A newly published meta-analysis of 22 randomized controlled trials has found no cancer preventive effect of antioxidant supplements. Neither primary cancer nor distance recurrence was prevented by such supplementation. Additional subgroup analyses found no preventive effect for individual antioxidants or for particular types of cancer. However, the use of antioxidant supplements was found to increase the risk of bladder cancer in a subgroup meta-analysis of four trials that evaluated this risk. The authors comment that benefits of antioxidant supplements on human health, particularly in relation to cancer, should not be overemphasized because their use might be harmful for some cancers.

Study conclusions and limitations

While meta-analyses of multiple studies can produce interesting conclusions, they can also obscure valid findings. For example, in the case of antioxidant supplements (normally thought of as "vitamins"), studies of levels of supplementation near the recommended daily allowance are likely to produce different results than those focusing on much higher levels. We take the study results to mean that supplementation with multivitamins is probably safe (except during radiation and chemotherapy — see below) and while it may support overall good health to the extent that it addresses any deficiencies, such supplementation is not likely to prevent cancer. However, the interactions between antioxidant supplementation and cancer risk are more complicated and nuanced than that suggested by the overall study conclusions.

The case for antioxidant supplementation is based, in part, on the observation that cancer patients tend to have low blood levels of antioxidants compared with healthy individuals, indicating that the patients have exhausted the antioxidant defenses of the body and are vulnerable to oxidative damage that could further promote cancer. In fact, the available evidence suggests that, generally speaking, antioxidant supplementation is more likely to promote cancer than to reduce cancer risk. For example, in addition to the evidence regarding bladder cancer mentioned by the meta-analysis authors, several studies have reported associations between beta-carotene, retinol, and lutein supplementation and increased risk of lung cancer. Apparently, some types of cancer cells are able to use concentrated antioxidants to support growth or resist cell death.

Other studies find antioxidant supplements have varying effects on risk

One major Chinese study found that vitamin E and vitamin B supplementation was associated with lower risk of breast cancer, but only among women with low dietary vitamin E and vitamin B intakes. The same study found an increase in breast cancer risk among women who frequently used a vitamin B supplement and who also had a high dietary vitamin B intake. Supplemental vitamin C intake has been found to be weakly associated with increased breast cancer risk. Multivitamin use appears to confer some protection against breast cancer among women who regularly consume alcohol. Vitamin D deficiency is prevalent in the U.S. population and supplementation with vitamin D can reduce the risk of breast cancer and breast cancer recurrence.

Avoid antioxidant supplements during breast cancer treatment

It is important for breast cancer patients to avoid taking supplements that will lessen the cytotoxic impact of radiation or chemotherapy on breast cancer cells. While various antioxidants found in fruits, vegetables and other foods have been shown to help protect against breast cancer development and metastasis, some of the same micronutrients might enable breast cancer cells to survive such treatment.