Plant-based fat may reduce risk of prostate cancer.

Plant-based fats may cut the risk of prostate cancer by reducing the levels of both testosterone and certain enzymes that metabolize testosterone into more active forms, according to a new study by University at Buffalo nutrition researchers (Department of Physical Therapy, Exercise and Nutrition Sciences, 415 Kimball Tower, Buffalo, NY 14214). The study reported that feeding rats a diet rich in phytosterols, the type of fat found in plants, reduced testosterone in the blood stream by 33%. A high level of testosterone has been implicated as a risk factor for developing prostate cancer.

The levels of two enzymes, 5-a reductase and aromatase, that metabolize testosterone into end products and which also are implicated in the development of prostate cancer, were reduced 44% and 55%, respectively. Investigators have been trying to understand the mechanisms responsible for vegetarians' lower rates of hormone-dependent cancers, and for the lower mortality rate from prostate cancer in Asian countries, where populations eat little meat.

With fats known to play a role in the development of several cancers, scientists have been focusing on the phytosterols for possible answers. The phytosterol B-sitosterol appears to play a role in inhibiting the growth of human prostate cancer cells by strengthening an intracellular signaling system that inhibits cell division. The objective of the current study was to examine the influence of vegetable fats on the metabolism of testosterone in liver, testis and prostate tissue in rats.

Prostate cancer doesn't develop in men castrated before puberty or in men who have low levels of 5-a reductase. This suggested that suppressing the action of the enzyme might be useful in preventing prostate cancer in high-risk groups. The enzyme aromatase converts testosterone to estrogen, also considered a possible risk factor for prostate cancer.

To determine if high levels of phytosterols could inhibit these enzymes, the researchers fed one group of rats a standard, or basal, diet. A second group, designated the control group, ate the standard diet plus cholic acid, which stimulates the absorption of vegetable fats. A third group ate a standard diet enhanced with a mixture of phytosterols, plus cholic acid. The trial lasted 22 days.

Results showed that rats fed the phytosterol diet had between 33% and 48% less testosterone than the animals that received no additional phytosterols. There was no difference in serum testosterone levels between the basal and control groups. The enhanced diet reduced the activity of 5-a reductase by 44% in the liver and 33% in the prostate, but did not affect the enzyme's activity in the testis. Phytosterols reduced the activity of aromatase 57% in the prostate, but had no effect in the liver or testis.

The University of Buffalo research team continues to examine exactly how phytosterols bring about these changes, but it believes one pathway involves the fluidity of membranes that harbor the enzymes. Every enzyme requires a specific fluidity in the membrane in which it resides to be activated. If the membrane is too fluid, the enzymes may not function properly. It's also possible that phytosterols compete with testosterone and its products of metabolism for the enzyme's active receptors, or that they reduce production of the enzymes themselves.

Further information. Atif Awad; phone: [phone]; fax: [phone]; email: [e-mail].