This article originally appeared on Tonic.
Low-fat was high fashion two decades ago. Cutting carbs and pounding protein was big in the 2000s. Now, fat is making a serious comeback: The ketogenic diet—comprised of 80 to 90 percent fat—the pitch goes, is the secret to everything from losing more weight a to adding 10 years to your life, increasing your productivity, and purging your body of cancer cells.
As of this writing, two of Amazon's top 10 nutrition books are ketogenic diet-focused. Google searches about the diet have grown nearly 20-fold over the last five years. Kim K, Lebron, Tim McGraw, and Gwyneth Paltrow have all gone keto. But just as other "don't eat this" or "eat more of that" diets of the past didn't cure all, so goes the ketogenic diet. "It's one of these diets where so many people are talking about losing tons of weight, improving their health risks, beating cancer, and all these other lofty claims," says Stephan Guyenet, an obesity researcher and author of The Hungry Brain. "But all those big claims are far ahead of the current scientific evidence."
Then there's this catch: Eating keto is a big, fat pain in the ass. Some background: The ketogenic diet "works" by altering how your body powers itself. By cutting carbs to nearly nothing, eating minimal protein and mostly fat, you shift from drawing energy primarily from glycogen—a sugar stored in your muscles and liver that you mainly draw from carbohydrates—to ketones, molecules your liver produces in the absence of glycogen. It takes about one to three days to enter this ketone-fueled state, called ketosis. Think of the process like converting your car's gas engine to a diesel engine. Your car and the experience of driving it is basically the same, it's just using a different fuel type.
Keto evangelists leverage this idea of an alternative fuel source as one of the diet's main selling points. It sounds "sciencey." It's sort of logical. And that makes the diet easy to buy into, says Krista Scott-Dixon, a nutrition consultant for Precision Nutrition. "Keto has this sort of 'truthiness' and contrarianism," she says. "But I think the interest in this diet comes from the basic human desire to find some magic formula that will take your problems away, make you feel good, transform your body, and give you health, energy, and confidence. It's an attempt to fill the basic human desire for transcendence."
The ketogenic diet is about a century old, but its seeds were planted in 500 BC, when humans first discovered that fasting can control epilepsy. In the 1920s, two Harvard Medical School doctors who studied the phenomenon found improvements in epileptics began two or three days into a fast—the amount of time it takes a person's body to burn through their glycogen stores and shift to using ketones.
With that info, two researchers at the Mayo Clinic in 1921 theorized you could reap the same seizure-controlling benefits over a longer period by eating in a way that kept your body in ketosis. After all, you can't fast forever. The doctors prescribed a daily diet of 90 percent fat, with the remainder coming from protein, save for a few grams of carbs. The approach seemed to work—and the ketogenic diet has been a valid tool for treating epileptics ever since. A 1972 study published in the journal Archives of Disease in Childhood, for example, discovered that a quarter of epileptic children placed on a keto diet experienced significantly fewer seizures, and half of them became seizure-free. The clinical use of the diet has since grown slowly, although many doctors consider it a next-to-last treatment for epileptics, according to a study in the journal Epilepsia.
Ketosis might be beneficial under other extreme circumstances, according to newer research. It may prevent oxygen toxicity among deep-water Navy SEAL divers, ease the fuel burden of ultra-endurance athletes, or augment cancer therapies. It's important to note, however, that this research is not only in its infancy, but also controversial. A recent review in Medical Oncology, for example, found the ketogenic diet isn't useful for treating cancer, and may come with potentially harmful side effects.
And here's the thing: these potential benefits are interesting, but the average person is not eating to control epilepsy, execute military deep dive missions, run 200-mile races, or improve the outcome of a scary cancer diagnosis. And any therapeutic aspects of the diet are mostly lost on "healthy" and "normal" people, says Trevor Kashey, an Ohio-based nutrition consultant.
Keto doesn't seem to be any better for weight loss compared to any other "diet," Kashey says. And that's due to how weight loss science works. "People get excited about the idea that the ketogenic diet burns fat stored on your body," Kashey says. "Yes, ketones can be made from your stored fat, but ketogenic diets are high in fat. Therefore, the fat you eat is what gets converted to ketones. How do you make sure you are burning more fat than you are eating? You eat less fat. That sounds an awful lot like reducing calories."
The brunt of scientific research suggests nearly every popular weight loss diet works by putting you in caloric deficit. When you cut out all carbs, your calories naturally drop, driving weight loss. The same effect has been shown when people remove all fat, Guyenet says. "There's something to being at the extremes of dieting that isn't the best for long-term health, but can give you some short-term benefits."
Whether the ketogenic diet is optimal for health depends on how you define the term. If "health" is ideal blood sugar, blood pressure, and blood cholesterol, the diet could make you healthy. Would it make you any healthier than any other sensible approach to eating? Probably not, Kashey says.
But it may also have some health-based drawbacks, Scott-Dixon says. "Especially in women, it can disrupt hormonal production," she says. "You also sometimes see immunity issues and nutritional deficiencies, so you're maybe risking getting sick, and becoming malnourished, which is sort of embarrassing if you live America." Guyenet agrees, saying "few human groups have ever lived in chronic ketosis, so I'm skeptical that it's a good idea."
And despite what biohacker fitness bros will have you believe, keto doesn't seem to deliver a significant performance benefit either. One study of endurance athletes found the diet led to performance declines, and another found that it hurt endurance race performance. One study on mountain bikers, however, did show some promise. Any benefits of exercising in ketosis are likely offset by the drawbacks, covered here. "You know what has been proven to improve exercise performance, though?," Kashey asks. "Carbs."
The hitch with all this research is that it's conducted in controlled settings. Dieting happens in the real world, and in the real world a diet of 80 to 90 percent fat is neither easy to consistently pull off nor all that appetizing.
Picture an average day. You'd eat something like a breakfast of eggs and a few vegetables fried in butter; a lunch of avocado and olive oil-drenched ground beef and greens; and a dinner of butter-saturated fatty fish and broccoli. There are plenty of keto cookbooks, but your list of approved foods is slim, and eating all that liquefied fat can get gross fast.
The success you have on any common diet is determined by whether you follow it day-in and day-out, not the diet itself, according to a study in the Journal of the American Medical Association. You might adhere to your complicated keto plan for awhile—until you go on a work trip and have to eat-out for a week, or a business lunch at a burger joint, or to a movie and are faced with the horror that you can't eat popcorn.
"The amount of disruption that a keto diet causes is significant. To me, that's the important piece," Scott-Dixon says. "It requires you to change your behavior, change your life, cook very particular foods, skip going out with friends, figure out how to make your diet work at restaurants, and more. And it's just not that good that it's worth all the tradeoffs."
Should we write off keto completely? Of course not. "I'm not going to close my mind to the diet," Guyenet says. "Anecdotes are often the starting points of science. If people report a certain benefit, you take a skeptical attitude and investigate. But I'm not going to believe all the claims until I see credible evidence, and I'd certainly be very shocked if they turned out to be true."