About half said that they were and subsequently began a strict low-carbohydrate, high-fat routine. The other men and women continued with a high-carbohydrate diet. Everyone's meals were matched in terms of how many calories, relative to body weight, they ate.
Before the diets kicked off, though, the researchers drew blood from the athletes before and after a workout, to establish their baseline bone health and other markers of their health and fitness. (The experiment was designed to look at many elements of the ketogenic diet in sports.)
Bone is an active tissue, constantly breaking down slightly and remodeling itself in response to the demands we place on it. Knowing this, the researchers checked for the levels of specific substances in the athletes' blood known to be associated with bone breakdown, rebuilding and overall metabolism.
Then the athletes embarked on three and a half weeks of intense training, while eating mostly fat or mostly carbs. Afterward, the researchers again drew their blood and rechecked the markers of bone health.
They found differences. The markers of bone breakdown were higher now among the athletes on the keto diet than at the start of the study, while those indicating bone formation and overall metabolism were lower. These same markers were generally unchanged in the high-carb athletes. The athletes on the ketogenic diet, in other words, showed signs of impaired bone health.
How their bones might have been affected by their eating is still unclear, says Louise Burke, the head of sports nutrition at the Australian Institute of Sport in Canberra and one of the lead authors of the new study.
“We believe that the keto diet may affect bone metabolism due to the downstream effects of low-carbohydrate availability on certain hormones, along with other factors,” she says. But more study, of course, is needed.