What Is Ketosis?

The keto diet is more popular than ever, but what exactly does it mean to be in ketosis? And, is it good for you?

The keto diet is the latest fad to hit the wellness world, and its adherents swear by the supposed benefits of the low-carb, high-fat eating style. "Keto" is short for ketosis, a metabolic process that involves burning fat instead of glucose (carbs) for energy. It sounds simple enough, but the process of getting into and staying in ketosis could be very difficult because there are foods that you can and cannot eat on a ketogenic diet. Thus, there are no proven health benefits for the vast majority of people to stay on this diet for long term. Read on to find out what ketosis actually is, how it happens, and whether or not it's healthy and sustainable.

What, when and how much you eat matters

Carbs are your body's preferred form of energy. There are a few different metabolic processes in place that can break down glucose molecules (what all carbs are made of) into energy that fuels the body's cells and functions. Which pathway your body chooses to use depends on what and when you last ate, and how much glucose is in your bloodstream versus how much is stored in other tissues (like your liver and your muscles, in the form of glycogen).

How does ketosis occur?

If you drastically cut back on eating carbs for a long enough period of time, your body will use up its stored glucose. And, since there's no new glucose coming in, your body is forced to find another way to fuel itself. This is when ketosis happens. Instead of breaking down glucose, your body will start breaking down stored fat and the fat you're eating as energy. When triglycerides (the molecules that make up fat) are broken down, some glucose is released and can be used for energy. In this process, ketones are released as a byproduct, and then eliminated in urine.

If you've ever heard of people on the keto diet peeing on an indicator strip to check that they're in ketosis, this is why—ketones are acids, so your urine is more acidic when you're in ketosis.

The side effects of ketosis

Unfortunately, those ketone bodies make their way to more than just your urine. Because of elevated levels of acetone (a type of ketone), one ongoing symptom of ketosis is bad, sour breath. Fatigue and impaired physical performance—in workouts, and also in daily life—are common when someone first goes into ketosis, although most people find that their energy comes back in a few weeks.

Of course, rapid weight loss is an early symptom of ketosis and the one that keeps people flocking to the keto diet. It's important to realize, though, that this initial weight loss isn't a fat loss—it's just water weight. Glycogen (the form glucose takes when it's stored in the body) binds water, so less glycogen means less water. Whether or not ketosis leads to long-term weight loss remains to be seen (more on that later).

sunny side up egg in a cast iron skillet
Getty Images / Bill Boch

Do you have what it takes to stay on a high-fat, low-carb diet long-term?

"When you eat a specific, long-term diet that is high in fat and low in carbs, your body goes into ketosis," says Amy Gorin, M.S., RDN, owner of Amy Gorin Nutrition in the New York City area. "It's important to note that once your body goes into ketosis, you need to consistently follow the ketogenic diet to keep your body in that state."

In other words, maintaining a specific weight long-term requires dedication and meticulous meal planning. Eating adequate carbs will immediately cause your body to revert back to using glucose for energy, and to stop breaking down fat and producing ketones.

But, OK—exactly how low-carb/high-fat do you need to eat?

"This varies from person to person based on a variety of factors, including activity level," Gorin says. "Typically, you'll be eating less than 50 grams of carbohydrates per day and about 60-80% of your daily calories will come from fat."

Most adherents eat between 75 and 80% of their calories from fat, 15 to 20% from protein, and less than 5% from carbs. For someone eating 2,000 calories a day, 5% is just 25 grams of carbs—about the amount in half a cup of rice. Needless to say, this is tough to do, and even tougher to maintain.

For the majority of people, there's no long-term benefit to following the keto diet.

Eating so few carbs is challenging. Carbs taste good and also make up a huge portion of the foods we eat. The type of carbs you choose makes a difference. Cutting them out, particularly the complex carbs and fruits, so drastically mean constantly reading food labels, cooking your own meals, and turning down classic treats.

There is substantial evidence that following a keto diet (and staying in ketosis) can help reduce seizures in children with epilepsy, but even researchers concede that many of these children stop following the diet because it's just too difficult. Researchers have also looked into the keto diet as a possible strategy for managing diabetes, but the results are very mixed. So, you may want to learn more about if keto is a good idea when you have diabetes.

While ketosis might help with blood sugar control, it often also leads to a higher risk for heart disease because of increased saturated fat consumption. Again, researchers point out that many people have a hard time sticking to the diet. And, while short-term weight loss is a common side effect of ketosis, studies show that it's generally not sustained over the long term. What's more, long-term ketosis has also been associated with dehydration and electrolyte imbalances.

Bottom line

Unless you're using the keto diet and ketosis to manage a medical condition (as advised by a doctor), there's probably not much benefit to making such drastic changes to your eating habits. Long-term weight loss is unlikely, and there are other nasty side effects like bad breath and potential electrolyte imbalances.

That said, if you decide that the keto diet is for you, learn more about how to start on the keto diet, including the pros and cons, with our ketogenic diet beginner's guide. More importantly, be sure to seek out expert guidance. "Because specific macronutrient breakdowns [for ketosis] vary from person to person, I'd recommend working with a registered dietitian nutritionist to create a specific plan," Gorin says. "You can find a dietitian using the "find an expert" tool at eatright.org."

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