At least 14 Olympic sports federations overseas and in the United States, including USA Gymnastics and USA Track & Field, have recently signed sponsorship deals with the makers of dietary supplements, putting their teams’ prestige and glamour behind powders and pills that promise to keep athletes in peak form — but that in many cases have not been validated by clinical trials.

Several hundred Rio-bound athletes from around the world have also endorsed shakes, drinks, and vitamins that claim, with little scientific backing, to provide a nutritional or energy boost, or to ward off common problems like muscle cramps.

Health experts who have studied the supplement industry worry that the tie-ins with popular Olympic teams and athletes will spur more consumers — including teenagers — to give such products a try.

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The supplement industry in the United States is only lightly regulated; manufacturers don’t have to run clinical trials, consult federal regulators, or produce data showing their products are safe or effective before shipping them to store shelves. The products have been linked to at least 23,000 emergency room visits annually in the United States, for reactions including heart palpitations and chest pain, a study found last fall.

And there may be special risks for Olympians: The US Anti-Doping Agency last fall warned about the dangers of athletes using supplements. Recent research has found that 11 supplement brands contain an amphetamine banned by the World Anti-Doping Agency, while 14 brands contain a banned heart stimulant.

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Some of the federations that oversee Olympic sports frown on such products. But others have traded sponsorship rights for free pills, powders, or drinks for their athletes — and in some cases, they’ve also received payments from supplement makers. (The terms of such deals are private, and none of the participants would provide financial details to STAT.)

Among the most popular industry partners: Herbalife, which just last month agreed to pay $200 million to settle federal allegations of deceptive sales practices.

The Olympic committees of Costa Rica, Italy, Vietnam, Israel, and India have all signed new deals with Herbalife, which sells a variety of fitness supplements, including a powder marketed with this slogan: “Compelling performance requires preparation.”

‘Massive market of products that don’t work’

Sports executives involved in these endorsement deals said they partnered only with supplement makers that they or another sports federation have rigorously vetted and that they trust to produce safe and effective products.

But experts say the flurry of sponsorship deals is an alarming sign of the growing clout — and perceived legitimacy — of the supplement industry.

Even supplements unlikely to contain banned or dangerous substances are part of “a massive market of products that simply don’t work, but can be dressed up and advertised to pretend that they do, so that consumers will buy them,” said Dr. Pieter Cohen, a Harvard Medical School professor and internist at Cambridge Health Alliance who studies supplement safety.

“I suspect that this type of Olympiad sponsorship of supplements will lead many young people to try new sports supplements,” he said. “If they don’t want to buy these expensive ones that are being advertised by the athletes, they might end up purchasing one sitting next to it in the store that’s spiked with drugs.”

At the Olympic level, concern about tainted supplements has been swirling in advance of this year’s Games, which begin on Friday.

After a Rio-bound wrestler from Australia failed a doping test, his coach recently suggested that supplements may have been to blame. Another would-be Olympic wrestler, from India, last week claimed his supplements had been spiked after he tested positive for a banned steroid.

Perhaps the highest profile case in recent years involved American swimmer Jessica Hardy, who had been poised for success in Beijing in 2008 until she tested positive for a banned drug used to treat asthma and build muscle.

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Disqualified from the Olympics and temporarily banned from the sport, Hardy blamed a drink mix she had taken; it was made by a supplement company with which she had an endorsement deal. (The company, AdvoCare, disputed those claims, and both sides sued each other before reaching an out-of-court settlement in 2012.)

Vetting supplements for safety

The US Olympic Committee has tried to ensure that the supplements American teams endorse are clean.

The committee, for instance, spent several years vetting Thorne Research, including inspecting the company’s Idaho-based manufacturing facility, before giving the thumbs up this past spring.

Thorne has since hammered out eight separate deals with US national sports governing bodies, which organize sports from youth athletics all the way up to the Olympic team. Under the deals, US Olympians and other elite athletes will be supplied with a line of Thorne’s amino acid powders and vitamins marketed with the slogan “NO limits on your potential.”

“We were just convinced that this is a very safe and effective option for athletes to enhance their health and their nutrition and their performance, ultimately,” said Chuck Menke, chief marketing officer for USA Triathlon, one of the US national governing bodies that forged a partnership with Thorne.

Thorne has tested some of its products in clinical trials, like a recent study of 40 patients, which found that people who took a supplement for several months had lower cholesterol and blood pressure, among other outcomes, than those who took a placebo. (The study did not look at athletic performance.) Thorne also has a partnership with Mayo Clinic to run randomized studies of its supplements, but none of those results are in yet.

As for Herbalife, its deal with Italy’s Olympic committee gives athletes access to a line of citrus-flavored sports supplements that promise to “help you train, recover, and perform like never before.”

Products designed to be taken after workouts promise to “rebuild muscle” and “reduce inflammation” — although Herbalife’s own website acknowledges that just two of its many products have been proven effective in clinical trials. (One, a protein shake, was shown to cause weight loss. The other, a citrus-flavored powder marketed as a way to keep blood vessels toned, boosted performance of “avid cyclists” over age 50.)

