Because the keto diet is a low-carb diet, your body may enter ketosis naturally. Symptoms may be similar to that of the flu. But you may be wondering if ketosis is safe for everyone.

A ketogenic diet induces a state called ketosis. This is different from ketoacidosis, a serious condition that can happen when a person is unable to manage diabetes.

Ketosis is a natural metabolic state that may have benefits for weight loss (1, 2).

It may also have therapeutic effects for people with epilepsy, type 2 diabetes, and other chronic conditions (3, 4, 5, 6).

Ketosis is likely safe for most people, especially if they follow it with a doctor’s supervision.

However, it can have some negative effects, especially at the start. It’s also unclear how a ketogenic diet may affect the body long term (7).

First, it’s necessary to understand what ketosis is.

Ketosis is a natural part of metabolism. It happens either when carbohydrate intake is very low (such as on a ketogenic diet) or when you haven’t eaten for a long time.

When this happens, insulin levels fall and the body releases fat to provide energy. This fat then enters the liver, which turns some of it into ketones.

During ketosis, many parts of your body are burning ketones for energy instead of just carbs. This includes your brain and muscles.

However, it takes your body and brain some time to “adapt” to burning fat and ketones instead of carbs.

During this adaptation phase, you may experience some temporary side effects.

Summary: In ketosis, parts of the body and brain use ketones for fuel instead of carbs. It can take some time for your body to adapt to this.

In the beginning of ketosis, you may experience a range of negative symptoms.

People often call these the “low carb flu” or “keto flu” because they resemble symptoms of the flu.

These may include:

  • headache
  • fatigue
  • brain fog
  • increased hunger
  • poor sleep
  • nausea
  • decreased physical performance (8)

These issues may discourage people from continuing to follow a ketogenic diet before they start noticing the benefits.

However, the “low carb flu” is usually over within a few days.

Summary: The “low carb flu” or “keto flu” is a set of symptoms that can occur in the initial stages of ketosis. While it may cause some people to discontinue the diet, it’s usually over in a short amount of time.

One of the more common side effects of ketosis is bad breath, often described as fruity and slightly sweet.

It’s caused by acetone, a ketone that’s a byproduct of fat metabolism.

Blood acetone levels rise during ketosis, and your body gets rid of some of it via your breath (9).

Occasionally, sweat and urine can also start to smell like acetone.

Acetone has a distinctive smell — it’s the chemical that gives nail polish remover its pungent odor.

For most people, this unusual-smelling breath will go away within a few weeks.

Summary: In ketosis, your breath, sweat, and urine may smell like acetone. This ketone is produced by the liver from fat and increases on a ketogenic diet.

In ketosis, some people may experience leg cramps. These can be painful, and they can be a sign that you need to drink more water.

Leg cramps in ketosis usually stem from dehydration and loss of minerals. This is because ketosis causes a reduction in water weight.

Glycogen, the storage form of glucose in muscles and liver, binds water.

This gets flushed out when you reduce carb intake. It’s one of the main reasons why people lose weight rapidly in the first week of a very low carb diet.

It’s important to continue to drink plenty of water to reduce the risk of dehydration, changes in electrolyte balance, and kidney problems (7).

Summary: Some people may experience muscle cramps in ketosis. Loss of water and minerals increases your risk of leg cramps.

Dietary changes can sometimes lead to digestive issues.

This is also true for ketogenic diets, and constipation is a common side effect in the beginning (10).

This is most commonly due to not eating enough fiber and not drinking enough fluids.

Some people may also get diarrhea, but it’s less common.

If the switch to a keto diet dramatically changes the way you eat, you’re more likely to have digestive symptoms.

Nevertheless, digestive issues are usually over within a few weeks.

Summary: Constipation is a very common side effect of ketosis. Diarrhea may also occur in some people.

Some people also experience increased heart rate as a side effect of ketosis.

This is also called heart palpitations or a racing heart. It can happen during the first few weeks of a ketogenic diet.

Being dehydrated is a common cause, as well as low salt intake. Drinking a lot of coffee might also contribute to this.

If the problem doesn’t stop, you might need to increase your carb intake.

Summary: A ketogenic diet can increase heart rate in some people, but staying hydrated and increasing your salt intake may help.

Other, less common side effects may include:

  • Ketoacidosis. A few cases of ketoacidosis (a serious condition that occurs in diabetes when it’s not properly managed) have been reported in breastfeeding women, likely triggered by a very low carb diet. However, this is rare (11, 12, 13).
  • Kidney stones. Although uncommon, some children with epilepsy have developed kidney stones on a ketogenic diet. Experts recommend regular kidney function monitoring while following the diet. (10, 14, 15, 16, 7).
  • Raised cholesterol levels. Some people get increased total and LDL (bad) cholesterol levels (17, 18, 19).
  • Fatty liver. This can develop if you follow the diet for a long time.
  • Hypoglycemia. If you use medications to manage your blood sugar levels, speak to a doctor before starting the diet, as they may need to adjust the dose.

Some of the negative effects, such as dehydration and low blood sugar can lead to emergency room visits (7).

The keto diet is not suitable for people with a number of conditions, including:

  • pancreatitis
  • liver failure
  • carnitine deficiency
  • porphyria
  • disorders that affect the way their body processes fat

Summary: Less common side effects include kidney stones high cholesterol levels.

Here’s how to minimize the potential side effects of ketosis:

  • Drink plenty of water. Consume at least 68 ounces (2 liters) of water a day. A significant amount of weight lost in ketosis is water, especially in the beginning.
  • Get enough salt. The body excretes sodium in large amounts when carb intake is low. Ask your doctor if you should be adding salt to your food.
  • Increase mineral intake. Foods high in magnesium and potassium may help relieve leg cramps.
  • Avoid intense exercise. Stick to moderate levels of exercise in the first week or two.
  • Try a low carb diet first. This might help you reduce your carbs to a moderate amount before moving onto a ketogenic (very low carb) diet.
  • Eat fiber. A low carb diet is not a no-carb one. Ketosis typically starts when your carb intake is less than 50 grams a day. Eat fiber-rich foods like nuts, seeds, berries, and low carb veggies (7).

Summary: There are a few ways to minimize the negative symptoms of ketosis. These include drinking enough water and eating foods rich in fiber and minerals.

Click here for more tips on how to stay safe while following the keto diet.

A ketogenic diet may benefit some people, such as those with obesity or type 2 diabetes and children with epilepsy.

However, it can cause some side effects, including the “low carb flu,” leg cramps, bad breath, and digestive issues, especially in the first few days or weeks.

Experts also note that, while the diet can help you lose weight in the short term, the weight can return when you stop the diet. Many people don’t manage to stick with the diet (7).

Finally, a keto diet may not suit everyone. Some people experience significant benefits, while others feel and perform better on a higher carb diet.

People who are thinking of starting a keto diet should first talk to a healthcare provider who can help them decide if it’s a good option for them.

A medical professional can also help you follow the diet safely to minimize the risk of adverse effects.

Summary: A keto diet may be safe and helpful for some people, but you should check with your doctor before starting this diet.

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