Higher blood levels of gamma-tocopherol, a form of vitamin E not usually included in vitamin supplements, is associated with a lower risk of developing prostate cancer than is alpha-tocopherol, the synthetic form of vitamin E most commonly found in supplements.
Although seldom studied, gamma-tocopherol is a natural form of vitamin E routinely found in the U.S. diet, notably in soy foods. The study also revealed that high concentrations of gamma-tocopherol appear to boost the prostate cancer-fighting abilities of both alpha-tocopherol and the micronutrient selenium.
The study, by scientists at the Johns Hopkins School of Public Health, appears in the December 20 issue of the Journal of the National Cancer Institute, where it is accompanied by an editorial.
"Given gamma-tocopherol's positive interactions with both alpha-tocopherol and selenium," says Kathy J. Helzlsouer, MD, MHS, professor, Epidemiology, the Johns Hopkins School of Public Health, and the lead investigator of the study, "its use should be considered in upcoming prostate cancer prevention trials."
A previous intervention trial, designed to look at the effects of supplemental alpha-tocopherol on the risk of lung cancer among smokers, had observed that the men taking the supplements had a lower risk of developing prostate cancer.
Little attention, however, has been paid to other forms of vitamin E such as gamma-tocopherol, which studies in cells suggest may have even stronger antioxidant properties than alpha-tocopherol.
The present study is the first to simultaneously investigate the association between the risk of prostate cancer and concentrations of alpha-tocopherol, gamma-tocopherol and selenium. It is also the first to simultaneously consider the association between the risk of prostate cancer and concentrations of these micronutrients when they are supplied primarily from normal dietary intake (i.e., not from supplements).
In a countywide campaign in 1989, researchers from the Johns Hopkins School of Public Health collected and froze blood samples from a total of 10,456 male residents of Washington County, Md., for future study.
At the time of blood donation, participants gave information about their medical and smoking histories; all supplements and medications taken within the past 48 hours; and their height and weight, both currently and at age 21. Each participant was also asked to mail in a nail clipping from the big toe so that selenium levels could be assessed.
Among the men who gave a blood sample and nail clipping in 1989, 110 went on to develop prostate cancer between January 1990 and September 1996. Each of these men with prostate cancer were matched with two controls, men who had remained cancer-free. Serum levels of alpha-tocopherol and gamma-tocopherol, as well as toenail selenium levels, were measured and compared between the two groups. The researchers then compared all three micronutrients' concentrations, both singly and in combination, with each man's risk of developing prostate cancer.
Median concentrations of both alpha-tocopherol and gamma-tocopherol were lower among the men with prostate cancer than among the control subjects, but these differences were statistically significant only for gamma-tocopherol.
Compared with the men with the lowest levels of gamma-tocopherol, men with the highest levels had a fivefold reduction in their risk of developing prostate cancer.
What's more, gamma-tocopherol appeared to boost the protective effects of both alpha-tocopherol and selenium. Compared with individuals with low concentrations of all three micronutrients, high concentrations of selenium and alpha-tocopherol were associated with a statistically significant decreased risk of prostate cancer only when high concentrations of gamma-tocopherol were also present.
The authors note that since alpha-tocopherol supplementation may lower gamma-tocopherol concentrations in plasma and tissues, supplementation with combined alpha- and gamma-tocopherol may be called for in future prostate cancer prevention trials.
Support for this study was provided by a Public Health Service grant from the National Cancer Institute at the National Institutes of Health and by grants from the Department of Defense.