The growth in popularity of dietary supplements has come largely despite a lack of scientific evidence to back up claims that they work.
There are, however, growing questions about their risks.
The latest: A new study, published in the British Journal of Cancer on Monday, found evidence of a troubling connection between men who took muscle-building supplements and their risk of developing testicular cancer.
Genes and family history factor into the likelihood that someone will develop testicular cancer, but those factors alone don't explain why rates of this form of cancer have increased in the last few decades.
So researchers conducted a survey of almost 900 men from Massachusetts and Connecticut, asking them questions about their habits (supplement use, smoking, drinking, exercise and other factors) and family history of testicular cancer. Among the men surveyed, 356 had been diagnosed with testicular cancer and 513 were not diagnosed with the cancer.
Researchers found a clear relationship between the use of muscle-building supplements and the risk of developing cancer even after controlling for other factors like race and age. Participants in the control group and the group with cancer had similar levels of education, prevalence of smoking, alcohol drinking and height.
“The observed relationship was strong,” Tongzhang Zheng, who led the study at Yale University, said in a statement. (Zheng is now a professor of epidemiology at Brown University School of Public Health.) “If you used at earlier age, you had a higher risk. If you used them longer, you had a higher risk. If you used multiple types, you had a higher risk.”
In this case, "use" means taking a supplement at least once a week for at least four consecutive weeks.
Duffy MacKay, senior vice president of scientific and regulatory affairs for the Council for Responsible Nutrition, a leading trade association for the supplement industry, said that the study's finding should be interpreted with caution:
There are several factors to consider here, including the fact that andro is not a legal dietary supplement, it is an illegal drug. This observational study purports to find an association, but not a causal relationship, between the ingredients identified and increased cancer risk, and while that’s more likely with andro or illegal steroid use by study participants, there is no scientific evidence to support the idea that protein or creatine are a risk for testicular cancer, and these products should not have been lumped in together. We should also take into account the habits of body builders as it is possible that they may be either knowingly or unknowingly taking other products, such as illegal steroids, that could be dangerous. We urge consumers taking supplements in this category to carefully research the companies they are buying products from and to be aware that if they are using products labeled supplements that create extreme muscle-building results, that they may be using a product that is illegally spiked with an illegal, non-supplement ingredient. If the claims or results are too good to be true, be cautious.
Overall, men who used muscle-building supplements increased their risk of developing testicular cancer by 65 percent. Men who used more than one type of muscle building supplement had a 177 percent increase in risk. Men who used the supplements for three years or more had a 156 percent increase in risk. And men who started using supplements at the age of 25 or younger had a 121 percent increase in risk.
“Considering the magnitude of the association and the observed dose-response trends, muscle-building supplements use may be an important and modifiable exposure that could have important scientific and clinical importance for preventing testicular germ cell cancer development if this association is confirmed by future studies,” the authors note in the study.
Critically, the study doesn't specifically name supplements that were used by the study's participants but they included 30 different types of powders and pills. Among the major ingredients were creatine, protein, and androstenedione. And the results suggest a relationship between supplement use and cancer, but it don't prove that the supplements cause cancer.
Additional clinical experiments will need to be conducted in order to confirm the results.
But recent research has raised questions about the potential harm posed by dietary supplements on the lightly regulated market, whose sales reached $13 billion in 2013. Previous research has linked ingredients in performance-enhancing supplements to a substance that caused testicular damage in rats. Another has shown a high prevalence of performance-enhancing supplement use among people diagnosed with testicular cancer.
And other studies have raised questions about unknown, unlabeled and untested ingredients that find their way into these products. So-called natural ingredients are particularly fraught, the study's authors note, because they can act as "artificial hormones" that may play a role in the development of cancer.
A recent study found that a synthetic, amphetamine-like chemical was found in weight-loss supplements advertised as a "natural ingredient." Two years earlier, the Food and Drug Administration found that the chemical, which has never been tested for safety in humans, could be found in supplements that were sold freely on store shelves.
The problem of mystery ingredients being sold as "natural" has prompted some lawmakers to call for changes in how the FDA regulates nutritional supplements, which is different from the way it regulates food or pharmaceutical drugs.
In the meantime, the new study's results suggest that further research could potentially provide much-needed insights into what causes this type of cancer.
“Our study found that supplement use was related to a higher risk of developing testicular cancer," Russ Hauser, professor of environmental health science at Harvard T.H. Chan School of Public Health and a co-author of the study, said in a statement. "These results are important because there are few identified modifiable risk factors for testicular cancer."
[This post has been updated.]