Inspired by that possibility, some athletes and nutritionists now tout diets that contain almost no carbohydrates and large amounts of fat. Over time, these diets lower the body’s stores of carbohydrates and require it to burn fat almost exclusively.
Such diets also result in an abundance of ketones, substances produced by the liver that the muscles and brain can use as fuel.
The problem is that it takes weeks or, more commonly, months for the body to adjust to a low-carbohydrate diet. In the interim, athletic performance suffers and most people feel sluggish and unwell, sometimes developing a condition known as “ketone flu” that resembles food poisoning.
So some athletes and their advisers have begun to wonder whether taking ketone supplements might allow athletes to gain the benefits of a high-fat diet without the long, difficult transition time. Various types of these supplements, in the form of powders or drinks, are chemically identical to the ketones made by the body. They are also widely available at many stores and, as of now, legal under international doping regulations.
Last year, a much-discussed study conducted by researchers at Oxford University found that a proprietary type of ketone supplement improved cycling performance in a group of trained riders.
But that study looked at relatively moderate exercise, not the kind of intense exertion required during competition.
So for the new study, which was published last month in Frontiers in Physiology, scientists at the Australian Institute of Sport and other institutions decided to create a more real-world test of ketone supplementation.
Conveniently, they had a ready-made group of volunteers in the 11 members of a men’s professional, world-class bicycling team that were about to begin their preseason training in Australia. The team’s members, who had ridden in the Tour de France and other international events, were keenly interested in the possible benefits of ketone supplements, according to Louise Burke, the head of sports nutrition at the Australian Institute of Sport and the study’s senior author.
The experiment the scientists concocted was simple. About an hour before a training session, they gave each rider a drink that contained either ketones or a placebo. The drinks looked and tasted much the same. The riders had a second dose of that same drink immediately before starting. (The drink they used is, so far, available only for research purposes and not for sale to the public.)
Then the men rode stationary bicycles programmed to recreate on a screen the 31-kilometer (about 19 miles) course of the 2017 road-cycling world championship. The riders virtually raced at the highest pace and power output that they could maintain.
Each rider rode after drinking the placebo and, on another day, after swallowing the ketones.
The results were unanimous. Every rider performed worse after drinking the ketone supplement. Their times were about 2 percent slower and their power output almost 4 percent less, declines that, in actual competition, would drop them from contention for medals.
Every rider also reported some degree of gastrointestinal upset after drinking the ketone supplement. One experienced such “prolonged vomiting and dizziness,” the scientists reported, that he could not start the time trial. Others had symptoms ranging from dry retching to relatively mild nausea.
Not surprisingly, the riders all guessed during which trial they had the ketones, based on their stomachs’ reactions. But most told the scientists that they thought they were riding faster during that trial, because the pedaling had felt so difficult. Told that their performance in fact had declined, almost all announced that they were no longer interested in using ketones for performance enhancement.
This study was very short-term, however. It is possible that the riders and their stomachs might adjust to the supplement over time and perhaps gain benefits, Dr. Burke says. The study also does not show whether stomach upset alone clobbered the riders’ performance or if other physiological processes were involved. The riders’ brains could have interpreted the sudden presence of ketones as indicating a metabolic problem, Dr. Burke says, and responded by sending messages to the muscles urging them to slow down.
The upshot is that far more research needs to be conducted before experts can make recommendations about ketone supplementation for athletes, Dr. Burke says. “Everyone wants simple answers,” she says, but the body’s mechanisms are too complex to allow for that, especially during sports.
In the meantime, there is one other practical caveat to bear in mind about the currently available supplements, she says. They almost all “taste very nasty.”