A ketogenic diet—which is very low in carbohydrates and high in fat—may be getting attention, but the evidence to support it is far from solid.

“Low-carb and low-fat diets equally effective for losing weight: Study.”

That headline, which ran in Newsweek in 2018, was reporting on DIETFITS, a year-long study on 609 people who were assigned to eat healthy diets with as little fat or carbs as possible.

It’s not just DIETFITS. The Pounds Lost trial tested diets that were higher or lower in carbs, fat, or protein on 811 people. After two years, weight loss was the same.

“The Pounds Lost results make a very strong case that it doesn’t matter which you cut—fat, carbs, or protein,” says lead author Frank Sacks, professor of cardiovascular disease prevention at the Harvard T.H. Chan School of Public Health.

But those and similar studies didn’t end the debate over low-carb diets and weight loss.

“What is this keto diet you keep hearing about, and is it healthy?” asked Bon Appétit in August. “With celebrities like Halle Berry and Kourtney Kardashian getting on board, we have a feeling that interest won’t be waning any time soon.”

Ketogenic diet basics

A ketogenic diet is not just low—it’s very low—in carbs: no more than 20 to 50 grams a day. (The average adult consumes about 245 grams of carbs a day.)

Without carbs to burn for fuel, the body breaks down fat—from food or the fat in your cells—into ketones (like acetone) that can be burned for fuel instead. When it does, you’re in ketosis.

A strict version of the diet has been used since the 1920s to treat children with drug-resistant epilepsy.

“It’s not an easy diet to follow even for a few months,” says Sacks. “Look at all the carbohydrate-containing foods you can’t eat.”

It’s goodbye burgers, fries, sodas, shakes, pizza, burritos, sandwiches, breads, bagels, buns, pasta, rice, cereals, chips, muffins, cakes, cookies, candy, and just about all junk food. It’s also farewell to carrots, oranges, beans, oatmeal, and other healthy carbs.

And it’s hello fats—cream, butter, oils, nuts, avocado—along with fatty meats and cheese. You can eat protein, but not too much, because it boosts insulin levels.

“One of the things that’s driving the ketosis is the low levels of insulin,” says Kevin Hall, senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases.

“Keeping insulin low drives the liver to take up the fatty acids from fat cells and produce ketones.” That doesn’t happen if insulin goes up.

But those ketones have a downside.

Side effects of a ketogenic diet

“People don’t feel so well,” says Sacks. “They have side effects like fatigue, bad breath, bloating, and constipation.” Some diet books call it the “keto flu.”

They’re constipated because they can’t eat fiber-rich foods like whole grains, beans, or some fruits and vegetables.

“A fiber supplement is generally recommended because most people don’t get enough fiber to move their bowels efficiently,” says Judith Wylie-Rosett, who heads the division of health promotion and nutrition research at the Albert Einstein College of Medicine in New York.

Can you eat fruits and vegetables on a keto diet?

You can eat some fruit (like berries) and some vegetables in some ketogenic diets.

“They tend to focus on low-carb vegetables, like broccoli or cauliflower,” notes Wylie-Rosett. “If you can’t have french fries, you may find broccoli much more attractive, because there’s only so much steak you can eat. And if you’re trying to eat more fat to get into ketosis, you need to put the fat on something. So oil and garlic on broccoli becomes extremely attractive.”

The long list of forbidden foods can help dieters, at least for a while.

So what's the deal with weight loss?

“People often consume fewer calories on a low-​carbohydrate diet because they have fewer foods to choose from,” says Wylie-​Rosett.

But in studies that last at least a year, any difference in weight loss between low-carb and other diets shrinks or disappears. Of course, you may have heard otherwise.

“A meta-analysis of 13 randomized controlled trials suggested that people on ketogenic diets tend to lose more weight and keep more of it off than people on low-fat diets,” reported a news article in the Journal of the American Medical Association in January.

What the reporter left out: The difference in weight loss after a year or more was only two pounds. And that was in people who typically had obesity.

“The differences appear to be of little clinical significance,” said the meta-analysis.

Photo: ricka_kinamoto/stock.adobe.com.