A big selling point for both sets of sponsorships, executives at Herbalife and Thorne said, was that their supplements were certified by an independent testing agency called NSF International, which tests for banned substances. Experts agree it’s one of a few gold-standard safety checks for supplements — but they stress that the testing in no way validates manufacturers’ claims that the supplements improve athletic performance.

Have you heard about our newest partners? You're going to want to read this one.. Link in bio ? A photo posted by Thorne Research (@thorneresearch) on Jun 13, 2016 at 7:44am PDT

Very little evidence that supplements work

Indeed, there is no clear evidence that consumers derive meaningful health benefits from taking supplements, other than for a few very specific goals, like reducing diarrhea from antibiotics and relieving symptoms of irritable bowel syndrome.

Elite athletes, of course, live different lifestyles and have different nutritional needs than most people. For some athletes, supplements can be an efficient way to get certain nutrients, according to a position paper published this year by the American College of Sports Medicine and two dietitian groups. And certain amino acids have been shown to modestly increase endurance.

But the paper also made clear that evidence of efficacy is limited and hobbled by small sample sizes and design problems. And some of these supplements cause side effects like weight gain and stomach pain that could impede performance.

If supplements could indeed deliver on their tacit marketing promises in a meaningful way, experts said, it would raise thorny questions about whether they ought to be banned.

“If it actually enhances performance — makes you stronger, increases your power, increases your speed, increases your endurance — when does that become a performance-enhancing supplement?” said Tim Caulfield, a lawyer and health policy professor at the University of Alberta.

‘Not the right fit’

Not all American national governing bodies are embracing supplements.

USA Swimming, for example, doesn’t have any supplement sponsors and “does not support nor endorse the use of supplements, particularly for athletes under 18 years old,” said Scott Leightman, a spokesman for the federation.

And USA Track & Field is so uneasy about supplements that it’s characterizing its new five-year deal with a company that sells mostly supplements as nothing of the sort. The deal with Florida-based Garden of Life centers on specific protein bars, protein powders, protein shakes, and probiotics.

“We wanted to focus on what we had confidence in and what we could best control and help our athletes with,” said Jill Geer, a spokesperson for USA Track & Field, noting that athletes sometimes have trouble finding a reliable source of protein on the road.

Supplements is “not a category that we would have a sponsor in. It’s just not the right fit,” Geer said.

But Garden of Life doesn’t make such a fine distinction. The company considers all its products to be supplements, with the exception of meal replacements, according to spokeswoman Rhonda Price.

Just in time for Rio Olympic marathon! I’m excited to announce my partnership with @Team_HOTSHOT, formally codenamed #ITSTHENERVE! I’ve been preventing and treating muscle cramps with this scientifically proven sports shot. Learn more and order now! www.TeamHOTSHOT.com! #TeamHOTSHOT A photo posted by Shalane Flanagan (@shalaneflanagan) on Jun 2, 2016 at 7:05am PDT

Athletes tweet their allegiances

Supplement makers’ existing partnerships with individual Olympians are also helping them drum up sales in advance of the Rio Games.

Such deals are nothing new, but they seem to be becoming more common. Herbalife, for example, is sponsoring more than 80 Olympians and Paralympians, approximately double the number it had during the London Olympics in 2012. Usana Health Sciences said it has deals with more than 70 Rio-bound athletes; Zoi Company said it was also providing “certified nondoping supplements to top Olympians.”

Don’t expect to see supplement makers tout these relationships much during the Olympics. Strict rules restrict nearly all marketing during the Games to big players like Coca-Cola and McDonald’s, which have paid a premium to be official sponsors of the Games, or official teams.

But that hasn’t stopped some athletes and their supplement sponsors from touting their relationships on social media as the Olympics approach.

Several American runners bound for Rio, for example, are promoting Hotshot, a drink made from ginger, cinnamon, and pepper and marketed by Flex Pharma to relieve cramps. It’s been tested in a few small trials; the largest was a field study of 31 athletes that didn’t include a control group.

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“Athletes heading to Rio this summer,” the long-distance runner Shalane Flanagan wrote in a recent tweet, “I recommend giving @Team_HOTSHOT a try now!!” (Marina Hahn, president of the Hotshot brand, said that “the athletes we are working with, like Shalane Flanagan, have had issues with cramps in the past and the opportunity seemed like a perfect fit for both of us.”)

Even AdvoCare, the supplement company that swimmer Hardy blamed for her 2008 positive drug test, has endorsement deals with four Rio-bound athletes.

Take Jake Dalton, an American gymnast who has had an endorsement deal with AdvoCare since 2012. He receives free products and has been paid for participating in some marketing campaigns, the company said, but he’s not paid to endorse the products on social media.

But Dalton, who couldn’t be reached for comment, does it all the same.

Late last month, he tweeted out a photo of an AdvoCare limeade-flavored “vitamin & amino acid supplement,” with a “US” logo in the background. The caption: “Started packing and definitely won’t forget this!”