Drinks, trainers and protein shakes which purport to be performance enhancing have a "striking lack of evidence" to back up their claims, a study suggests.
Researchers said that it was "virtually impossible" for the public to make informed choice about the benefits and harms of advertised sports products.
The study's authors examined the science behind the marketing and found that more than half of 104 performance enhancing products made claims which were not substantiated by any evidence.
The researchers from the Oxford Centre for Evidence Based Medicine judged that of those that do, half of the evidence is not suitable for critical appraisal.
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The vast majority of the critically appraised studies (84%) were judged to be at high risk of bias, according to the research which is published in the online journal BMJ Open.
The absence of high-quality evidence is "worrying", the authors conclude.
"There is a striking lack of evidence to support the vast majority of sports-related products that make claims related to enhanced performance or recovery, including drinks, supplements and footwear," they write.
"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.
"No systematic reviews were found, and overall, the evidence base was judged to be at high risk of bias."
Deborah Cohen, BMJ investigations editor, said: "These misleading messages filter down to everyday health advice by company-sponsored scientists who advise high-profile sports bodies.
"For instance, fear about the dangers of dehydration has become gospel and now influences what and how we drink when we exercise. It's a triumph of marketing over science."
A BMJ and BBC Panorama investigation, which examines the research, also looks at claims that found that while sports drinks may be helpful for elite athletes, the drinks companies rarely study ordinary gym goers.
The Panorama investigation also suggests that the European Food Safety Authority should do more to regulate these marketing claims.
Dr Matthew Thompson, senior clinical scientist at Oxford University's department of primary health care sciences, told the programme he would like to see "a more scientific and rigorous approach" to assessing the basis of food claims in Europe.
Professor John Brewer, head of sport science at the University of Bedfordshire, said: "There is a wealth of high-quality scientific evidence supporting the sensible use of sports drinks and nutritional supplements in appropriate environments.
"Whilst those who exercise as a means of keeping fit and losing weight may find that water is a useful and simple means of staying hydrated and do not therefore require sports drinks, those who compete intensively for long durations have been shown by numerous studies to improve their performance through the sensible consumption of sports drinks.
"It is essential that athletes who use sports supplements chose reputable manufacturers who can justify their claims with scientific evidence, and have their products screened to ensure they do not contain WADA (World Anti-Doping Agency) banned substances."
A spokesman for the British Soft Drinks Association said: "It is well established that one of the factors that can help sporting performance is drinking the right amount of the right kind of drink, and there is a range of sports drinks available which are designed to meet the varying requirements of different kinds of exercise.
"Their formulation is based on the latest scientific evidence and medical knowledge, and sports drinks carry nutritional information on the label, including the calorie content.
"By helping people participating in sport to perform better and to recover more quickly, sports drinks can encourage people to exercise more."
Panorama - The Truth About Sports Products airs tonight at 8pm on BBC One.
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