Designer anabolic steroids, also known as androgens, pose a potential health risk and, for athletes, may lead to a positive doping test and a ban from competing in a sport.
The lead author of a new trans-Tasman study, Prof Alison Heather of Otago University's Department of Physiology, said athletes use supplements hoping they will make them more competitive and healthy.
"Sports supplements, as well as other forms of nutritional supplements, are very popular with athletes, at the professional, amateur and recreational levels," Prof Heather said.
In some sports, such as body-building, all athletes used such supplements.
"Some members of the general population also take them in the hope they might improve body image or vitality."
The sports supplement market was not well regulated and manufacturers could market their products with unsubstantiated claims, she said.
"Unfortunately, sports supplements are usually marketed without rigorous scientific, clinical safety and efficacy data."
There was no legal requirement for proof-of-benefit claims.
"Supplements can contain ingredients that may have useful properties," she said.
"However, due to poor manufacturing practices or adulteration, sports supplements can contain compounds that are banned for use in sports, but that are not included on the label as an ingredient."
European studies had shown up to 16% of sports supplements contain a banned substance.
Heather's research focused on developing cell bioassays that can detect any bioactive androgen that is able to switch on an androgen response.
These bioassays provided a generic detection for any androgen, regardless of chemical structure.
Using such an assay, members of her laboratory team screened 116 sports supplements bought over-the-counter in Australia, many of which were also marketed in New Zealand.
One of the products that tested positive is actually made in, and distributed from, New Zealand.
But none of the products were named by the researchers, which is conventional for such studies.
"We simply went into supplement retailers, or bought online, supplements that were being marketed for athletes as 'performance-enhancing' products.
"We specifically chose supplements that did not state on their labels that they contained an androgen."
They found that 5.4% of the supplements tested positive for an androgen.
This meant there was more than one in 20 contained anabolic steroids that the users would be unaware of if they were relying on information on their labels.
"The supplements we tested included protein powders, pre-workout formulations, fat metabolisers, vitamins and herbal extracts."
"Sports supplements contaminated or adulterated with androgens are a major concern because, for the unsuspecting consumer, they pose a potential health risk, while for an athlete it could result in a positive doping test."
In 2015, a report showed 95% of elite New Zealand athletes consumed sports supplements, which followed a report a year earlier that 70% of New Zealand First XV players regularly took supplements.
"Our research shows that there is a real risk for health and doping violations that athletes must consider when taking sports supplements, even those sold over the counter."
The study, carried out with researchers from the University of Technology Sydney, has been published in the International Journal of Sport Nutrition and Exercise Metabolism.
