Let's Talk About Ketosis

This chemical reaction in your body is at the heart of the keto diet. We asked our health and nutrition experts for a more detailed look at what it is, and how it works.

by Tula Karras Health Writer

If you’ve been thinking about trying the trendy keto diet, you’re far from alone. Millions of people around the world have given this approach to weight loss a try. And if you’re not exactly sure how it works—well, you’re far from alone there, too. Let’s just say, it’s complicated. The keto diet’s starring role goes to ketosis, a chemical process in your body that helps it burn fat for fuel. Why does that matter? Here’s what the experts have to say.


Our Pro Panel

We went to some of the nation's top experts in the keto diet to bring you the most up-to-date information possible.

Lisa Sasson, R.D.

Lisa Sasson, R.D.

Associate Dean for Global Affairs and Experiential Learning; Clinical Professor of Nutrition and Food Studies

New York University

New York, NY

William S. Yancy, Jr., M.D.

William S. Yancy, Jr., M.D.

Director; Associate Professor of Medicine

Duke University Diet and Fitness Center; Duke University School of Medicine

Durham, NC

Elisabetta Politi, R.D.

Elisabetta Politi, R.D.

Nutrition Director

Duke University Diet and Fitness Center

Durham, NC

Frequently Asked Questions
What’s the difference between the keto diet and ketosis?

The keto diet refers to a way of eating that requires people to derive about 70% or more of their daily calories from fat, while cutting way back on the amount of carbs they eat. This style of eating results in ketosis, a metabolic state where the body burns fat (in the form of ketones), not sugar, for fuel.

Does ketosis cause side effects?

Yes. The cluster of side effects from ketosis is known as the “keto flu.” Despite its name, however, there is no fever or infection associated with it. Symptoms of fatigue, headaches, sleep disturbance, nausea, and foggy thinking are common with the keto flu. Fortunately, it usually disappears on its own within a week of starting the diet.

Is ketosis good for my health?

Sometimes, yes. For instance, in people with type 2 diabetes, ketosis may lead to an improvement in blood sugar levels, making them less dependent on insulin medication. And some research suggests that people with Alzheimer’s may experience neuroprotective aspects of the ketones produced as part of a keto diet.

Is ketosis dangerous?

No. Ketosis simply refers to the process of burning ketones—which are converted from fat—for fuel. However, ketoacidosis—a condition where there is an unsafe high level of ketones present—is the leading cause of death in people under the age of 24 who have type 1 diabetes. If you have type 1 diabetes, the keto diet is probably not advisable.

Remind Me Again, What’s Keto?

Keto, short for ketogenic, is a type of diet that emphasizes eating fats over all other sources of energy. Typically, the quickest type of fuel for your body comes from the carbohydrates you eat, which your body can easily convert into glucose (a.k.a. blood sugar) for instant energy. In the case of the keto diet, the premise is that if you can teach your body to burn ketones—chemicals your liver converts from fat—instead of the glucose from carbs, it will help you lose more weight, faster.

The keto diet is trending right now, thanks to celebrity endorsements and clever marketing. But it’s actually been around since in the 1920s, when doctors discovered that children with epilepsy had a sharp drop in seizures after their carbohydrates were restricted. (Scientists still aren’t sure exactly why it works.) With the introduction of anti-epileptic drugs, the keto diet was used less and less, although some doctors today still recommend it for kids and adults with drug-resistant epilepsy.

It popped up again in the early 1970s, thanks to cardiologist Robert Atkins, M.D., who published a popular diet book emphasizing a ketogenic phase in the first two weeks of the diet plan. The plan became more popular throughout the 1990s and 2000s, but because it ran afoul of growing recommendations from major medical organizations to limits fats in order to protect against heart disease, the diet fell out of favor…until now.

What’s changed to bring keto back into mainstream acceptance? The science, for one. An increasing body of research suggests that for people with type 2 diabetes (nearly 33 million Americans and counting), the keto diet could be the perfect antidote: By following an extremely low-carb plan, your body needs less insulin to balance blood sugar levels, thus dramatically decreasing the need for medication over time.

Get All the Details on the Keto Diet

How Does Ketosis Work?

The metabolic state that causes your body to use fat for energy is known as ketosis. How you get there is a little tricky. Follow along while we explain.

  • Your body has three options for fuel: carbohydrates, fat, and (in certain cases) protein.

  • Carbohydrates (grains, fruits, and vegetables) are the preferred form of energy, but when you restrict carbs to a very low level, your body resorts to fat.

  • Your liver converts the fat into ketones, molecules which can be used for fuel.

  • This process requires a bit more energy and therefore, the theory goes, you burn more calories than you would using carbs. In addition, the keto diet’s ability to lower insulin levels may be at play, since insulin tells the body to hold onto fat rather than burn it.

The sequence of carb-restricting that leads to fat-burning is called ketosis, and it takes about two to four days (more or less depending on the individual) for your body to shift into ketosis. One of the biggest challenges with the keto diet, however, is that the moment your carb intake goes up, you get bounced out of ketosis because your body will immediately start using glucose (or protein, if it can get it) instead of fat for energy.

People who are committed to getting into and staying in ketosis often use special urine strips that detect the amount of ketones in their pee. Like pregnancy tests, the strips turn a certain color when you are in ketosis. But you don’t always need chemical evidence to show you’ve achieved the desired metabolic state: You might also know you’re in ketosis if people back away when you talk, since bad breath (or at least fruity or metallic breath) is one side effect for a small percent of dieters.

What Are the Benefits of Ketosis?

