Medically Reviewed by Minesh Khatri, MD on June 19, 2022
4 min read

Ketosis is a process that happens when your body doesn't have enough carbohydrates to burn for energy. Instead, it burns fat and makes things called ketones, which it can use for fuel.

Ketosis is a word you'll probably see when you're looking for information on diabetes or weight loss. Is it a good thing or a bad thing? That depends.

Ketosis is a popular low-carb weight loss program. In addition to helping you burn fat, ketosis can make you feel less hungry. It also helps you keep muscle.

For healthy people who don't have diabetes and aren't pregnant, ketosis usually kicks in after 3 or 4 days of eating fewer than 50 grams of carbohydrates per day. That's about three slices of bread, a cup of low-fat fruit yogurt, or two small bananas. You can start ketosis by fasting, too.

A diet high in fat and protein but very low in carbs is called a ketogenic or “keto” diet.

Ketosis can have some benefits beyond weight loss. Doctors may put children who have epilepsy on a keto diet because it can help prevent seizures. Adults who have epilepsy sometimes eat modified Atkins diets.

Some research suggests that ketogenic diets might help lower your risk of heart disease. Other studies show that specific diets very low in carbs help people who have diseases such as:

Researchers are also studying the effects of these diets on conditions including:

  • Acne
  • Cancer
  • Polycystic ovary syndrome (PCOS)
  • Nervous system diseases like Alzheimer's, Parkinson's, and Lou Gehrig's disease

During the first week of a keto diet, you might start to feel bad. Some people call this the "keto flu," but it isn’t an official medical condition. Some doctors think this is due to sugar and carbohydrate withdrawal. Or it could be because of changes to your gut bacteria or an immune system reaction. You might notice temporary side effects such as:

  • Headache
  • Fatigue
  • Brain fog
  • Irritability
  • Constipation
  • Trouble sleeping
  • Nausea
  • Stomachache
  • Dizziness
  • Sugar cravings
  • Cramps
  • Sore muscles
  • Bad breath, also known as ketosis breath

Drinking plenty of water can ease or prevent some of these symptoms.

About 1 in 20 children who have epilepsy and are on the keto diet get kidney stones. A supplement called potassium citrate can help prevent them. Talk to your doctor about the kidney stone risk if your child is on the keto diet.

If you’re a new mom and don’t get enough calories or fluids, it could affect your breast milk supply. Doctors generally recommend that you wait until you’re done breastfeeding if you want to start the keto diet.

Some over-the-counter supplements claim to raise your ketone levels. They come as pills, powders, oils, and other forms. There isn’t much research on whether they work or if they’re safe. Talk to your doctor before you start taking any supplements.

If you're healthy and eating a balanced diet, your body controls how much fat it burns so you don't usually make or use ketones. But when you cut way back on your calories or carbs, your body will switch to ketosis for energy. It can also happen after you exercise for a long time and during pregnancy.

If you have diabetes that isn’t under control, ketosis can become dangerous when ketones build up. High levels lead to dehydration and change the chemical balance of your blood. It becomes acidic and can cause a coma or death.

People who have diabetes can get ketoacidosis, or diabetic ketoacidosis (DKA), when they don't take enough insulin. They can also get DKA when they're sick or injured or when they don't get enough fluids and become dehydrated.

Some people who don’t have diabetes can get ketoacidosis, too. It's caused by alcoholism, starvation, or an overactive thyroid. A healthy low-carb diet shouldn't cause a problem.

If you have these symptoms, call your doctor:

  • Thirstiness or a dry mouth
  • Peeing a lot
  • Feeling tired
  • Dry or flushed skin
  • Upset stomach
  • Throwing up
  • Trouble breathing
  • Confusion
  • Fruity-smelling breath
  • Pain in your belly

When you have diabetes, throwing up can be especially dangerous. Although DKA usually starts off slowly, throwing up can speed up the process so that it happens in just a few hours. Call your doctor if you've been throwing up for 2 hours or more.

You can find out how much ketosis is going on in your body by testing for ketones in your blood or urine. You can buy test strips to check your pee at home. Some blood sugar meters can measure ketones in your blood.

If you don't know how and when to test your ketones, talk to your doctor or diabetes instructor. High levels of ketones are dangerous.