Despite reported widespread use of dietary supplements by cancer patients, few empirical data with regard to their safety or efficacy exist. Because of concerns that antioxidants could reduce the cytotoxicity of chemotherapy, a prospective study was carried out to evaluate associations between supplement use and breast cancer outcomes.
Patients with breast cancer randomly assigned to an intergroup metronomic trial of cyclophosphamide, doxorubicin, and paclitaxel were queried on their use of supplements at registration and during treatment (n =1,134). Cancer recurrence and survival were indexed at 6 months after enrollment.
There were indications that use of any antioxidant supplement (vitamins A, C, and E; carotenoids; coenzyme Q10) both before and during treatment was associated with an increased hazard of recurrence and, to a lesser extent, death. Relationships with individual antioxidants were weaker perhaps because of small numbers. For non-antioxidants, vitamin B12 use both before and during chemotherapy was significantly associated with poorer disease-free survival and overall survival. Use of iron during chemotherapy was significantly associated with recurrence as was use both before and during treatment. Results were similar for overall survival. Multivitamin use was not associated with survival outcomes.
The authors concluded that associations between survival outcomes and use of antioxidant and other dietary supplements both before and during chemotherapy are consistent with recommendations for caution among patients when considering the use of supplements, other than a multivitamin, during chemotherapy.
These data are interesting but, for a range of reasons, not compelling. There might have been several important confounding factors distorting the findings. Even though clinical and life-style variables were statistically adjusted for in this study, it might still be possible that supplement users and non-users were not comparable in impotant prognostic variables. Simply put, sicker patients might be more likely to use supplements and would then have worse outcomes not because of the supplements but their disease severity.
Moreover, it seems important to note that other research showed the opposite effects. For instance, a study prospectively examined the associations between antioxidant use after breast cancer (BC) diagnosis and BC outcomes in 2264 women. The cohort included women who were diagnosed with early stage, primary BC from 1997 to 2000 who enrolled, on average, 2 years postdiagnosis. Baseline data were collected on antioxidant supplement use since diagnosis and other factors. BC recurrence and mortality were ascertained, and hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated.
Antioxidant supplement use after diagnosis was reported by 81% of women. Among antioxidant users, frequent use of vitamin C and vitamin E was associated with a decreased risk of BC recurrence. Vitamin E use was associated with a decreased risk of all-cause mortality. Conversely, frequent use of combination carotenoids was associated with increased risk of death from BC and all-cause mortality.
The authors concluded that frequent use of vitamin C and vitamin E in the period after BC diagnosis was associated with a decreased likelihood of recurrence, whereas frequent use of combination carotenoids was associated with increased mortality. The effects of antioxidant supplement use after diagnosis likely differ by type of antioxidant.
Yet another study provided limited support for the hypothesis that antioxidant supplements may reduce the risk of breast cancer recurrence or breast cancer-related mortality.
What is needed, it seems, is a systematic review of all these contradicting studies. A 2009 review is available of the associations between antioxidant supplement use during breast cancer treatment and patient outcomes.
Inclusion criteria were: two or more subjects; clinical trial or observational study design; use of antioxidant supplements (vitamin C, vitamin E, antioxidant combinations, multivitamins, glutamine, glutathione, melatonin, or soy isoflavones) during chemotherapy, radiation therapy, and/or hormonal therapy for breast cancer as exposures; treatment toxicities, tumor response, recurrence, or survival as outcomes.
A total of 22 articles met the criteria. Their findings did not support any conclusions regarding the effects of individual antioxidant supplements during conventional breast cancer treatment on toxicities, tumor response, recurrence, or survival. A few studies suggested that antioxidant supplements might decrease side effects associated with treatment, including vitamin E for hot flashes due to hormonal therapy and glutamine for oral mucositis during chemotherapy. Underpowered trials suggest that melatonin may enhance tumor response during treatment.
The authors concluded that the evidence is currently insufficient to inform clinician and patient guidelines on the use of antioxidant supplements during breast cancer treatment. Thus, well designed clinical trials and observational studies are needed to determine the short- and long-term effects of such agents.
Antioxidants seem to have evolved as parts of elaborate networks in which each substance plays slightly different roles. This means that each antioxidant has a different spectrum of actions. And this means that it is probably not very constructive to lump them all together and excect to see uniform effects. What we would need to create more clarity is a series of RCTs on single antioxidants. But who is going to fund them? We might be waiting a long time for more clarity. Meanwhile, consuming a healthy and well-balanced diet might be the best advice for cancer patients and everyone else.