When your body burns fat instead of glucose, this leads to elevated ketone levels, which is called ketosis. Symptoms can include changes in the way your breath smells, appetite, and energy levels.

The ketogenic, or keto, diet is a popular, effective way to lose weight and improve your health (1, 2).

When followed correctly, this low carb, high fat diet will raise your blood levels of ketones. Ketones are chemical byproducts that provide a new fuel source for your cells. They’re responsible for most of the unique health impact of the keto diet.

On a keto diet, your body undergoes many biological adaptations. This includes a reduction in insulin levels and the increased breakdown of fat (3, 4).

When this happens, your liver starts producing high numbers of ketones to supply energy for your brain. However, it can often be hard to know whether you’re in ketosis or not.

The keto diet should be avoided if you have certain medical conditions, including pancreatitis, liver failure, fat metabolism diroders and others (2, 5).

However, it is important to understand the difference between ketosis and ketoacidosis. The latter is a very serious condition that can affect people with diabetes.

Here are 10 common signs and symptoms of ketosis, both positive and negative.

1. Bad breath

Many people on keto and similar diets, like the Atkins diet, report that their breath takes on an unusual, fruity smell.

Bad breath is actually a common side effect of reaching full ketosis.

Elevated ketone levels cause the bad breath. The specific culprit is acetone, a ketone that exits the body in your urine and breath (6).

While this breath may be less than ideal for your social life, it can be a positive sign for your diet. Brushing your teeth several times per day or using sugar-free gum can resolve the issue. Staying hydrated by drinking a lot of water may also help.

Check the nutrition label for carbs if you’re using gum or other alternatives like sugar-free drinks. These products may raise your blood sugar levels and reduce your ketone levels.

2. Weight loss

Keto diets, along with other low carb diets, are highly effective for weight loss, though it should be noted that in one study, 8 participants dropped out because of safety concerns such as increased LDL cholesterol or increased creatinine (4, 7).

As many weight loss studies have shown, you may experience both short- and long-term weight loss when switching to a keto diet (4, 8).

Fast weight loss can occur during the first week. While some people believe this to be fat loss, it’s primarily water weight (4).

After the initial rapid drop in water weight, you should continue to lose body fat consistently as long as you stick to the diet and remain in a calorie deficit.

3. Increased ketones in the blood

One of the hallmarks of a keto diet is a reduction in blood sugar levels and an increase in ketones.

As you progress into a keto diet, you’ll start to burn fat and ketones as your main fuel sources.

The most reliable and accurate method of measuring ketosis is to measure your blood ketone levels using a specialized meter. This is the method is used in most research studies (9).

The meter measures your ketone levels by calculating the amount of beta-hydroxybutyrate (BHB) in your blood. This is one of the primary ketones present in the bloodstream.

According to some experts on the keto diet, nutritional ketosis is defined as blood ketones equal to or above 0.5 millimolar (mM) (10).

The main downside of this test method is that it requires pricking your finger to draw blood.

What’s more, test kits can be expensive. For this reason, most people will just perform one test per week or every other week.

4. Increased ketones in the breath

Another way to measure blood ketone levels is a breath analyzer.

It monitors acetone, one of the three main ketones in your blood during ketosis (6).

This gives you an idea of your body’s ketone levels since more acetone leaves the body when you’re in nutritional ketosis (11).

The use of acetone breath analyzers has been shown to be fairly accurate, though less accurate than the blood monitor method (12).

5. Appetite suppression

Many people report decreased hunger while following a keto diet.

The reasons why this happens are still being investigated.

However, it’s been suggested that this hunger reduction may be due to alterations to your body’s hunger hormones, along with an increased intake of proteins (13, 14).

The ketones themselves may also affect your brain, helping to reduce appetite (14) (15).

6. Increased focus and energy

People often report brain fog, tiredness, and feeling sick when starting a very low carb diet. This is termed the “low carb flu” or “keto flu” (16).

However, long-term keto dieters often report increased focus and energy.

When you start a low carb diet, your body must adapt to burning more fat for fuel instead of carbs.

When you get into ketosis, a large part of the brain starts burning ketones instead of glucose. It can take a few days or weeks for this to start working properly (17).

Ketones are an extremely potent fuel source for your brain. They have even been tested in a medical setting to treat brain diseases and conditions such as concussion and memory loss (18, 19, 20, 21).

Eliminating carbs can also help control and stabilize blood sugar levels. This may further increase focus and improve brain function (22, 23).

7. Short-term fatigue

The initial switch to a keto diet can be one of the biggest issues for new dieters. Well-known side effects can include weakness and fatigue (16).

These often cause people to quit the diet before they get into full ketosis and reap many of the long-term benefits.

These side effects are natural. After several decades of running on a carb-heavy fuel system, your body is forced to adapt to a different system.

As you might expect, this switch doesn’t happen overnight.

To reduce fatigue during this switch, you may want to take electrolyte supplements. If you do, be sure to read the labels on any electrolyte supplements since they may contain added carb content.

Electrolytes are often lost because of the rapid reduction in your body’s water content (4).

When adding these supplements, make sure to include sodium, potassium, and magnesium.

8. Short-term decreases in performance

As discussed above, removing carbs can lead to general tiredness at first. This includes an initial decrease in exercise performance.

It’s primarily caused by the reduction in your muscles’ glycogen stores. They provide the main and most efficient fuel source for all forms of high intensity exercise.

After several weeks, many keto dieters report that their performance returns to normal. In certain types of ultra-endurance sports and events, a keto diet could even be beneficial (24).

What’s more, there are further benefits — primarily an increased ability to burn more fat during exercise.

One small yet famous study found that athletes who had switched to a keto diet burned as much as 230% more fat when they exercised compared to athletes who weren’t following this diet (25).

While it’s unlikely that a keto diet can maximize performance for elite athletes, once you become fat-adapted, it should be sufficient for general exercise and recreational sports (26).

9. Digestive issues

A keto diet generally involves a major change in the foods you eat.

Digestive issues such as constipation and diarrhea are common side effects in the beginning (17).

Some of these issues should subside after the transition period, but it may be important to be mindful of different foods that may be causing digestive issues.

Also, make sure to eat plenty of healthy low carb veggies, which still contain plenty of fiber.

Most importantly, don’t make the mistake of eating a diet that lacks diversity. Doing so may increase your risk of digestive issues and nutrient deficiencies.

10. Insomnia

One big issue for many keto dieters is sleep, especially when they first change their diet (17).

A lot of people report insomnia or waking up at night when they first reduce their carbs drastically.

However, this usually improves in a matter of weeks.

Many long-term keto dieters claim that they sleep better than before after adapting to the diet (27).

The bottom line

Several key signs and symptoms can help you identify whether you’re in ketosis.

Ultimately, if you are following the guidelines of a keto diet and staying consistent, you should be in some form of ketosis.

If you want a more accurate assessment, monitor ketone levels in your blood or breath every week. Be sure to discuss with a medical provider before starting the keto diet as there are some medical conditions where it should be avoided.

That said, if you’re losing weight, enjoying your keto diet, and feeling healthier, there is no need to worry about your ketone levels.