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? 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.

Although you may think of the keto diet as a new trend, it’s actually been around for nearly 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, an egg scramble 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.

Cutting backs on certain carbs can also be good for you.

“Eliminating processed carbohydrates has many benefits and no risks,” 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.

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.

But eliminating carbohydrates altogether is a different matter, Ludwig says.

For one thing, the keto diet may be an overly restrictive form of a low-carb diet. It can be a very difficult eating plan to follow, and most people “may not need to go to that extreme to get the benefits,” Ludwig says.

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.

And the keto diet has a number of other drawbacks. For example, in the BJN study, LDL (“bad”) cholesterol increased in people following it.

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, Ludwig says. 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, 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.

And some research has suggested that a low-carb diet might be especially useful in helping people get diabetes under control. This makes sense, Ludwig notes, because people with diabetes don’t process carbohydrates normally.

But 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

Consumer Reports’ nutritionists say cutting back on or even eliminating processed foods made with refined grains and added sugars is smart for weight loss and your overall health. But carbohydrates as a group shouldn't be vilified, Vallaeys says. A healthy diet includes minimally processed whole grains, legumes, fruit, and vegetables.

It’s also crucial, if you’re eating more fats, to make sure you eat the right kinds. A study published last year in Lancet Public Health of 15,428 adults found that low-carb diets were linked with a higher risk of dying during the study period if people replaced carbs with 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.  

The bottom line? It’s not just whether you prioritize carbs or fats in your diet—it matters what type of each you’re getting.