What Is the Keto Diet?

While it’s not as popular as it once was, the keto diet is still firmly in the mainstream, making the list of the top 15 most common eating patterns of 2024, according to an International Food Information Council survey of 3,000 adults. Much has been written about the keto diet pros and cons, but the divisive diet continues to inspire strong reactions. Fans say it’s transformed their health, and detractors stress that the extremely low-carb, high-fat diet could have long-term health repercussions. So who’s right? Is the keto diet safe?

The bottom line is that despite its following, the keto diet isn’t well understood, even by people who claim to follow it. A true keto diet doesn’t just involve throwing out bread and calling it a day. And, contrary to popular belief, it isn’t an all-you-can-eat bacon buffet either. Overall, the keto diet requires a dramatic lifestyle change that completely alters the way your body produces energy. Most importantly, while it may have some benefits for very specific subsets of the population, it’s not appropriate for many people to follow — and it certainly shouldn’t be put forth as a quick and easy weight loss diet.

To help us understand how this extreme eating plan works, we spoke with two dieticians and a cardiologist who offered their take on the keto diet, its potential risks, and who — if anyone — it’s best for.

Experts Featured in This Article

Abigail Collen, MS, RD, CDN, is a registered dietician and nutritionist at the Mount Sinai Health System.

Gabrielle Gambino is a senior clinical dietitian of Advanced Heart Failure Service at NewYork-Presbyterian.

Karen Aspry, MD, MS, FNLA, FACC, is a cardiologist with the Cardiovascular Institute, certified lipidologist, and co-chair of the American College of Cardiology Prevention Council, Nutrition, and Lifestyle Workgroup.

What Is the Keto Diet?

In simple terms, the ketogenic (aka keto) diet is a very restrictive, high-fat, low-carb eating pattern. A typical macronutrient ratio per day on the keto diet consists of around 70 percent fat, 15 to 20 percent protein, and just 5 to 10 percent carbohydrates — meaning, carb intake could be as low as 20 to 50 grams per day on a standard 2,000-calorie diet. High-fat foods such as meat, eggs, butter, cheese, avocado, oils, and nuts are popular options on the keto diet.

“Keto is a low-carb diet that shifts the way the body uses energy,” says Abigail Collen, MS, RD, CDN, a registered dietician and nutritionist at the Mount Sinai Health System. She explains that the body is forced to rely on fat — rather than carbs — for its primary energy source. “When there’s not enough sugar in the body from carbs, the body is reliant on other types of energy, which is created from the breakdown of fats and fatty acids.”

In other words, the human body typically uses carbohydrates for energy, breaking them down into glucose (or blood sugar), which is then absorbed into the bloodstream for fuel. Without carbs, it produces ketone bodies from fat that are used for energy — a process known as ketosis.

Because of this metabolic shift, the keto diet was initially implemented in the 1920s as a way to improve symptoms of epilepsy and reduce seizures, and it’s still used today, says Gabrielle Gambino, a senior clinical dietitian of Advanced Heart Failure Service at NewYork-Presbyterian. The theory is that “by shifting the energy source to ketone bodies, it also changes the way cells metabolize energy, which may lower excitability of the brain and remove certain metabolites connected to seizure activity,” Gambino says.

It appears to be effective, although more research is needed to explain exactly how it works —and a review published in 2019 in the Frontiers in Neuroscience noted that “each patient must have an individually designed diet,” meaning that there’s no one ratio that works for all people, and that “”adult patients have more difficulty in maintaining the [classic ketogenic diet].”

It’s Not Your Typical Low-Carb Diet

While the keto diet is known for being low-carb, many people tend to underestimate just how much carb-cutting it truly entails. (And depriving the body of carbs is no easy feat.)

“A lot of people think they go on a keto diet when it’s really better classified as a low-carb diet,” explains Karen Aspry, MD, MS, FNLA, FACC, a cardiologist with the Cardiovascular Institute, certified lipidologist, and co-chair of the American College of Cardiology Prevention Council, Nutrition, and Lifestyle Workgroup. “In order to really enter ketosis, there has to be a very low carb amount and a very low protein amount.”

Along with 10 percent or less of your daily carb intake, the keto diet entails a daily protein intake of just 1.2 to 1.5 grams of protein per kilogram of body weight. “Anything above that, you’re not going to be in ketosis,” she says. “There are some people that may say they’re following it, but when you ask them what they’re eating, it’s high in protein. So they’re probably not in ketosis to any degree, or at least not continually.”

