Most people consider dietary supplements as likely to be beneficial but at the very least ‘there not going to make you worse.’ That sentiment is wrong. A review recently published has shown that some dietary supplements may increase the risk of cancer (Journal of the National Cancer Institute 2012; 104: 732-39).
Nearly half of the US adult population uses one or more dietary supplements but there is very little evidence that these supplements reduce cancer risk; in fact, the contrary is true. Based on numerous studies, the authors make extensive comments regarding the studies of antioxidants, folic acid, and vitamin D/calcium which are summarized below.
While observational data has suggested benefits from fruit and vegetable consumption, data on antioxidant supplement consumption has not shown a beneficial effect. The review highlights a number of studies with regard to β-carotene, vitamin A, vitamin C, and vitamin E/α-tocopherol. Specifically, vitamin C and E do not protect against total cancer incidence. α-tocopherol and β-carotene do not protect against cancer or cancer mortality.
- The Selenium and Vitamin E Cancer Prevention Trial (SELECT) followed 35,533 men at average risk for prostate cancer for approximately 5.5 years. This study was halted due to lack of benefit. In addition, the extended followup reported that α-tocopherol significantly increased the risk of prostate cancer by 17%.
- The β-carotene and Retinol Efficacy Trial (CARET) had a 39% increase in lung cancer incidence compared to the placebo arm.
- In two of three large studies of β-carotene, the intervention increased the risk of all-cause mortality.
- The Nutritional Prevention of Cancer (NPC) extended followup found that selenium supplementation statistically increased the risk of squamous cell skin cancer by 25% and non-melanoma skin cancer by 17%.
Folic acid which is a synthetic oxidized form of folate is commonly used in fortification and supplements. Recent meta-analysis of randomized controlled trials (RCTs) has found no effect of folic acid supplementation on the risk of colorectal adenomas over a 3-year treatment period. In addition, one study demonstrated an increased risk of advanced colorectal adenomas (relative risk = 1.67). Also, in observational studies, higher intake of folic acid has been linked with increased prostate cancer risk.
Vitamin D and Calcium
The Institute of Medicine published recommendations with regard to Vitamin D and calcium intake in 2011 and found that “there is not enough evidence to state that there is a causal association between low vitamin D intake and increased cancer risk.” The authors summarize several conflicting results with regard to breast, colorectal, and prostate cancers. In addition, a recent meta-analysis of RCTs indicated that calcium supplementation was associated with a statistically-increased myocardial infarction risk.
Why are supplements so widespread if they are not beneficial and potentially dangerous?
- The authors also summarize regulatory efforts. In 1990, due to unsubstantiated health claims by food manufacturers, Congress passed the Nutrition Labeling Education Act (NLEA).
- To limit FDA authority over supplements, at the behest of nutritional supplement manufacturers, in 1994 Congress passed the Dietary Supplement Health and Education Act. This classified supplements as food and limited the role for the FDA.
- In 2006, in reaction to deaths from ephedra, Congress passed the Dietary Supplement and Non-prescription Drug Consumer Protection Act. This allows the FDA to collect adverse reports on supplements but did not give additional regulatory powers.
Conclusions from this review
- In populations with a high background of normal nutrient status, risk is accentuated if there can be harm at higher doses. For selenium (in the NPC study), apparent benefits have been confined to individuals with the lowest baseline blood selenium levels.
- It is not reasonable to assume that consumption of a single nutrient would exert a chemopreventive effect equally in all tissues. In addition, there are substantial variations in formulations and doses of supplements available.
- Efficacy and harm are typically tested over several years. Given the natural history of cancer, it may take decades to assess supplement impact.
- Multiple consensus recommendations have indicated that supplements do not prevent cancer and do not prevent chronic disease (Table 1 in reference). The most recent was from the American Cancer Society in 2012. “Present knowledge indicates that dietary supplements do not lower cancer risk.”
- Despite the evidence, the authors note that believers in supplements are unlikely to accept ‘mainstream’ science. Some may think that unconventional treatments are ignored by science for monetary reasons. Some may think that these products are regulated and would not be offered if they were not beneficial.