Animal Fat and Cancer

AMERICAN men whose diets are rich in animal fats, and particularly fats from red meat, face nearly an 80 percent greater risk of developing potentially fatal prostate cancer than do men with the lowest intake of such foods, a major new study has found.

The study, which has been following the medical fates of more than 51,000 male health professionals since 1986, provides the strongest evidence yet linking dietary fat to the chances of dying of prostate cancer, which is the second leading cause of cancer deaths in American men, after lung cancer.

The finding strengthens the credibility of earlier observations derived from studies of cancer-diet relationships in laboratory animals, international comparisons of prostate cancer rates, studies of cancer rates among immigrants from countries where prostate cancer is rarely fatal and examinations of the diets of men with advanced prostate cancer.

As in the laboratory studies, the new research suggests that fats derived from animal foods promote rather than initiate the development of prostate cancer and may be the crucial factor determining in which men prostate cancers change from a dormant, symptomless condition to a spreading and possibly lethal malignancy. The study, published in today's issue of The Journal of the National Cancer Institute, was conducted by Dr. Edward Giovannucci and colleagues at Harvard's Medical School and School of Public Health.

Cancer researchers have long been puzzled by the fact that when men from different countries are examined at autopsy, 15 to 30 percent are found to have had latent prostate cancer that caused no symptoms. Yet there is as much as a 120-fold difference in prostate cancer death rates, with the rate for American men among the highest in the world.

U.S. Rise in Death Rate

This year the American Cancer Society estimates that 35,000 men will die of the disease, and it will be diagnosed in another 165,000. In the last three decades, the prostate cancer death rate among American men has risen more than 17 percent.

Studies have also shown that when men move here from Japan, where prostate cancer is a rare cause of death, their chances of developing the disease increase with the length of their stay, eventually equaling those of native Americans. Such evidence strongly suggests that environmental factors, rather than genetic differences, account for Americans' high rate of fatal prostate cancer.

In an editorial in the same issue, Dr. Kenneth J. Pienta of the Michigan Cancer Center and Peggy S. Esper of Harper Hospital in Detroit said "dietary fat currently seems to be the most likely environmental culprit."

Dr. David Rose, who conducts diet-cancer studies at the American Health Foundation in Valhalla, N.Y., said: "A number of different fatty acids drive prostate cancer cells once they've developed. Fats can accelerate the growth of tumors and increase their propensity to metastasize."

An ability of dietary fats to stimulate cancer growth in animals has been demonstrated for cancers of the breast, ovary, colon and uterus as well as for cancer of the prostate. While most fats have been implicated as cancer promoters in the laboratory studies, fats from fish -- the so-called omega-3 fatty acids -- have the opposite effect, suppressing cancer growth and metastases.

In the new study, researchers conducted careful assessments of the usual diets of the 47,855 participants who were free of recognizable cancer when the study began. In the next four years 300 cases of prostate cancer, including 126 cases of advanced cancer, were diagnosed among these participants.

While no relationship was found between fat and prostate cancer over all, a definite link was established between consumption of animal fat and the likelihood of being diagnosed initially with advanced prostate cancer.

Among the various sources of animal fat, fat from red meat was most strongly linked to advanced cancer. Those who consumed the most red meat were two and a half times more likely to be found to have advanced cancer or to die of prostate cancer than those who ate meat infrequently. The investigators found a tenfold difference in intakes of animal fat between low and high consumers, ranging from an average of 3.2 grams of fat from red meat each day to 30.5 grams a day.

Total fat intake was also associated with the risk of advanced cancer. Those who consumed an average of 88.6 grams of fat a day had a 79 percent greater chance of developing advanced prostate cancer than those who averaged 53.2 grams of fat daily. The greatest risk -- nearly three and a half times higher -- was associated with the highest intake of a monounsaturated fatty acid called alpha-linolenic acid. But Dr. Giovannucci said most of this fatty acid was derived from animal fats rather than vegetable oils in the men's diets.

Need to Confirm Findings

Both the writers of the editorial and the authors of the new study pointed out that the relationship between fat intake and the promotion of prostate cancer was complex and in need of much further research. Dr. Giovannucci said, "The findings need to be confirmed in similar prospective studies in different populations, and more research is needed into how animal fats might promote prostate cancer."

But Dr. Ernst Wynder, director of the American Health Foundation, called for more aggressive intervention studies, in which men with early prostate cancer would be placed on a carefully controlled low-fat diet to see if it reduced their chances of dying of prostate cancer. Such a study is about to be begun by the foundation among 2,000 women who have been treated for early-stage breast cancer. Half the participants will be instructed to cut their usual fat intake by more than half, to just 15 percent of daily calories.