From a survival standpoint, ketosis allows us to burn energy from stored fat in times of food scarcity. From a weight-loss standpoint, there is some evidence that your metabolism ticks up when you burn ketones as opposed to glucose, an increase of anywhere from 50 calories to 200 calories a day, depending on which study you look at. In addition, ketones may also suppress your appetite.

From a health standpoint, there are a few ways that the diet and its ketones—which go up significantly on the plan—might give you an advantage. The following conditions have some strong science behind them:


People with type 2 diabetes who follow this diet plan often see a dramatic change in their ability to manage insulin, though whether that’s from ketosis or simply from a decrease in the amount of carbs they are taking in (and thus lower blood sugar and improved insulin management) is debatable.


The OG disease responsible for the creation of the keto diet a century ago, epilepsy can still be managed, to a certain degree, through the keto diet. In children with epilepsy, ketosis can significantly reduce seizures even when medications fail.

Alzheimer’s Disease

New research shows there might be an Alzheimer’s risk advantage to having certain types of ketones in your blood. A study of patients with mild to moderate Alzheimer’s disease who were put on a keto-style diet for three months performed better on a cognitive test measuring memory and language skills compared to those who were not on the diet. Researchers speculate the neuroprotective aspect of ketones might be responsible (the effects were most dramatic in those who had the Apoe4 allele gene, which raises your risk for Alzheimer’s).

What Are the Side Effects of Ketosis?

Revamping your diet to the point of triggering a metabolic change in your body is bound to have a few side effects—some more pleasant than others. These are some of the not-so-fun things you can expect when you enter a state of ketosis. (Typically, the side effects are temporary and will resolve themselves as your body adapts to its new state.)

Keto Flu

After the first several days of a carb-starved existence, many people experience what’s been termed the “keto flu,” which can include fatigue, headache, foggy thinking, nausea, and sleep problems. (Despite its dramatic name, there’s no fever or infection involved.)

No one is certain what causes this constellation of symptoms, though theories run from a form of carb-withdrawal (and related drop in blood sugar levels) to an electrolyte imbalance. Some researchers speculate that the keto flu may also have to do with a change in the gut’s microbiome, which can be altered dramatically as a result of the diet and ketosis.

The symptoms usually resolve within a week or two—and certain side effects can be avoided altogether by staying hydrated and keeping up electrolyte levels. In fact, some people report feeling more energized and clear-headed than ever after the flu phase passes.

Learn More About the Keto Flu


The constipation that people report at the outset of ketosis can continue, unfortunately, thanks to the dearth of fiber-rich whole grains and fruit fiber on the keto diet, which usually keep things moving right along. Experts say to drink a lot of water and include plenty of fibrous vegetables, which are allowed on the plan.


If it’s not one thing, it’s the other. Some people can experience diarrhea when following the keto diet. Your risk is higher if you are replacing sugar and sugar-filled products with artificial sweeteners or artificially sweetened keto products. Another culprit for the runs: All that fat, which can take more than a hot minute for your body to learn how to process.

To deal, stick with natural sweeteners like stevia, and pile keto-friendly fibrous produce on your plate, like broccoli—fiber holds onto water and bulks things up. And because you can lose a lot of water with a bout of diarrhea, be sure to gulp more H2O when you’re following this plan. You can also try adding sugar-free psyllium fiber supplements, which can bulk up loose stool. If you can’t beat diarrhea with these changes, you may have to let it go (the diet, that is) and try a different weight loss approach.


Imagine filling your car’s tank with gallons of cooking oil instead of gasoline. Think it would go over well? Of course not. Neither—at first—does telling your body it’s got to find a whole new source of fuel to carry you through the day. That shock to your physical system can often carry over to your psychological state, resulting in irritability, malaise, and the classic “short fuse.”

That moody phase is common with most dieting strategies, not just keto, and generally disappears as your body adapts to a new way of eating. However, at least one study found that people on very low-carb, high-fat eating plans were more likely to still experience negative moods a year into the plan, especially when compared to those following a low-fat diet instead. It’s likely this side effect is less due to ketosis itself and is more a reflection of the emotional stress of trying to follow a highly restrictive eating plan for months at a time.

What Is the Difference Between Ketosis and Ketoacidosis?

Ketosis—the process of burning ketones from stored fat—is what you’re shooting for with the keto diet. But ketosis shouldn’t be confused with ketoacidosis, also called diabetic ketoacidosis, a dangerous condition seen mostly in people with type 1 diabetes in which a person doesn’t have the insulin necessary to process glucose. Left untreated, ketones build up in the bloodstream too quickly of type 1 diabetics, turning their blood acidic. This is known as ketoacidosis, a life-threatening condition that can lead to a coma or even death and needs immediate emergency care and additional insulin.

If you are concerned about ketoacidosis (an extremely rare occurrence in otherwise healthy people), these are the signs to look for:

  • extreme thirst

  • vomiting

  • confusion

  • shortness of breath

  • weakness

People with type 1 diabetes are much more susceptible to ketoacidosis than those with type 2, which is why the keto diet—and its hallmark elevated ketone levels—is usually not recommended for those with type 1 diabetes, except in cases of very close medical supervision. Although ketoacidosis is rare in someone with type 2 diabetes, you should always check with your doctor before changing your diet.

Think the Keto Diet May Be Right For You?
Tula Karras
Meet Our Writer
Tula Karras

Tula Karras is an award-winning journalist, editor, and content creator who specializes in health, nutrition, and profile writing. She has written for numerous print and online publications, including Self, Parents, Real Simple, National Geographic, Prevention, Cosmo, Consumer Reports, Health, Good Housekeeping and Yoga Journal. She has ghost-written two books and is based in Austin, Texas.