The studies mentioned by those sports drinks products with performance-enhancing claims did provide them had a high risk of bias in 84%. In addition, 41.9% of the studies had no randomization, allocation concealment was apparent in 6.8% of studies, and only 27% of the research included blinding of participants, investigators, or outcome assessors, Heneghan and co-authors wrote online in BMJ Open.
Researchers found 431 claims about the products online on the manufacturers' websites after reviewing 1,035 web pages related to those products. They recorded data on each product's category, number and type of performance claims attributed to it, references to claims made about it, and qualifiers related to claims (such as "should be used in conjunction with a healthy diet"). If a reference paper supporting a claim was not available, the company was contacted and asked for any supporting literature.
Literature, if available, was evaluated for quality through the Center for Evidence-Based Medicine Levels of Evidence and Cochrane method of risk of bias, as well as whether participants were professional athletes or not.
Of 431 advertising claims made, only 74 studies referenced by those claims were appropriate for critical appraisal.
There were 2,031 participants between the 74 studies, two of which made up a quarter of total participants (total n=505). Around half of total participants were classified as "regular people who exercise" (48.6%), another 39.2% were considered endurance or serious athletes, and 10.8% were considered professional athletes.
A total of three of the studies (4.1%) were judged to be of "high quality and at low risk of bias," and none of the studies included systematic review, Heneghan and colleagues wrote.
"Half of all websites for these products provided no evidence for their claims, and of those that do, half of the evidence is not suitable for critical appraisal," they added.
Most of the studies reported a surrogate, rather than direct, outcome of performance or recovery (83%, 95% CI 73% to 92%) and two studies repeated the study protocol intervention (2.7%, 95% CI 0% to 25%). Only 11% of studies reported study limitations (95% CI 0% to 33%).
In addition, small samples and laboratory environments had a negative effect of the validity of study findings, the investigators wrote.
The authors noted several limitations in their own review, including the possibility that the evaluated products were "at the worst end of the spectrum," subjective criteria to determine whether a product was billed as "performance-enhancing," and a lack of investigation into the heterogeneity of effects or publication bias.
Based on these results, current research by sports product manufacturers does not sufficiently inform potential customers of risks and benefits to using products sold with performance-enhancing claims, they concluded.
The authors had no conflicts of interest to declare and received no funding support.
Reference: Heneghan C, et al "The evidence underpinning sports performance products: a systematic assessment" BMJ Open 2012; DOI: 10.1136/bmjopen-2012-001702.