What Happens in Ketosis? 5 Scientists Reveal the Risks and Benefits

Over the past few years, the ketogenic diet has skyrocketed in popularity. With the diet’s low-carbohydrate, high-fat guidelines, people are melting butter into their morning coffee, tossing out their strawberries, and eating bacon with reckless abandon. It’s tempting to jump on this newest diet trend, with celebrity devotees and your neighbor down the block praising its effects. But the diet’s critics question whether drastically cutting carbs and eating so much fat may lead to nutrient deficiencies and detrimental long-term effects. Some dietitians worry that the psychological impact of restricting broad categories of food could damage people’s body image, relationship with food, and social relationships.

“I think we need to look at the ketogenic diet as a valid and useful tool that will work for some percentage of the population rather than it being either demonized or artificially elevated,” Dr. Brianna Stubbs, lead translational scientist at the Buck Institute for Research on Aging, tells Inverse. “It’s not necessarily better. It might work for some people and not for others, but it is a valid tool.”

And the keto diet requires more than moderate adjustments. It’s a serious lifestyle change with transformative physiological effects. New research about the keto diet is challenging conventional nutritional wisdom, but before you hop on the bandwagon, let’s explore the science behind the keto diet, what it really means to be “in keto,” and its physiological effects.

What Does Eating Keto Even Mean?

Keto devotees replace the bulk of their carbs with fats and a moderate level of protein. The diet restricts carbohydrate consumption to approximately five percent of daily caloric intake, or less than 20 to 50 grams per day. (For reference, a single apple has around 25 grams of carbohydrates.) That means no starchy vegetables, grains, legumes, sugar, and limited fruit. Instead, keto dieters eat lots of greens, vegetables, and healthy fats like olive oil and avocados.

"A healthy ketogenic diet is probably more like spinach or arugula with oil and vinegar dressing.

“The popular conception of the ketogenic diet is bacon and eggs, but actually, a healthy ketogenic diet is probably more like spinach or arugula with oil and vinegar dressing,” Dr. John Newman, a geriatrician at the University of California, San Francisco, and researcher with the Buck Institute for Research on Aging, tells Inverse.

Following a keto diet can mean eliminating some of the most-loved foods, like pasta, whole grains, dessert, and even fruit. Even the most enthusiastic keto devotee can find the diet difficult to maintain. Some try the diet and stick to it consistently, while others start and stop sporadically. The research isn’t clear on whether going in and out of ketosis over the long-term has negative effects.

Physicians, researchers, and nutritionists are quick to stress that no single diet or nutrition plan works for everyone. Choosing what to eat is an individual decision with unique outcomes for every human body.

Fat as Fuel, Not Sugar

To understand “being in keto,” you first have to understand how and where the body derives energy. To fuel our daily activities, the human body relies primarily on glucose or blood sugar — mostly from carbs like bread, pasta, starchy vegetables, and fruit. Carbohydrates are broken down into glucose and converted into energy to support our hungry organs, or stored as glycogen in our liver or muscle tissue. But what happens when you deprive your body of carbs? That’s where ketogenesis comes in, the metabolic process the keto diet is named for.

Without carbohydrates to convert into glucose, the body goes into a state called ketosis, in which it breaks down stored fat to produce an alternative fuel source: ketones. Basically, your body begins burning fat for energy instead of sugar from carbs. As quickly as two to four days after switching to a low-carb diet, the body can transition to a state of ketosis, depending solely on ketones to function. A simple breath, blood, or urine test can determine whether the body is in ketosis.

Making the transition into ketosis can be daunting. During the first few days with limited carbs, keto dieters often report experiencing the “keto flu”: fatigue, muscle weakness, insomnia, and digestive issues like constipation, diarrhea, dehydration, and brain fog. Another side effect can be bad breath, resulting from elevated ketone levels. Acetone, a ketone excreted from the body in your breath and urine, gives breath a slightly sweet smell.

"If you can survive the one week of feeling lousy, most people who get through that don’t look back.

“If you can survive the one week of feeling lousy, most people who get through that don’t look back,” Dr. Ethan Weiss, a cardiologist at the University of California, San Francisco, and founder of Keyto, a company manufacturing breathalyzers to monitor ketone levels, tells Inverse.

In the first few weeks, some dieters, primarily elite and endurance athletes, also experience decreased athletic performance. This occurs from depleted glycogen stores in the muscles as ketones are produced, which are typically a critical energy reserve for high-intensity exercise.

After a few days, dieters report these symptoms subsiding and being replaced with increased focus and energy. And the reason many people start in the first place? Many keto dieters report rapid weight loss while still feeling full.

Emerging research suggests the ketogenic diet may affect females and males differently. Female dieters may have more trouble losing weight than their male counterparts.

Why Is Everyone Talking About Keto?

Keto proponents call it the miracle diet. Skeptics say it’s dangerous, extreme, and immoderate. Most people, unfamiliar with the nutritional approach, are perplexed — conventional wisdom rules, that carbs are fuel and fat is bad. A growing body of research shows the reverse may be true: some fats are a critical part of a healthy diet, and limiting carbohydrates could have positive physiological effects in the short- and long-term.

"There’s this almost evangelical quality to the people that get on it.

