The ketogenic diet - or 'keto' for short - is a high fat, low carb, restrictive eating plan that promises to help you lose weight.
It has also been deemed effective in treating epilepsy, but has been widely criticised by nutritionists for being restrictive and extreme.
Now a study published in Frontiers in Endocrinology, warns of another concern - keto may have an unexpected side effect of weakening athletes' bones during intense training.
Athletes on keto diet showed greater signs of bone breakdown
Researchers from the Australian Institute of Sport and Australian Catholic University looked at 30 elite race walkers (25 male and five female) over three and a half weeks of intense athletic training.
About half of the group were assigned to a low carb, high fat ketogenic diet, getting between 75 and 80 per cent of their daily calories from fat. The other half stuck to a high carb diet. Both groups consumed the same amount of calories, relative to their body weight, and both groups ate a moderate amount of protein.
The team found that the athletes on the keto diet showed greater signs of bone breakdown than they had at the start of the study. Athletes on a high carb diet, however, showed no significant difference, according to the test results.
After reintroducing carbs into their diet, the keto athletes saw some improvement in their bone health, but they weren't back to full strength, the researchers found.
These results suggest that the ketogenic diet somehow sped up the breakdown of athletes' bones during intense exercise, and inhibited recovery, although it's not exactly clear how.
Criticisms of keto diet
The new findings come alongside other criticisms of the diet, with experts suggesting that keto is only safe and effective over a short period of time.
Some experts have even warned that a longterm keto diet can damage the heart muscle.
"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," said Louise Burke, lead author of the study and head of sports nutrition at the Australian Institute of Sport in Canberra
It is not clear from this study how the keto diet will affect bone health over a longer period of time and whether athletes may be able to adapt the diet to reduce symptoms.