A diet rich in antioxidants like vitamins C and E, and beta-carotene has been reported to protect certain sub-populations, particularly smokers, against prostate cancer, a disease that is becoming more common, with incidence rates haven risen by almost two per cent over 15 years.
"Vitamin E supplementation in smokers and beta-carotene supplementation in men with low dietary beta-carotene were associated with reduced risks of this disease," wrote lead author Victoria Kirsh from US National Cancer Institute.
The Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, published in the Journal of the National Cancer Institute (Vol. 98, pp. 245-254), surveyed the diet and supplement use of 29,361 men with an average age of 63, and with an average follow-up time of 4.2 years.
Former smokers made up about 50 per cent of the population, while 30 per cent of the volunteers were non-smokers, and about 10 per cent current smokers. Ninety per cent of the study population was white.
Dietary information was collected by a self-administered 137-item food frequency questionnaire (FFQ) that assessed the volunteer's diet over the previous year.
"There was no statistically significant association between the risk of prostate cancer [for the general population] and the duration of use of supplemental vitamin E, beta-carotene, or vitamin C," reported the researchers.
However, current and recent-former smokers taking 400 IU or more vitamin E daily were 71 per cent less likely to develop prostate cancer than those who took no vitamin E.
Differing vitamin C intake, dietary and supplemental, was reported to have no effect on any of the sub-populations.
"Our cohort findings do not provide strong support for population-wide implementation of high-dose antioxidant supplementation for the prevention of prostate cancer.
They do suggest, however, that in certain population subgroups there was an association between supplement intake and reduced risks of prostate cancer," concluded Kirsh.
These results agree with those of the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) study (1998), which reported that smokers who received 50 micrograms of vitamin E had a lower incidence of prostate cancer than the non-supplemented group.
However, the Heart Outcomes Prevention Evaluation (HOPE) Trial (2005) reported that a daily supplement of 400 IU of vitamin E had no effect on either prostate or any other type of cancer.
The majority of studies have looked at alpha-tocopherol, the form of vitamin E found mostly in supplements, but seven other forms of the vitamin exist, with gamma-tocopherol being the most abundant from diet.
The current study attempted to address if a lower risk of prostate cancer was linked to other tocopherols but found no statistically significant associations. A previous study (American Journal of Epidemiology, Vol.157, pp. 335-344), on the other hand, reported that higher blood concentrations of gamma-tocopherol were associated with lower risks of prostate cancer.
Gamma-tocopherol has been shown to stop the growth of cancer cell cultures in a laboratory study.
Similar uncertainty exists for beta-carotene supplements. The ATBC trial reported that 20 mg supplements were linked to a small, non-significant increase in risk. The Physician's Health Study reported that, like the PLCO trial, volunteers who had low serum beta-carotene levels had less risk of prostate cancer if they took beta-carotene supplements.
It is clear that results in this field, particularly for vitamin E, are contradictory, and "may reflect the limitations inherent in studies that use a food frequency questionnaire to collect data," according to Kirsh and colleagues.
Such questionnaires, and the associated nutrient databases, have a limited ability to quantify specific tocopherol intake, for example, suggesting a well-designed intervention trial is badly needed to clarify the uncertainty with vitamin E, beta-carotene and prostate cancer.
Henry Scowcroft, science information officer at British charity, Cancer Research UK, echoed this view when he said that the jury was still out on cancer prevention by vitamin E supplements.
Several studies which may provide a clearer verdict, including the Selenium and Vitamin E Cancer Prevention Trial (SELECT), are ongoing and the results are due out soon.
Scowcroft told NutraIngredients.com: "The main finding of this paper, that smokers who have a high intake of vitamin E have a lower risk of advanced prostate cancer, needs to be interpreted with caution.
The best health advice to smokers is simply to give up smoking if they want to reduce their risk of cancer and other chronic diseases."
The European School of Oncology currently recommends consuming more antioxidants, like selenium and vitamins C and E as a way of preventing the disease.