“Part of the reason why it’s become so popular is that there’s this almost evangelical quality to the people that get on it,” Weiss tells Inverse. “It’s almost like people have felt they have discovered the fountain of youth, and they want to share it with everybody. That’s very unusual in weight loss.”

Most people keep their food choices private, Weiss explains. But not keto fans.

“I’ve been dealing with this on a clinical level for 25 years, and no one wants to talk about dieting,” Weiss says. “No one wants to talk about whether they’re going to Weight Watchers or doing any of these other things.”

Keto enthusiasts are loud and proud, broadcasting their results across social channels and telling friends and family.

How do you know if somebody is on keto?

You don’t. They’ll tell you.

“The big joke,” Weiss says, “is ‘How do you know if somebody is on keto? And the answer is, you don’t. They’ll tell you.”

Evelyn Tribole, registered dietitian and author of The Intuitive Eating Workbook, is skeptical of the keto diet and diet culture in general. The dietitian instead advocates for an introspective approach to food, encouraging people to tune into their hunger and satiation cues to guide their food choices. “Intuitive eating is a self-care eating framework, where you’re the expert in your body,” she tells Inverse.

“Not only do diets not work, they cause harm. They increase the risk of eating disorders and disconnect people from their bodies,” Tribole says. “You see more binge eating and more emotional eating after trying diets like keto.”

Tribole says the keto diet and other diets with strict guidelines are incredibly distracting and can impact relationships with friends and family. “It’s like when you’re in a relationship, and it feels like you’re talking to someone who’s constantly on the phone,” she says. “They’re there, but they’re not really showing up. They are not present.”

Cavemen in Ketosis

While the word may seem foreign, ketosis is a natural part of our biology, Stubbs, the Buck Institute researcher, explains. Throughout human evolution, humans entered ketosis during periods of fasting or — in extreme cases — starving.

“You’ve got your evolutionary hunter-gatherer man wandering around, and if he goes more than 48 hours without food and then can’t make ketone bodies, he dies,” Stubbs says. The brain cannot be fueled by fat; it needs a way to convert fat to usable molecules. “The ability to turn fat stores into ketones starts to appear as our brains start to get bigger and bigger and become a big energy consumer.”

But today, when food is abundant and the average person rarely fasts, ketogenesis is most often kick-started during exercise, when carbs are so depleted that the body starts to burn or oxidize fat to keep going.

Surprisingly, ketosis is also a normal part of healthy infant development, Stubbs says. Newborns drink milk, which is very high in fat and rich in medium-chain triglycerides, which are then converted into ketone bodies.

It’s essential to our survival, Stubbs says.

Exploring the Risks and Benefits

Even though its popularity has ballooned in the past few years, the keto diet is nothing new. Since the ‘20s, doctors have been prescribing a ketogenic diet to help treat children with drug-resistant epilepsy. Ketones have been shown to stabilize neuronal activity, which helps prevent seizures.

But beyond epilepsy, research in recent decades explored whether ketogenic diets, low-carb diets, or simply elevated ketones in the body have other neuroprotective effects. Some research, primarily based on animal models and mice studies, shows potential protective benefits for other neurological disorders and neurodegenerative diseases, like Alzheimer’s and Parkinson’s disease, traumatic brain injuries, and stroke.

"Ketone bodies also help to dampen inflammation and signal repair pathways.

“Ketone bodies also help to dampen inflammation and signal repair pathways,” Newman says.

In a 2017 study, a team led by Newman found that aging male mice experienced improved memory, healthspan, and midlife survival from a keto-like diet.

And the keto diet impacts more than just the brain. Some research shows that a low-carbohydrate diet can be powerful in managing type 2 diabetes and reducing medication dependence. By limiting carb intake, patients can control their blood sugar and insulin production. Even the American Diabetes Association, while not endorsing any one diet over another, includes a low-carb diet as a potential tool for patients to manage their type 2 diabetes.

Some of these health benefits can’t be attributed to the ketogenic diet alone. Weight loss caused by the ketogenic diet triggers a long list of positive health effects, including lower blood pressure, decreased risk of cancer and heart disease, better sleep, and improved mobility. So perhaps the benefits are shared, with elevated ketones and weight loss working in tandem.

More Work Is Needed

"I think the best thing is to eat a wide variety of food.

Some physicians, like Dr. Mark Mattson, professor and researcher in neuroscience at Johns Hopkins University, have reservations about the ketogenic diet. “I think the best thing is to eat a wide variety of food, especially fruits and vegetables,” Mattson says. “It’s hard to find vegetables that do not have carbohydrates.” Mattson also notes that the high saturated fat intake could exacerbate cardiovascular disease.

Tribole agrees: “Show me the five-year data,” she says. “I haven’t seen it.” Tribole warns people to use caution when interpreting research and making dietary decisions. “I worry it could cause harm long-term,” she says.

More randomized controlled human trials, the gold standard of medical research, must be completed to make conclusive claims about the keto diet’s effects. Researchers predict that as the diet penetrates the mainstream and the public’s knowledge about ketones and ketosis grows, funding will also increase to support this research.

“I think we’re just getting to the point now where the kind of mainstream medical community is beginning to be aware and take interest,” Weiss says. “I think the numbers of studies that we will see in the next five years, 10 years is going to explode, because everybody wants to know the answers to these questions.

Ultimately, deciding what and how to eat is up to the individual. The keto diet is just one way to go about it.