An array of keto-friendly foods, including fish, nuts, eggs, cheese, and avocado.

The ketogenic diet is currently one of the hottest weight-loss plans around. It involves consuming very few carbohydrates—typically  20 grams (about the amount in a small banana) to 50 grams (about the amount in 1½ cups of cooked penne pasta) per day—and getting 70 percent or more of your calories from fats.

But can eating so much fat really help you lose weight? And even if it does, are there risks that might outweigh the benefits? According to Carol Kirkpatrick, Ph.D., M.P.H., clinical associate professor and director of the Idaho State University Wellness Center, there’s still a lot we don’t know about this diet. “We just don’t have evidence to say that it's a ‘safe’ dietary pattern to follow long term,” she says. Here's what you need to know.

What Happens to Your Body

The premise behind the keto diet is that it forces your body to draw energy from the fat in the foods you eat and from stored body fat rather than from carbohydrates.

Usually, your body converts the carbohydrates you eat into glucose, its preferred source of fuel. Your brain, the most active organ in your body, consumes about two-thirds of the glucose you produce, and the rest is used by your other organs, muscles, and cells.

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At very low levels of carbohydrate intake, however, your body can’t make much glucose, and your liver begins to convert fatty acids into substances called ketone bodies. The ketone bodies provide an alternative source of energy for your brain and other cells.

You may think of the keto diet as a new trend, but it has actually been around for almost 100 years. Its original purpose wasn’t weight loss, though, but to control epileptic seizures before medications for epilepsy were developed.

Although scientists aren’t completely sure why the diet reduces seizures, even today it's an option for treating epilepsy in children and adults, says Kelly Roehl, M.S., R.D.N., an advanced-level dietitian and instructor at Rush University Medical Center who works with patients to manage epilepsy, other neurologic disorders, and weight.

What You Eat on a Keto Diet

Typically, the few carbs you eat on a keto diet come from non-starchy vegetables. Roehl says that a meal plan for her clients might include:

  • For breakfast, a scrambled egg cooked with butter, heavy cream, feta cheese, spinach, and mushrooms.

  • For lunch, a salad made with leafy greens, avocado, a hard-boiled egg, bacon, and crumbled or shredded cheese, dressed with olive oil and red wine vinegar.

  • For dinner, spiralized zucchini “pasta” and baked chicken topped with an Alfredo sauce made with heavy cream and Parmesan cheese, or an olive oil and pesto dressing.

  • For snacks, almonds, celery and cream cheese, or plain 4% fat Greek yogurt and fresh strawberries.

Keto’s Effect on Your Health

Research suggests that people who follow a keto diet do drop pounds. A 2013 analysis published in the British Journal of Nutrition (BJN) that evaluated 13 studies found that people who adhered to it lost more weight than those who followed a low-fat diet, at least in the short term. And Roehl notes that her patients who follow the diet for epilepsy tend to lose weight as a side effect.

But following this highly restricted form of a low-carb diet may not be worth it for most people. For one thing, some carbohydrates—especially fruits, vegetables, beans, and whole grains—are important parts of a healthy diet and are known to reduce the risk of a number of chronic diseases.

A "well-formulated" keto diet can be effective for obesity and type 2 diabetes control, and can even be used as a first treatment option for these conditions, says David Ludwig, M.D., a professor of pediatrics at the Harvard Medical School and a professor of nutrition at the Harvard T.H. Chan School of Public Health. He notes that more research is needed to determine effects on LDL ("bad") cholesterol levels and whether there are some people, such as pregnant women, who shouldn't follow this eating pattern. 

Still, the keto diet can be hard to stick to and many people “may not need to go to that extreme to get the benefits,” Ludwig says. Some evidence suggests keto may not be better than other diets for long-term weight loss. According to a recent scientific statement about carb-restricted diets from the National Lipids Association published in the Journal of Clinical Lipidology, very-low-carb diets led to more weight loss than low-fat diets in the short term (about six months), but only when both diet plans also severely restricted calories. And beyond six months, “that difference was essentially gone,” according to Kirkpatrick, who is the lead author of the report, and the two types of diets produced similar results.

