So, Where Did We Land on Ketosis?

You've likely heard about the keto diet (it was all the range in 2019), which involves eating fat — and lots of it — while limiting carbs. By doing so, your body, which typically turns carbohydrates into glucose for energy, has to pivot. The body starts to burn fat for energy instead, producing ketones, in a process called ketosis. Sound familiar? Makes sense since there's no shortage of keto fans out there.

The diet is still popular today, with plenty of content creators touting the benefits of going into ketosis. Still, it can be difficult to wrap your head around what really happens during ketosis — and whether or not it's safe for your body. Ahead, we tapped top experts to weigh in on everything you need to know about ketosis before hopping on the bandwagon.

Experts Featured in This Article:

Beth Zupec-Kania, RDN, CD, the owner of Ketogenic Therapies.

Abigail Collen, MS, RD, CDN, is a dietitian at Mount Sinai Health System.

Kristin Kirkpatrick, RD, is a dietitian at Cleveland Clinic Department of Wellness and Preventive Medicine.

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

Shayonne Morrison, MS, RD, LD, a registered dietitian at Nourish.

Julie Stefanski, RDN, is a spokesperson for the Academy of Nutrition and Dietetics.

What Is Ketosis?

''Nutritional ketosis is a metabolic state in which the body primarily uses ketones, a byproduct of fat metabolism, as its primary source of energy instead of glucose," says Beth Zupec-Kania, RDN, CD, the owner of Ketogenic Therapies. This occurs when carbohydrates aren't available and the body starts to rely on fat instead, resulting in the production of ketones (acids that are produced in the liver when the fats are broken down). It's worth noting that our bodies naturally produce ketones when we sleep or haven't eaten for a long time; once we eat, ketone levels decrease. When you're following a ketogenic diet, the goal is to remain in a state of ketosis longer.

And how fast your body gets into ketosis depends on many factors, including age, gender, health status, and physical activity levels. In one recent study of about 1600 people, more than 95 percent had elevated ketone levels (a sign of ketosis, more on that point in a moment) starting on day four.

That said, the only way to know for sure that you're in ketosis is via an at-home blood or urine test. Both of these methods are FDA-approved for ketone testing. Breath tests are also available, but aren't not as accurate as urine or blood tests (and also aren't FDA-approved). According to Stefanski, breath testing of ketones only detects acetone, the cause of the sweet nail polish smell that is sometimes present when there are high levels of ketones in the blood. Since this is a newer form of testing ketone bodies, there is less research on the ideal level of ketones exhaled.

What Are the Side Effects of Ketosis?

There are several side effects that signal your body is in ketosis, from the famous keto flu to other symptoms such as constipation, fatigue, and bad breath (also called keto breath). Here's a full breakdown:

The good news? These irksome symptoms are typically temporary. ''Most people will feel better after a few days, but some people complain of continued fatigue and may abandon strict carbohydrate limits due to lack of energy," Stefanski points out.

So, Is Going Into Ketosis Actually Good For You?

The benefits of being in ketosis can vary from person to person. But here are a few of the potential perks:

May result in weight loss: Ketogenic diets can result in weight loss and/or muscle loss, though the former may not be sustained. One study followed participants after a six-month weight-loss period on either a ketogenic diet or a low glycemic index diet. The keto group lost more weight initially, but during the 18-month follow-up, the low glycemic index group continued to lose weight, while the keto group regained some weight.

May improve blood glucose and cholesterol: "There have been several controlled studies which showed beneficial impacts [of ketosis] on lowering blood sugar levels, insulin resistance, and LDL cholesterol levels," says Stefanski. The ketogenic diet has been also shown to improve some lipid levels, including HDL (also known as good cholesterol) and triglycerides in people with type 2 diabetes. ''As a registered dietitian working with patients using a ketogenic diet, I was surprised at the changes in lipid levels in the blood for some individuals. Some patients did have a rise in their cholesterol levels, but others did not," Stefanski says. "Monitoring lab work is really the key to see if eating an extremely high fat diet will impact your own personal lab values." That said, there's also research showing that the ketogenic diet can negatively impact blood LDL (AKA bad cholesterol) levels. At the end of the day, more studies and individualized approaches are needed to determine the overall effects of high-fat diets on improving blood lipid levels and chronic diseases.

May benefit polycystic ovary syndrome (PCOS): A 2023 study found that a short-term practice of the ketogenic diet can help improve hormonal imbalances associated with PCOS. Improvements in body weight, blood glucose, and hormone levels can be positive contributing factors.

May help with neurodegenerative diseases. The ketogenic diet has been shown to improve the mental state of people with Alzheimer's disease. But since these types of neurodegenerative diseases are generally diagnosed later in life, these individuals are more likely to have chronic diseases and maintaining adequate nutrition and malnutrition can be crucial. Therefore, the suitability of the ketogenic diet should be assessed by healthcare professionals for not only cognitive benefits but also its effects on overall health, Stefanski says.

May boost athletic performance: The ketogenic diet and ketone supplements are also used in sports nutrition, especially in endurance sports. This is because burning fatty acids for energy is more advantageous in aerobic endurance sports since it's not as limited as glycogen (a glucose reserve) stored in the liver and muscle.

Risks of Ketosis

There are just as many risks of going into ketosis as there are benefits, per the experts. The ketogenic diet can cause long-term conditions such as:

Also worth noting: people with diabetes should be extremely cautious about ketogenic diets. Those with type 1 diabetes are more at risk of developing ketoacidosis, a high-risk condition caused by low glucose levels and altered acid-base balance, Gabrielle Gambino, a senior clinical dietitian of Advanced Heart Failure Service at NewYork-Presbyterian, previously told PS.

The Bottom Line on Ketosis

Ask a healthcare professional to weigh in before ditching the carbs. Ketosis can be effective for weight loss, but it doesn't come without some (potentially serious) pitfalls. "Ketogenic diets can be restrictive and expensive. While many people find success with the keto diet, it is not sustainable long-term for most," says Shavonne Morrison, MS, RD, LD, a registered dietitian at Nourish. Anyone considering going keto should talk with a doctor or dietitian beforehand, and it's especially imperative for diabetics to speak with a physician or endocrinologist in order to ensure it's safe for them, she adds.

Merve Ceylan is a dietitian and freelance writer with published scientific articles in peer-reviewed journals. She has expertise in nutrition and health and has written more than 100 articles for well-known outlets such as PS, Health.com, Signos, Healthnews, and WellTheory. She writes with a passion for making science-based health information accessible to the public.