Roxanne Nelson

October 24, 2011

October 24, 2011 (Boston, Massachusetts) — A high intake of dietary selenium — but not selenium from supplements — might reduce the risk for ovarian cancer, according to data presented here at the Tenth Annual American Association for Cancer Research International Conference on Frontiers in Cancer Prevention Research.

The study found that dietary selenium was associated with a 60% reduced risk for ovarian cancer. A strong protective effect was observed for the highest tertile of dietary selenium intake, compared with the lowest (odds ratio, 0.41; 95% confidence interval [CI], 0.20 to 0.85).

"But after adjustment for major risk factors, we didn't find any evidence of an association with total antioxidant indices for ovarian cancer risk," explained lead author Dina Gifkins, MPH, from The Cancer Institute of New Jersey in New Brunswick.

Conversely, she said, "we found an increased risk for all of the [antioxidant] supplements we looked at — vitamin E, vitamin C, beta-carotene, and selenium — of about 1.6."

The researchers found that supplement use was associated with significant increased risks for all micronutrients. Vitamin C supplement use was associated with an increased risk of 1.63 (95% CI, 1.01 to 2.62), as was vitamin E supplement use (95% CI, 1.02 to 2.63). The risk for beta-carotene supplement use (1.69; 95% CI, 1.08 to 2.66) and selenium supplement use were slightly higher (1.64; 95% CI, 1.05 to 2.56).

Needs Validation

The results are interesting, said Karen H. Lu, MD, professor in the Department of Gynecologic Oncology at the University of Texas M.D. Anderson Cancer Center, Houston. However, "it's way too preliminary for the general public to change what they do based on these findings, or for physicians to make recommendations," she told Medscape Medical News. "The data need to be validated."

Dr. Lu was not involved in the study.

Etiologic theories of ovarian cancer indicate that oxidative stress might play a role its development. "It has been suggested that limiting oxidative stress to the ovarian epithelium could be considered a first-line defense against ovarian cancer," said Ms. Gifkins. "However, information on individual antioxidant micronutrients in ovarian cancer prevention is considered to be limited and inconclusive."

Individual contributions from micronutrients and synergistic combinations can alter risk, she explained. "There have only been a few studies looking at total antioxidant intake in any cancer risk, but most have shown strong protective effects."

Study Details

Ms. Gifkins and colleagues evaluated the effect of total dietary antioxidant capacity and the intake of individual antioxidants on ovarian cancer risk in a population-based case–control study conducted in New Jersey. The cohort consisted of 233 women 21 years and older with newly diagnosed epithelial ovarian cancer and 467 control subjects in the same age range and who resided in the same geographic area.

The characteristics were similar between cases and controls, explained Ms. Gifkins, although cases tended to have a higher body mass index and were more likely to have a family history of ovarian cancer, and controls were more likely to have had a tubal ligation.

The dietary intake of the participants was assessed using the Block Food Frequency Questionnaire (FFQ), and total antioxidant indices were constructed by linking FFQ-derived estimates to 2 standardized antioxidant capacity databases — the USDA Oxygen Radical Absorbance Capacity Database, and the University of Olso's Antioxidant Food Database.

Multivariate logistic regression models were used to calculate odds ratios and confidence intervals while controlling for major ovarian cancer risk factors. "To calculate the total antioxidant indices, we looked at the total daily intake of over 50 food items in the food questionnaire," she pointed out. "We calculated daily consumption based on the frequency and portion size reported."

An analysis was done for each of the individual antioxidants, and there were no significant differences in antioxidant indices between cases and controls, although cases had a slightly lower intake of vitamin E than controls, Ms. Gifkins noted.

Controversy Over Supplements

The proposed mechanism of action for selenium in cancer prevention is its antioxidant properties, Ms. Gifkins said. "Animal studies have shown that selenium reduces tumor yield, inhibits cell growth, and stimulates programmed cell death," she added. "Human studies have shown protective effects of selenium in prostate, lung, and colorectal cancers."

She pointed out that several recent reports have raised concerns about the safety of vitamin supplements. "In the latest World Cancer Report, there is a warning to cancer patients and survivors against taking supplements until further research is conducted," she said.

Studies of supplement intake and their relation to cancer have been mixed. For example, as previously reported by Medscape Medical News, a recent study showed that vitamin E supplementation in men might increase the risk for prostate cancer. In addition, a Cochrane Systematic Review recently reported that, thus far, there is no "convincing evidence" that selenium supplements can prevent cancer in any population.

However, another study found that vitamin use — vitamins C and E in particular — appears to be associated with a reduced risk for mortality and recurrence among patients with breast cancer who underwent chemotherapy.

"Our findings warrant further investigation to better understand the role of selenium and supplement intake on ovarian cancer risk," Ms. Gifkins concluded.

Ms. Gifkins have disclosed no relevant financial relationships.

Tenth Annual American Association for Cancer Research (AACR) International Conference on Frontiers in Cancer Prevention Research: Abstract PR-02. Presented October 23, 2011.

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