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This past February, the latest findings investigating the use of a low fat diet to decrease the risk for breast cancer were announced and reported by the various media outlets. Various headlines proclaimed that the consumption of a low fat diet didn’t prevent breast cancer, colorectal cancer, or heart disease. The results that were announced came from the Women’s Health Intervention (WHI) clinical trial, clinical trial that began in 1992 involving postmenopausal women, of all ethnicities, aged 50 to 79 years of age. The WHI trial was the first large prospective randomized controlled clinical trial to address whether a low fat diet reduced the incidence of breast cancer in postmenopausal women (48,835 women). This was because the issue of whether dietary fat plays a role in the risk for breast cancer or not has been debated for several decades. Previous studies have yielded mixed results with some finding that fat intake was significantly associated with postmenopausal breast cancer incidence while other studies found no association.1,2  Therefore, in an effort to determine whether dietary fat consumption does play a role in the incidence of breast cancer, the WHI trial included a low fat dietary intervention.3

The women in the low fat dietary intervention group were encouraged to consume a diet not only low in fat (20% of total daily calories), but also consume increased amounts of fruits and vegetables, (³6 servings/day) and grains (³ 6 servings/day).

What was not discussed by the media:

The media failed to announce that the incidence of breast cancer was 9% lower for the women in the low fat dietary group in comparison with the women who continued to consume their usual diet. Although this reduction did no reach statistical significance, it was very close. Many investigators think this the failure to reach statistical significance was due to the fact that these women were only followed for about 8 years. Therefore, study investigators will follow these women until 2010.

Furthermore, women in the low fat group who entered the study consuming the most fat in their diets (37% of calories), had a 22% lower risk of breast cancer than women who continued to consume their normal diets.

Study Weaknesses

There are several factors to consider when interpreting the study results. First, relatively few women in the intervention group actually achieved the goal of consuming a low fat diet as defined in the study (20% or less of total daily calories from fat). Secondly, a significant different in breast cancer incidence between the two groups may not have been found because women may have actually been consuming more fat than they reported. Therefore, who knows what the results might have been had the women actually consumed a low fat diet. The fact that there was a borderline difference in the rates of breast cancer despite the fact that women in the intervention group did not reach the study goal of consuming a low fat diet is significant in or itself.
Lastly, this study did not encourage women to differentiate the type of dietary fat consumed. This study was designed in the mid-1980s when the emphasis was on total fat rather than on what type of fat was consumed. Therefore, in order to achieve the goal of reducing total fat calories to 20%, all fats across the board were reduced. Whether a higher consumption of good fats (e.g., omega-3 fatty acids) vs. bad fats (e.g., saturated and trans fatty acids) would have had a beneficial impact on reducing the risk for breast cancer is not known. Unfortunately in this study, the women in the low fat intervention group consumed almost as much bad fat as the control group.

The Bottom Line

Because this study came close to finding a significantly lower risk for breast cancer in the low fat group, and women in the low fat group who reduced their fat intake the most did have a 22% lower risk of breast cancer, women should follow the dietary guidelines for fat intake (see below). Women in the low fat intervention group also had fewer precancerous colon polyps which may decrease their risk for colorectal cancer overtime.

To decrease the risk for heart disease, stroke, and other types of cancer, consumers should limit their daily fat intake to 20-35% of total daily calories. When choosing fats, an emphasis should be placed on consuming healthy fats such as omega-3 and monounsaturated fatty acids. Nuts, seeds, avocados, olives, whole grains, olive oil, and cold-water fish (salmon, sardines, trout) are good choices.

The Dietary Guidelines for Americans recommends the following:

 

• Reduce saturated fatty acid intake (less than 10% of total calories/day)
• Reduce trans fatty acid intake to as little as possible
• Limit cholesterol intake to < 300 mg/d
• Consume 9 or more servings of fruits and vegetables daily
• Limit sodium intake to 2,300 mg/day
• Maintain a healthy weight
• Obtain at least 30 minutes of physical activity daily

References:

• Howe GR et al. Dietary factors and risk of breast cancer: combined analysis of 12 case-controlled studes. J Natl Cancer Inst 1990;82:561-569.
• Hunter DJ et al. Cohort studies of fat intake and the risk of brast cancer: a pooled analysis. N Engl J Med 1996;334:356-361.
• Prentice RL 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.
• Dietary Guidelines for American. Available at www.health.gov. Retrieved March 28, 2006.