Keto Diet Weight Loss May Hide Metabolic Damage in Mice

A high-fat, extremely low-carb diet prevented mice from putting on weight, although the trade-off was significant. Over several months, it raised blood-fat levels and disrupted blood-sugar regulation.

The findings indicate that sustained weight loss may mask underlying metabolic harm, which becomes apparent when carbohydrates are reintroduced.

Damage concealed by weight loss

In a long-term mouse experiment, the diet continued for more than 9 months before the animals were given a carbohydrate challenge.

During her postdoctoral research, Dr Molly Gallop monitored the mice to investigate how prolonged dieting changes organs over time.

The University of Utah Health team observed the animals for long enough to identify metabolic harm that developed gradually.

By measuring blood fats and responses to sugar, the researchers identified metabolic difficulties that would not have been apparent from body weight alone, even after months of dieting.

Keto required continuous fat burning

Reducing carbohydrates to nearly nothing meant the mice had to use fat as their energy source around the clock.

This condition, called ketosis – in which ketones take over from glucose as the body’s main fuel – may lower blood sugar in the short term.

The method was originally adopted by clinicians for epilepsy treatment, while updated guidance now stresses close monitoring and firm limits on duration, particularly for children.

It rapidly became popular for weight loss, yet its restrictive nature makes prolonged adherence challenging and increases the metabolic consequences when people alternate their carbohydrate intake.

Weight gain concentrated in fat tissue

Over the study period, mice on keto gained considerably less weight than those fed a Western-style high-fat diet.

However, nearly all of the weight they did gain was fat tissue rather than muscle, altering their body composition.

The conventional ketogenic formula used in the study provided roughly 90% of calories from fat and virtually no carbohydrates.

Although this extreme ratio curbed weight gain, it also made adverse changes in blood chemistry more likely.

Harmful blood fats rose

Despite appearing lean, the mice had excess fat circulating in their blood, a condition known as hyperlipidaemia, meaning abnormally high blood-fat levels.

Triglycerides, the primary form of fat in the blood, increased early and remained high throughout the extended feeding period.

“One thing that’s very clear is that if you have a really high-fat diet, the lipids have to go somewhere,” said Dr. Amandine Chaix, an assistant professor of nutrition and integrative physiology.

Because these fats can place the body under strain, the blood findings were important even before any shift on the scales.

The liver took up surplus fat

Male mice also accumulated fat in the liver, resulting in fatty liver disease. Further damage emerged when normal liver function declined, potentially interfering with the way the body handles fats.

Female mice were mostly protected from liver-fat accumulation while on keto, a sex-based difference the researchers were unable to explain.

This contrast suggests hormones or liver enzymes could influence risk, while making any straightforward application to people more complicated.

Carbohydrates revealed a vulnerability

Following 2 to 3 months on keto, the mice had low blood sugar and insulin levels, initially appearing beneficial.

But a modest dose of carbohydrates caused glucose intolerance, where blood sugar stays elevated for too long, because the pancreas produced insufficient insulin.

“Their blood glucose goes really high for really long, and that’s quite dangerous,” Chaix said.

The prolonged high glucose drew focus to the pancreas, which would normally deliver a rapid burst of insulin.

The pancreas lost its timing

The researchers established that the mice continued to respond to insulin, but produced too little following glucose exposure.

Insulin is a hormone, a chemical messenger that carries sugar into cells, and the pancreas normally releases it within minutes.

In mice fed keto, the beta cells, pancreatic cells responsible for making insulin, experienced stress in the internal compartments that handle proteins.

Electron microscopy linked this stress to an enlarged Golgi apparatus, the cell structure that packages proteins, which probably delayed insulin secretion.

Stopping keto restored sugar regulation

Ending the diet produced an unexpected benefit: 4 weeks on a low-fat diet returned normal glucose control in males.

This rapid improvement implied that the pancreas could restore its insulin-release machinery once exposure to high levels of fat was reduced.

The weight loss was not maintained when the mice went back to a Western-style diet, demonstrating that keto did not prevent subsequent weight regain.

This cycle is relevant to real life, as many people relax dietary restrictions and resume eating carbohydrates, requiring the body to adjust again.

Evidence in humans

A large meta-analysis reported greater diabetes remission after 6 months on low-carb diets, although that advantage had reduced by 12 months.

Even advocates acknowledge that long-term safety evidence is incomplete, and one frequently cited perspective cautioned that enthusiasm had exceeded the evidence.

“We’ve seen short-term studies and those just looking at weight, but not really any studies looking at what happens over the longer term or with other facets of metabolic health,” Gallop said.

A safer way forward

The mouse findings demonstrated that a diet may hold down weight while driving fat and glucose problems into less visible areas.

“I would urge anyone to talk to a health care provider if they’re thinking about going on a ketogenic diet,” said Dr. Gallop.

Before keto is considered for routine use, long-term studies in humans need to assess more than weight loss and closely track liver and pancreatic health.