QUESTIONS ABOUT potential bone loss caused by a ketogenic diet (KD) arose with a recent Australian study of athletes, which at first glance looked half-baked: a small sample of 30 competitive athletes; short duration of three weeks; and the self-selection by the athletes of which diet to follow.
But when further digging led to a tangle of earlier KD research that documented bone weakness and loss—and a few small studies that did not—the question came up: Why with the popularity of ketogenic diets starting in the early 2010s have there been no better, more thorough investigations into possible risks?
Ketogenic diets, in which up to 80% of food consumed daily and 90% of calories come from fats, have since the 1930s been the best treatment for those with epilepsy—before the advent of effective drugs and, afterwards, in about 30% of cases where medications fail to reduce seizures.
Renewed interest in the keto diet began in the early 1990s, leading to the creation of the Charlie Foundation for Ketogenic Therapies, both to promote the diet for epilepsy and to spur research into its effectiveness for neurological conditions, including Alzheimer’s disease, sleep disorders, headache and pain.
About 10 years ago, people began turning to the keto diet for weight loss based on “the carbohydrate-insulin hypothesis.” According to the theory, dietary carbohydrates raise insulin levels, leading to excess accumulation of fat and contributing to rising rates of obesity.
For most carbohydrates—whole grains, fruits and legumes—investigators have since raised questions about the hypothesis, but excess consumption of highly processed and refined carbs may contribute to obesity via other mechanisms.
Side effects of the keto diet, familiar since the early 20th century for children with epilepsy, include kidney stones and constipation— and effects on bone growth and health. Children on the diet longer than six years tend to remain in the fifth percentile of the growth curve and have lower bone mineral density with higher risk of fractures.
The International Society of Sports Nutrition’s position paper on the keto diet describes the “ergolytic,” or performance-impairing, effects on workouts: increased perceived effort during physical activity (related to excess oxygen required for the digestion of fat), as well as fatigue and mood disturbance.
And of two groups on an eight-week training course at CrossFit, those randomized to follow the keto diet lost about 3.5 ounces of muscle while the other group eating their preferred diet had average muscle gain of three pounds.
In the Australian study, 30 racewalkers training for international competitions had the choice of either a high-carb or ketogenic diet. After three and a half weeks, markers of bone breakdown were higher and those indicating bone formation lower among athletes on the keto diet than at the start of the study. The same markers for athletes consuming high-carb diets remained unchanged.
“Bone is an active tissue, constantly breaking down slightly and remodeling itself in response to the demands we place on it,” explains New York Times Well columnist Gretchen Reynolds, who describes the Australian study as “thought-provoking.”
Why the keto diet altered the Australian athletes’ bone metabolism remains unclear, as does how exactly it decreases seizures in those with epilepsy. For athletes, the Australian researchers hypothesize that low carbohydrate availability may affect circulating hormones—notably calcium-regulating hormones like estrogen —that inhibit bone breakdown and stimulate its formation.
A 2019 study of rats on a ketogenic diet for 12 weeks found “significant bone loss and reduced biomechanical function” in some bones and a lesser impact on others.
The caveat for all conclusions of existing KD research: Neither children on the keto diet for six years, nor competitive athletes consuming high fats for three weeks, nor rats may be valid yardsticks for typical healthy individuals who go on the keto diet to lose weight. For them, the greatest challenge may arise in maintaining weight loss after discontinuing the KD’s drastic ratios of fat.
“The greatest risk [of the ketogenic diet] may be…the opportunity cost of not eating high-fiber, unrefined carbohydrates,” according to an essay in the Journal of the American Medical Association Internal Medicine. “Whole grains, fruits and legumes are some of the most health-promoting foods on the planet.”
Every Tuesday, well-being editor Mary Carpenter delivers health news you can use.