Bacon is often a staple of the keto diet.
Photo: Brian Arch (Getty Images)

Ketogenic diets (keto for short) have become the fad health trend of the moment, with adherents claiming that they can help you lose and keep off weight faster than anything else, as well as provide a bevy of health benefits. But a new mouse study out of Switzerland suggests that keto diets might have an unintended effect on the ability to process insulin, which could possibly raise the risk of type 2 diabetes. The findings, however, may only be relevant to mice.

Keto diets exploit a genuine quirk of human biology. The fuel we use to keep our bodies running is usually taken from the stockpile of glucose that’s metabolized from our food. And the most easily available source of glucose comes from the carbohydrates we eat. But when the body doesn’t have glucose around, we instead break down our stores of fat for fuel. The liver carries out this function, creating chemicals called ketones that are then broken down into glucose.

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We occasionally produce ketones normally, such as during intensive exercise or while fasting, but when the body is in this emergency mode, it’s called ketosis. Ketosis usually happens because we’re in full-out starvation, but in a keto diet, you’re forcing it by eating little to no carbs, some proteins, and mostly fats while still providing the necessary amount of daily energy (calories) to keep yourself nourished.

Keto diets are already used to treat some medical conditions, such as seizures, and there is some evidence they can promote faster weight loss, at least in the short term (like most diets, the success tends to wane over time as people find the restrictive routine hard to keep up).

Insulin gets glucose in the bloodstream to the cells in our body that rely on it for energy. But people with diabetes can’t use insulin normally, meaning their bodies are constantly saturated with sugar in their blood, which can damage blood vessels and the rest of the body. People with type 2 diabetes can stop producing insulin, but more often, it’s that their bodies stop responding to the presence of insulin, which is known as insulin resistance.

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It’s well known how insulin affects our blood vessels, but the Swiss researchers wanted to see how fats broken down in the blood affected them as well, independently of insulin. And since a keto diet causes low levels of circulating glucose and insulin, it seemed a perfect way to study that very thing.

They fed one group of mice a diet high in fat but with plenty of carbs as well, and the other group a keto diet. Then they measured their metabolism and blood sugar. As expected, the carbs-and-fat dieting mice had signs of insulin resistance in the liver, and worse control of their blood sugar. But surprisingly, so did the keto mice, and to a noticeably greater degree.

The study was published Wednesday in the Journal of Physiology.

“Although ketogenic diets are known to be healthy, our findings indicate that there may be an increased risk of insulin resistance with this type of diet that may lead to type 2 diabetes,” study author Christian Wolfrum, a researcher at the University of Zurich’s Institute of Food Nutrition and Health, said in a statement.

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The study’s findings, the authors admit, are preliminary. Leaving aside that they looked at mice, not humans, they also only studied them for three days. And it’s likely that the dramatic effects on insulin resistance, if they happen in people, are probably weaker the longer someone stays on the diet.

“Clearly the detrimental effects seen in the early phase are not happening to the same extent in the later phase, otherwise nobody would use such a diet,” Wolfram told Gizmodo via email.

There’s also the very relevant fact that some research, in both people and animals, has suggested that ketogenic and low-carb diets might actually have a positive effect on blood sugar control, at least in people who already have type 1 or type 2 diabetes. And as with any successful diet, losing weight can also lower the risk of diabetes, if you’re overweight or obese.

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Nutritional science is notoriously messy, so the conflicting evidence isn’t anything new. And even the authors aren’t advocating that people throw out their keto cookbooks.

“I don’t think there is a need for worry, but the uncritical use of a ketogenic diet without careful monitoring is not advisable in my opinion,” Wolfram said. “In any case, any diet which restricts certain foods can be prone to cause malnutrition so I am personally in favor of more balanced diets.”

Wolfram has a point: People on keto routinely develop fatigue and other symptoms, especially at the start of their diet. This unofficial condition, referred to as the “keto flu,” is thought to be caused by a lack of nutrients and electrolytes.

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The team next plans to study whether people also develop insulin resistance early on in their keto diet, as the mice did. “Furthermore, we are interested to understand if this [liver] insulin resistance is part of a physiological adaptation process,” Wolfram said.

[Journal of Physiology]