The analysis also noted that the initial weight loss from low- and very-low-carb diets appears to be primarily due to loss of body water, and that restricting carbohydrates appears to result in greater loss of lean body mass—meaning muscle—as opposed to loss of fat mass. 

The keto diet also has a number of drawbacks. For example, in the BJN study, people who followed the diet had increased LDL cholesterol, the kind of cholesterol that leads to a buildup of plaque in your arteries and can raise your risk of cardiovascular disease.

Feeling foggy or lethargic is common during the first few weeks of following the plan, a result of significantly reducing glucose (which fuels the brain and muscles). It takes a few days for the body to switch over to using ketones for energy.

You may also be more prone to dehydration and electrolyte imbalances, because following a keto diet can cause your body to excrete more water than you otherwise would. There’s an increased risk of kidney stones, and, like other low-carb diets, the keto plan may cause bad breath, constipation, and headaches.

More serious: Scientists have documented several cases in which a condition called ketoacidosis occurred as a result of a very-low-carb diet. In ketoacidosis, the body produces more ketones than it can use for energy, and they build up in the blood, becoming toxic. If untreated, ketoacidosis can cause heart attacks, kidney failure, or fluid buildup in the brain.

The long-term effects of the keto diet aren’t clear. The fact that the body can derive energy from ketones is an evolutionary adaptation that helps people survive during periods of starvation, when glucose isn’t available to power the brain, points out Charlotte Vallaeys, M.S. a nutritionist and a senior food and nutrition policy analyst for Consumer Reports.

“We don’t yet know the impact of keeping the body in that state over long periods of time,” she says.

Is Keto Right for Anyone?

The keto diet has a good track record for helping children and adults with epilepsy, Roehl says. A 2016 analysis by the independent Cochrane Collaboration found that it stopped seizures in about half of those on it after 3 months.

Some experts believe a keto diet may be useful in controlling blood sugar levels in people with type 2 diabetes. This makes sense, according to Kirkpatrick. “Carbohydrate directly impacts glucose [blood sugar] levels,” she says. And diabetes disrupts the body’s normal processing of glucose. However, the National Lipids Association review found that while a low-carb diet may help reduce the need for diabetes medication, it didn’t have to be as low in carbs as what would be considered a keto level. What’s missing, Kirkpatrick says, are studies to illuminate how low carb intake needs to go for people with diabetes to get maximum benefit.

For epilepsy and diabetes, says Vallaeys, it’s best to think of keto as a medical diet—one that you shouldn’t take on without supervision from a healthcare provider experienced with it, who can help you avoid unpleasant or even dangerous side effects.

A Healthier Way to Lose Weight

Carbohydrates as a group shouldn't be vilified, Vallaeys says. A healthy diet includes minimally processed whole grains, legumes, fruit, and vegetables. But it does make sense to cut back on or even eliminate certain carbs—refined grains and added sugars—and highly processed foods for weight loss and your overall health.

“Eliminating processed carbohydrates has many benefits and no risks,” Ludwig says. Refined carbs, such as white flour and sugar, lack nutrients and fiber. And those, along with starchy processed foods such as some breads, cereals, and chips, have been linked to an increased risk of heart disease, type 2 diabetes, and weight gain.

Kirkpatrick agrees. “If there was one step we could take for improving our health and promoting success with managing weight, I think just switching to less processed foods would be beneficial,” she says. 

A diet that includes a moderate amount of healthy carbs is likely the best way to go, according to a study published in 2018 in Lancet Public Health. The researchers looked at the diet patterns and death rates in more than 430,000 people over a 25-year period. They found that people who ate a lot of carbohydrates (more than 70 percent of daily calories) as well as those who ate very little (less than 40 percent of calories) had shorter life spans than those who consumed a moderate amount. The sweet spot was 50 to 55 percent of calories from carbs, which is 250 to 275 grams of carbohydrates for someone eating 2,000 calories a day. 

But if you do decide to replace some of the carbohydrates you eat with fat, it is crucial that you choose the right types. In the study, low-carb diets were linked with a higher risk of death during the study period if people swapped carbs for animal-based fats and protein. But those who replaced carbs with fat and protein from plants—such as avocados, olive oil, nuts, and seeds—had a lower risk of dying.

Editor's Note: This story has been updated with additional comments from Harvard's David Ludwig, M.D.