Worth noting: ketosis seems to play a role in the keto diet’s ability to reduce seizure symptoms. There isn’t necessarily a benefit for someone without a seizure disorder. And in fact, ketosis can come with some unpleasant symptoms such as nausea and headache, sometimes called the “keto flu.”

Is the Keto Diet Safe?

For the average, healthy adult, the keto diet may be considered safe to try in the short term, but it’s certainly not suitable for everyone.

“There are risks, particularly with long-term adherence to a high-fat diet,” says Gambino. “This meal pattern may put individuals at risk for developing or worsening cardiovascular diseases, liver issues, kidney stones, and nutritional deficiencies if not followed correctly or if the individual is prone to these conditions genetically.”

She states that there are certain groups of people who should especially steer clear of the keto diet, including:

What Are the Risks of the Keto Diet?

According to our experts, these are the most notable risks associated with the keto diet’s low-carb, high-fat eating pattern:

It Goes Against Standard Guidelines for Healthy Eating

“The keto diet is inherently not balanced when looking at the standard guidelines for healthy eating,” says Gambino. “Keto-style meal plans call for a very high percentage of dietary fat. People can often over-indulge in animal-based, saturated fats, processed foods, and sodium-laden items while staying within the keto-macronutrient guidelines.”

The keto diet strays drastically from the Dietary Guidelines for Americans (DGA), which include limiting saturated fat to less than 10 percent of calories per day and keeping sodium intake to less than 2,300mg daily.

“Increased dietary fat intake, along with decreased intakes of whole grains, fruits, and vegetables may put many of us at risk for developing kidney stones, liver conditions, and heart conditions,” says Gambino. It could also increase the risk of nutritional deficiencies.

It Can Hurt Your Gut Health

One of the most essential nutrients for gut health is fiber — and the keto diet’s restriction on foods like beans, lentils, and starchy vegetables makes it hard to come by.

“Nutrition science has coalesced around plant-rich diets and fiber-rich diets in terms of reducing cardiometabolic disease and death from all causes,” Dr. Aspry explains. “You need to make sure your gut is healthy, and the best way to ensure that is with fiber-rich, phytonutrient-rich foods.”

The keto diet’s extreme carb reduction means whole grains are off the table too, which is also a problem for your gut. “The effects [of whole grains] are profound in terms of reducing inflammation and improving gut health,” says Dr. Aspry. “I tell patients to think of them like scrubbing brushes going through [your gut] – they’re removing cholesterol, toxins, and overgrowth of bacteria, even from your intestines.”

It Increases Saturated Fat Intake and LDL Cholesterol Levels

Increasing your fat intake isn’t necessarily a bad thing as long as you’re sticking to healthy, unsaturated fat sources such as olive oil, avocado, and nuts that can help raise your “good” HDL cholesterol levels.

While the keto diet does include foods rich in unsaturated fat, it also takes a liberal approach to saturated fat in the form of butter, fatty cuts of meat, cheese, and processed meat such as bacon. Dr. Aspry explains that an increase in unhealthy saturated fat can lead to higher levels of “bad” LDL cholesterol, which has been shown to increase your risk of heart disease.

Still, there is some debate among experts concerning the overarching link between saturated fat, LDL cholesterol, and heart problems — but as Dr. Aspry points out, “All the data trace LDL and causality in terms of cardiovascular disease. [When] you lower LDL cholesterol, your risk goes down; when you raise it, your risk is higher.”

She points to a recent large-scale cohort study by UK Biobank study that surveyed 100,000 participants on a low-carb, high-fat diet. She explains, “[The participants] had significantly higher levels of LDL cholesterol and more than two-fold high risk of cardiovascular events. Cardiovascular mortality was also increased.” She notes that this was an observational study and cannot prove causality, but it highlights the importance of more research on the long-term effects.

Gambino adds that other studies have suggested worsening cholesterol levels with longer-term keto adherence, especially in those who are genetically predisposed to struggle with cholesterol control.

Are There Any Benefits to the Keto Diet?

One of the biggest draws of the keto diet is weight loss. It’s gotten a reputation as a diet that delivers quick weight loss. And it’s true that many people see a dramatic drop in weight in the early weeks, but not for the reason you might think.

