Over the past few years, there has been growing evidence of a link between high levels of testosterone and the risk for developing prostate cancer. Recently, scientists at the University at Buffalo contributed to this area of research when they found that plant-based fats may reduce the levels of testosterone in the blood, as well as certain enzymes that metabolize testosterone into more active forms.
Specifically, the scientists discovered that by feeding rats a diet rich in phytosterols-the type of fat found in plants-they were able to reduce testosterone in the bloodstream by 33 percent.
In addition, they found that the levels of two enzymes, 5-alpha reductase and aromatase, were reduced by 44 percent and 55 percent, respectively, which is significant because these enzymes metabolize testosterone into end-products that also are implicated in the development of prostate cancer.
The study was led by Atif Awad, PhD, head of the nutrition program in UB's Department of Physical Therapy, Exercise and Nutrition Science, whose work was supported by a $45,000 grant from the Allen Foundation. Awad and his colleagues 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. Because fats are known to play a role in the development of several cancers, his group has been focusing on the phytosterols for possible answers.
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. During metabolism, interaction with the enzyme 5-alpha reductase changes testosterone into more active forms, while interaction with the enzyme aromatase converts it to estrogen, also considered a possible risk factor for prostate cancer.
Knowing that prostate cancer doesn't develop in men castrated before puberty or in men who have low levels of 5-alpha reductase suggested to Awad's group that suppressing the action of the enzyme might be useful in preventing prostate cancer in high-risk groups.
To determine if high levels of phytosterols could inhibit these enzymes, the researchers fed one group of rats a standard diet; a second, 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.
Results showed that rats fed the phytosterol diet had between 33 and 48 percent 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-alpha reductase by 44 percent in the liver and by 33 percent in the prostate but did not affect the enzyme's activity in the testis, results showed. Phytosterols reduced the activity of aromatase by 57 percent in the prostate but had no effect in the liver or testis.
Awad says his research team is continuing to examine exactly how phytosterols bring about these changes, but he believes one pathway involves the "fluidity" of the membranes that harbor the enzymes. "Every enzyme requires a specific fluidity in the membrane in which it resides in order to be activated," he says. "If the membrane is too fluid, the enzymes may not function properly."
Other options include the possibility 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. "Either way, the combined effect of reducing levels of testosterone and the activity of its two main enzymes suggests that a diet high in foods containing phytosterols could help reduce the risk of prostate cancer," Awad says.
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