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As the pace quickened during a mountain stage of the 2013 Tour de France, a jolt of excitement rippled through the peloton: one of the leaders had fumbled the feed bag containing his next drink. “I’d never, ever seen so much chatter on the radio,” Greg Henderson, a veteran bike racer from New Zealand, later told an interviewer. “It was like full panic. And I was thinking, Mate, it’s a drink bottle. Go back and get another one.”

Eventually, a team car stopped to collect the bag from the side of the road where it had fallen, and another rider shuttled it back up to the cyclist. “I was gobsmacked,” said Henderson. “I was just like, What can be so important in this drink bottle?”

The answer, he now suspects, was ketones, the much hyped “fourth macronutrient” that offers a sustained—and legal—source of energy for muscles beyond the traditional carb­ohydrates, fats, and proteins. For several years, whispers of an astronomically ex­pensive, ketone-based superdrink have percolated through the elite sports world, par­tic­ularly among cyclists. Now, courtesy of San Francisco biohacking startup (pro­nounced “human”), that elixir is on the market. The debate, however, has shifted to a knotty question: Does it work?

The ������԰�’s origins date back to a 2003 request for proposals from the Defense Advanced Research Projects Agency, the U.S. military’s idea factory, for ways to keep soldiers physically and cognitively sharp through multi-day battles. A team of scientists, including Richard Veech of the National Institutes of Health and Kieran Clarke of Oxford University, floated a novel concept. You already have a form of emergency fuel called ketone bodies, produced in your liver from fat, that kicks in when you’re approaching starvation. These ketones provide energy for the brain and muscles and can also alter your metabolism to draw more energy from fat stores while conserving precious carbohydrates. Perhaps soldiers could get a similar result by drinking ketones, the scientists hypothesized. The military gave them $10 million to find out.

While Veech and Clarke were plodding through the painstaking process of developing a drink, proving its safety, and getting the FDA to sign off, the once obscure topic of ketones went mainstream. Following a low-carb, high-fat diet, it turned out, could dramatically elevate ketone levels even in the absence of starvation. Ketogenic diets became popular among ultra-endurance athletes, and companies began marketing a variety of ketone-based concoctions that promised all the benefits of going low-carb without having to give up pasta and beer.

To put it politely, these drinks had varying levels of credibility. “Raspberry ketones are ludicrous,” Clarke says, recalling a fad sparked by Dr. Oz in 2012. “They’re not naturally found in the body, and you can’t metabolize them, so they’re a total waste of money.” Ketone-salt drinks offered a way of getting the real thing, but with a massive, unhealthy dose of sodium in each bottle—and no evidence of athletic enhancement.

Then, in 2016, Clarke and her colleagues published a in the journal Cell Metabolism. By combining ketones with an alcohol compound, they managed to create a ketone ester that people could consume like an ordinary drink. Once inside the body, the ketones provided fuel for muscles, allowing limited carbohydrate stores to last longer. In a 30-minute time trial after an hour of hard riding at a steady pace, eight elite cyclists improved their performance by an average of 2 pe­rcent over a placebo ride, completing roughly an extra quarter-mile. The problem? The drink tasted like gasoline and cost $100 per serving to produce, down from an initial $25,000.

In November, Hvmn rolled out Hvmn Ketone, a commercial version of Clarke’s ketone-ester drink. (It’s available for preorder, with an expected ship date of early 2018.) Manufacturing improvements have made it somewhat cheaper at three bottles for $99, and it’s still not for the faint of stomach: “metallic almond milk” is how one tester characterized an early version, and that’s among the nicer descriptions. According to Geoff Woo, the company’s CEO, Hvmn Ketone is already being used in Grand Tour cycling, in the NFL, and at the Ironman World Championship—with the identities of the athletes and teams who’ve adopted it, perhaps conveniently, protected by nondisclosure agreements.

Elite endurance athletes are paying close attention. A few weeks before Hvmn unveiled its drink, researchers in Australia published the results of a study in which 11 cyclists from the Orica-BikeExchange team rode a 19.25-mile time trial of a computer-simulated version of the 2017 World Championships course, with and without having consumed a rival ketone-ester drink, developed by University of South Florida researcher Dominic D’Agostino. Instead of boosting endurance, the ketone drinkers slowed down about 2 percent—perhaps because every single one of them experienced gastrointestinal side effects that ranged from mild nausea to prolonged vomiting and dizziness.

Another study, conducted by researchers at the University of British Columbia–­Okanagan, found that ­boosting ­ketone levels with a ketone-salt drink slowed cycling performance by 7 percent in an 11-minute time trial. The problem, lead sci­en­tist Jonathan Little explains, is that ele­vated ketone levels seem to inhibit access to the quick-burning carbohydrates needed for shorter, high-octane efforts. Clarke concurs, pegging the threshold of usefulness at sustained efforts lasting at least 20 minutes. “As soon as you’re up to 75 percent of your maximum workload, I wouldn’t even go near a ketone,” she says.

Woo notes the criticisms and caveats but says it’s wrong to think that all ketone drinks are created equal—just because one ketone ester makes people vomit doesn’t mean they all do. Clarke and her colleagues, meanwhile, continue to churn out studies showing the ������԰�’s potential—faster post-workout replenishment of carb stores in depleted muscles, reduced appetite hormones, and even (in maze-running rats, at least) better cognitive performance.

So should you shell out for a boost? That’s a tough one, because the true size and nature of the edge—or lack thereof—that ketones offer in real-world settings will likely take years to sort out. After all, the margins are small. “For a healthy human adult,” Woo admits, “it’s hard to enhance performance.” The whispers in the peloton will likely continue—but at least if another Tour rider drops his bottle of superfuel, someone else will probably be able to lend him a spare. 

The Rise of Superdrinks

Hvmn isn’t the only sports-drink company playing the science card, but giving the human body a reliable boost remains a challenge

Maurten: This Swedish burst onto the scene in 2017 with claims that its hydrogel-encapsulated carbohydrate drink could deliver a concentrated dose of energy to endurance athletes without upsetting their stomachs. It has won over top marathoners like Eliud Kipchoge—but the company has yet to publish research backing up its claims.

Generation Ucan: This sports secret is its SuperStarch, a corn-based carbohydrate that’s supposed to release glucose slowly and steadily into the bloodstream instead of in a rapid spike. Runner Meb Keflezighi swears by it, but whether the lab results translate to faster race times remains contested.

Gatorade Gx: In 2016, Gatorade this “sports fuel customization platform,” which includes a digital sweat patch that communicates wirelessly with the cap of your water bottle to enable real-time customized hydration. The launch has been pushed back several times, and the science is—well, you get the picture.