According to Collen, “The [initial] rapid weight loss people are seeing is really related to water weight and, to an extent, dehydration. Carbohydrates are stored in the body with water. [The keto diet] is depleting your carbohydrate stores and all the water that’s stored with it.”

Whether this weight loss is sustainable, however, is questionable. “It may be helpful for short-term weight loss, but for many, this diet is too difficult to keep up with forever,” says Gambino. Once you add carbs back into the diet, the numbers on the scale may go up too. That’s not a bad thing — weight alone isn’t an accurate measure of health. But if weight loss is one of the main reasons someone wants to try out the keto diet, it’s important to be aware of this.

If there’s one bright side to the keto diet (besides its anti-seizure benefits), it’s that “it really does get people to completely stop eating all the sugars in the American diet,” says Dr. Aspry. “Sugar is a big contributor to cardiovascular morbidity and mortality.”

That said, it’s important to note that some keto dieters tend to eliminate all forms of sugar, including natural sugars found in fruit and dairy. Natural sugar is not the same as the added sugar found in things like sweets, pastries, and condiments — it comes packaged with vitamins, minerals, and nutrients, along with fiber that causes the body to absorb sugar at a slower rate.

So unless a medical doctor is recommending you go on the keto diet for a very specific reason and offering personalized support, there probably isn’t any benefit you’d get from eating this way — especially one that would outweigh the potential risks.

Is the Keto Diet Good for Diabetics?

Gambino explains that, naturally, blood sugar levels will decline when eating less carbohydrates and more fats. “Studies have shown a decrease in fasting glucose levels and A1C percentages with the introduction of keto diets,” she explains. “Keto diets tend to help keep glucose levels at a stable state pre- and post-meal, which may reduce the amount of medication needed to correct glucose spikes.”

While this is certainly encouraging, she notes that many of the studies lacked control groups to compare the diet with other diabetes-related eating patterns, meaning there’s not enough evidence to show that the keto diet is superior to other standard low-carb diets.

“Additionally, most studies discussed were not long-term, with an average follow-up of one year or less. It will be important to better observe the impact of keto diets on diabetes management for longer periods of time,” she adds.

Since the keto diet is so hard to stick to, someone looking to help manage their diabetes through diet may be better off using a more sustainable eating plan. But ultimately, it’s best to come up with a plan with your healthcare provider.

The Bottom Line

While more research is needed to truly understand the long-term effects of the keto diet, one thing is for certain: it isn’t a “quick fix” for weight loss. This type of diet requires major lifestyle modifications and commitment, and Gambino recommends doing your due diligence before making the carb-free leap.

“The first step would be to discuss this plan with your medical team and dietitian to ensure you are not going to unintentionally do more harm than good, especially if [you have] a pre-existing medical condition like type I diabetes or heart failure.”

It’s also important to keep in mind the restrictive nature of the diet, which eliminates carbs in any form, regardless of whether they’re from healthy sources such as whole grains, beans, fruit, and vegetables — putting you at risk for nutritional deficiencies and gut issues. Moreover, an increased saturated fat intake can raise your LDL cholesterol levels and increase the risk of heart disease.

If you’re committed to a true keto diet, Dr. Aspry suggests making small modifications to protect your heart and your gut, such as adding a couple of tablespoons of brown rice every few days, eating more green vegetables, and swapping saturated fat with healthy fat whenever possible, such as butter for olive oil.

“Within keto, there are variations,” she says. “[For example], is the person eating nuts, avocados, and olive oil, or are they eating butter, animal fat, bacon, and coconut or palm oils? That has a completely different effect on their LDL cholesterol.”

And remember — if you’re looking to cut back on carbs, the keto diet isn’t the only way to do it. Even a moderate carb reduction can make all the difference since most Americans are overindulging, says Dr. Aspry. “A moderate intake of carbs will be around 25 to 45 percent of your total daily calories. A higher carb diet is 50 to 65 percent, which is what most Americans eat.”

Ultimately, the keto diet is not a one-size-fits-all solution, and we’re still learning about its long-term impact. Until then, it’s a good idea to stick with what we do know: extreme diets aren’t always the answer. The best type of diet is one that keeps you happy, healthy, and energized — even if that means eating a few carbs every now and then.

Alex Vance is a freelance health and parenting writer featured in PS, Fortune Well, Today.com, BuzzFeed, Parents, Verywell Family, and more. Currently, she writes part-time for baby-tracking app Huckleberry and the health and fitness website Get Healthy U.