Print

Email story

Comment

Email editor

Guest column

Low-fat diets fail to prevent some health risks, study says

By V. UPENDER RAO
Published February 13, 2006

Heart disease and cancer are the two most common causes of death. Together, breast and colorectal cancers account for the largest number of cancer cases and deaths among women in the Unites States.

Breast cancer is at least 40 percent more prevalent in Western women than Asian women. Apart from the obvious racial and ethnic variance, diet and lifestyle differences are also prominent. Many epidemiological and observational studies have suggested that a higher fat content in Western diets might be responsible for increased incidence of cancer and cardiovascular disease.

An increased occurrence of these diseases in the populations of the East that have migrated to the United States lends credence to this observation. Therefore, researchers tested the relationship of a low-fat diet to breast and colorectal cancers and heart disease.

The dietary modification component of the Women's Health Initiative study comprised 48,835 postmenopausal women ranging in age from 50 to 79. The total number of patients entered on this study amounted to 102 percent of the planned accrual. There were 19,541 women randomized to the intervention arm and 29,294 women to the comparison arm.

Patients on the intervention arm were placed on a low-fat diet and observed for eight years. Although researchers found a trend toward a benefit to low-fat diet in breast cancer, colorectal cancer and heart disease, the magnitude of the observed benefit did not reach statistical significance.

Several explanations are possible. The study diet mandated 20 percent energy from fat content of the daily diet. However, the successful adherence to this goal was measured at only 70 percent, which means there was a 30 percent variance from the study protocol. Only 31.4 percent of women adhered to this diet in the first year, and by the sixth year only 14.4 percent were compliant.

In the case of breast cancer, early menarche, increased estradiol levels and prolonged exposure of the breast tissue to unopposed estrogen are probably related, at least in part, to dietary fat content and carcinogenesis. It is probable that breast cancer evolves over a much longer period of time than the period of observation in this study, which was only eight years. A longer period of observation might have yielded a significant benefit.

As for colorectal cancer, no benefit to a low-fat diet was noted in this study. One of the explanations that the authors of the study gave was that perhaps it is not the fat but the iron content of the red meat that is carcinogenic. If this were true, then one has to explain why the iron content of green leafy vegetables is not harmful.

I discussed this issue with the editor of the Journal of the National Cancer Institute . There is some evidence in the literature that it is not so much the fat but the high calorie diet that is carcinogenic. High caloric food ingestion leads to excess insulin secretion. Insulin and insulinlike growth factors, as well as epidermal growth factor and platelet-derived growth factors, have been implicated in many cancers.

These studies appeared in the Feb. 8 issue of the Journal of the American Medical Association.

What appeared to be an astute epidemiological observation - that high-fat diets cause breast cancer, colorectal cancer and heart disease - failed the test of prospective randomized trials. Similarly, readers will remember that hormone replacement therapy for postmenopausal women, which was supposed to restore youth and prevent heart disease, bone deterioration and certain types of dementia, also not only failed to show benefit in randomized trials but increased the probability of cardiovascular disease and breast cancer.

Beta carotene, likewise, was supposed to protect from lung cancer but increased the lung cancer incidence when tested in randomized trials.

Epidemiological observation of benefit can at best be hypothesis generating and should not be recommended as routine interventions or treatments until and unless prospective randomized clinical trials ratify their benefit.

--V. Upender Rao, MD, FACP, practices at the Cancer and Blood Disease Center in Lecanto.

[Last modified February 13, 2006, 00:45:19]