Ask the Doctor

Low-Fat Diet

Q. I am a 60 year old female and have been eating a low-fat diet for years now because I was told it would reduce my risk of heart disease and cancer. Now I've read that a new study indicates that's not necessarily true. Why is there so much conflicting information out there on diet?

A. In 1991 the National Institutes of Health launched the Women’s Health Initiative (WHI) to study the effects of various interventions on the rate of death and disability of post menopausal women. The study enrolled over 100,000 participants. The recently released clinical trial looked at dietary modification and its effect on three factors; cardiovascular disease, breast cancer, and colon cancer.

The Dietary Modification Trial of 48,835 women revealed some surprising results. The trial was a controlled trial. That means 50% of the participants received an intervention and the other 50% were asked to continue their usual diet. This study followed these women for 8.1 years. The intervention group was asked to increase their intake of fruits/vegetables to 5 servings/day, reduce the dietary intake of fat to less than 20% of energy intake and increase grain intake to at least 6 servings/day.

The study found no significant impact from diet modification on the risk for coronary artery disease, stroke, or total cardiovascular disease. It is common in many studies of this kind (large clinical trials) to measure secondary endpoints, which in this study was cardiovascular risk As a result, other risk factors that contribute to stroke and heart disease (excess weight, lack of exercise, smoking, high salt diet) were not controlled. Therefore, this does not mean that a diet rich in fruits, vegetable and whole grains is not helpful, but it needs to be combined with a healthy lifestyle of not smoking, exercising regularly and weight control. People should still continue to try to reduce fat intake to less than 7% of their total energy intake. In addition, you should reduce the amount of saturated fats, monounsaturated fats and maintain a healthy weight.

The Dietary Modification Trial primarily looked at the effects of diet modification on the risk of breast cancer. This study did not show a statistically significant difference in the rate of invasive breast cancer between the diet modification group and control group. This is in slight contradiction to other studies that have shown some benefit. One study showed an improvement in the relapse-free survival of post menopausal breast cancer patients. Additional studies are ongoing to determine if there are any specific elements of diet that may be helpful in reducing the risk of breast cancer.

The second primary endpoint was the effect of a low-fat diet on the risk for colon cancer. This study did not show any statistically significant difference between groups. There are three criticism of the above finding:

1. The dietary modification in the intervention group was not as significant as initially intended. Participants did not make all the advised changes in diet and did not closely adhere to them for the length of the study.

2. The study length of 8.1 years was not long enough to measure an appreciable difference in rates of colon cancer.

3. Dietary modification in the post menopausal years may be too late to have an effect on the rate of colon cancer. People should modify their diet to high fiber, low fat, with increased portions of fruit and vegetables at a much earlier age.

At first glance, one might conclude that this data indicates little reason to observe a healthy diet, for it seems to have minimal impact. Most experts, however, believe that these studies do not lead to that conclusion. Eating healthy a low-fat, high-fiber diet), exercising regularly, maintaining a healthy weight, not smoking and controlling other risk factors (hypertension, diabetes), remain the foundation of good health.

More results will be published from the WHI and other studies in the next several years. Stay tuned. I hope this answers your question about this important study.