, death due to cardiac causes, cardiac bypass surgery, and stroke were not significantly different between the groups. Diastolic blood pressure and blood levels of LDL cholesterol were significantly lower in the intervention group; however, levels of HDL cholesterol, triglycerides, glucose, and insulin were similar between the groups.
The results of these studies should be interpreted in the light of several key points. Although the women in the intervention group had lowered their fat intake, the reduction did not meet the projected goals, making it harder to draw accurate conclusions about the incidences of the diseases in question.
In particular, the study was poorly designed to detect the effects of dietary fat on CVD incidence. Women in the intervention group did decrease their total fat intake, but along with reductions in the unhealthy trans and saturated fats, they also lowered their intake of heart-healthy mono- and polyunsaturated fats. In addition, there is evidence that high-temperature cooking causes chemical changes in some fats that might increase the risk of both heart disease and some cancers. Thus, the type of fat that a person consumes may be more important than total fat intake.
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Kimberly Beauchamp, ND, earned her bachelor’s degree from the University of Rhode Island and her Doctorate of Naturopathic Medicine from Bastyr University in Kenmore, WA. She cofounded South County Naturopaths in Wakefield, RI. Dr. Beauchamp practices as a birth doula and lectures on topics including whole-foods nutrition, detoxification, and women’s health.
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