Staying on the keto diet long term could carry health risks

Maybe you’ve seen an influencer make French fries out of almond flour. Or a sandwich recipe that swaps bread for fried cheese. They’re called keto meals, and they’re largely shared for one reason: to help people lose weight.

In the ketogenic diet, fat is king, and carbs are public enemy number one. Going keto means restricting carbs to the bare minimum and replacing those lost calories with fat. It’s the antithesis of the low-fat diet craze of the 1990s. Losing fat on keto diets typically means eating fat — and lots of it. The idea may sound paradoxical. But without our typical go-to energy source (sugar), our bodies learn to rely on a different type of fuel. In keto dieters, the liver converts fat into molecules called ketone bodies, which the body can burn instead of sugar.

That can lead to weight loss, despite an unusually high intake of fat. Such results may explain why so many Americans have tried the keto diet on for size. “I think a lot of people look at a ketogenic diet and think, ‘I’ll lose weight, I’ll be healthier,’” says Molly Gallop, a physiologist at Earlham College in Richmond, Ind.

On the surface, they may be right. But staying on the diet long-term could carry some risks, a new study in mice suggests. Mice fed a ketogenic diet for up to about a year — decades in human time — experienced health problems including glucose intolerance and signs of liver and cardiovascular disease, Gallop and her colleagues report September 19 in Science Advances.

The work uncovers some potential hidden costs to going keto, says physiologist Amandine Chaix, at the University of Utah in Salt Lake City. “It’s a cautionary tale,” she says. People sticking to this high-fat plan need to be careful, she says, “because this is not a magical dietary approach.”

Before the keto diet was a fad, it was a medical treatment. About a century ago, doctors started using the diet to treat people with epilepsy, says Johns Hopkins neurologist Tanya McDonald. Symptoms of the seizure condition can improve after short stints on the diet. Even today, McDonald still uses ketogenic diets as a treatment option for epilepsy patients.

The classic keto diet specifies that roughly 90 percent of a person’s calories come from fat. That’s close to double the fat proportion of a typical American diet. A raft of studies in humans have shown that keto diets can prompt weight loss in overweight adults. But scientists don’t know much about the diet’s long-term effects in the general population, McDonald says. It can be difficult for people to adhere to such a restrictive diet.

That’s easier studied in mice, Chaix says. Her team put mice on one of four different diets for roughly eight months and followed their health. Body weight was lowest in animals on low-fat diets, the team found. But mice on a keto diet still weighed less than those on a Western-inspired one. The finding supports the idea that ultra-high-fat, low-carb food intake could help keep body weight down.

Other results raised some red flags. Mice on the keto diet had excess fat in their blood, a hallmark of cardiovascular disease. Male mice also had fatty livers; lab tests indicated that the organs weren’t working properly. But what stood out most was the animals’ trouble disposing of sugar in the blood, Gallop says.

Normally, the pancreas secretes the hormone insulin, which tells tissues to draw sugar from the blood and store it in cells for use as energy. But mice on the keto diet had difficulty doing so — their insulin-making cells struggled to release the hormone. The researchers traced this trouble to the cellular machinery that pumps out insulin. Why it’s not working remains a mystery, Chaix says.

It’s also unclear whether the effects her team saw translate to humans, she says. But it’s something that warrants further investigation.

The team’s results track with McDonald’s thoughts on keto diet use. “We don’t recommend that the general public use ketogenic diets without medical supervision,” she says. And if people do decide to load their diets with nut-based fries and extra cheese, they should let their doctors know so they can be on the lookout for potential problems.

Even if problems crop up, Chaix’s results suggest a surprise silver lining: Quitting the keto diet solved the mice’s glucose issue, the team discovered. This keto-caused health problem, at least, appears to be temporary. And that, Chaix says, “is a really good thing.”