Keto is good for controlling blood sugar, but a low-fat vegan diet is better for weight loss, study finds

• A new study found a low-fat, plant-based diet could help people lose weight and burn body fat.
• But an animal-meat-based, high-fat ketogenic diet was found to be better for managing insulin and blood sugar.
• It's still not clear why some diets cause weight gain, but processed food is a major risk factor for obesity.

When it comes to eating healthy, which diet is the best can be hotly contested.

Advocates for the ketogenic diet propose that low-carb, high fat eating is superior, and that butter, bacon, and eggs can be health foods. But other experts argue we should instead aim for a plant-based, low-fat diet of whole grains and veggies.

The two diets have been pitted face-to-face in a new study published January 21 in Nature Medicine.

According to the research, by the US National Institutes of Health (NIH), a low-fat diet may lead to more fat loss, while a keto diet can better manage blood sugar.

Read more: A nutrition journalist and keto advocate defends 'fad' diets, saying mainstream advice doesn't work for everybody

The key to this study is to help us understand what nutrients may drive hunger and weight gain, according to Dr. Kevin Hall, lead author of the study and NIH senior investigator.

"It's not designed to test whether one diet is better than another, it's designed to test the biology of what's regulating appetite in people when they're not actively trying to change body weight," Hall told Insider.

A plant based diet can boost weight loss, even with lots of carbs

Hall studied 20 adults without diabetes who, over a two-week stay in the NIH Clinical Center, were randomly assigned either a low-fat, plant-based diet, or a low-carb, animal-based diet.

Then they switched to the other diet, and stayed for another two weeks.

Hall's study found that participants ate about 500-700 fewer calories a day on a plant-based diet, even though they ate more food - mainly high-carbohydrate, high-fiber foods.

They also lost a significant amount of weight and body fat, compared to the keto diet, suggesting that carbs alone aren't responsible for weight gain.

These results are consistent, however, with many previous studies that have linked vegan and vegetarian diets to weight loss, and other health benefits such as better digestion and a healthier gut microbiome.

However, it contradicts a theory, popular among keto carb advocates, that eating too many carbs causes insulin spikes that triggers fat storage.

"We were testing a couple different ideas of what causes people to overeat and gain weight, theories of obesity. The popular recent idea is that's all about insulin, that didn't turn out to be true," Hall said.

A high-fat keto diet may be better for stabilizing blood sugar

Hall's study found that the keto diet helped participants shed pounds too, but mostly water weight, without significant reductions to body fat.

The keto diet did have an advantage for helping people manage their blood sugar (glucose) levels, though. Study participants produced about 60% less insulin on a low-carb diet than a low-fat diet. That's because eating carbs raises your blood sugar, so eating more carbs requires more insulin to balance things out.

These findings are consistent with existing evidence that support the use of low-carb diets for managing diabetes.

"One of the exciting things about the potential for ketogenic diets is that you don't need to produce very much insulin," Hall said.

People could eat more calories and maintain weight with keto

While people on the low-fat, plant-based diet could eat more food and still consume fewer calories, people on the keto diet had their own benefit.

Participants on the keto diet could consume more calorie-rich foods like butter, cream, and cheese without gaining weight.

Previously, nutrition experts have theorized that high-calorie foods make it easier to over-eat and therefore gain weight.

But that didn't happen with a ketogenic diet, the study found.

"Data suggests people more or less ate the same number of calories that they were eating at baseline," Hall said.

These results suggest that high-calorie foods don't automatically cause us to overeat. If those high-fat foods are satisfying, dieters will end up eating less of them, and therefore not consume extra calories that could lead to weight gain.

Both keto and plant-based are healthier than a diet full of processed foods

The advantages of either keto or plant-based diets, though, might be less about carbs or fat and more than they include whole foods, instead of ultraprocessed foods like chicken nuggets, soda, and white bread.

"A diet high in ultraprocessed food is the only one we found results in overeating and weight gain," Hall said.

In addition to weight gain, processed food is also linked to higher risk of chronic illnesses such as cancer and heart disease, more severe symptoms of aging, and even a shorter lifespan.

But, Hall said, he knows first-hand how difficult it can be to avoid processed foods.

As the father of two young sons, his family tries to get more veggies on their plates, but they do rely on convenience foods that are "cheap, available, easy to prepare, and safe from a microbial perspective."

"I end up eating more chicken nuggets than I'd like, but we do eat a lot of broccoli and green beans," he said. "Lifestyle factors influence people's food choices more than any study I could produce."