The ketogenic diet is a popular, effective way to lose weight and improve your health.

When followed correctly, this low-carb, high-fat diet will raise blood ketone levels.

These provide a new fuel source for your cells and cause most of the unique health benefits of this diet (, , ).

On a ketogenic diet, your body undergoes many biological adaptations, including a reduction in insulin levels and increased fat breakdown.

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.

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

People often report bad breath once they reach full ketosis.

It's actually a common side effect. Many people on ketogenic diets and similar diets, such as the Atkins diet, report that their breath takes on a fruity smell.

This is caused by elevated ketone levels. The specific culprit is acetone, a ketone that exits the body in your urine and breath ().

While this breath may be less than ideal for your social life, it can be a positive sign for your diet. Many ketogenic dieters brush their teeth several times per day or use sugar-free gum to solve the issue.

If you're using gum or other alternatives like sugar-free drinks, check the label for carbs. These may raise your blood sugar levels and reduce ketone levels.

Summary The ketone acetone is partly expelled via your breath, which can cause bad or fruity-smelling breath on a ketogenic diet.

Ketogenic diets, along with normal low-carb diets, are highly effective for weight loss (, ).

As dozens of weight loss studies have shown, you will likely experience both short- and long-term weight loss when switching to a ketogenic diet (, ).

Fast weight loss can occur during the first week. While some people believe this to be fat loss, it’s primarily stored carbs and water being used up ().

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.

Summary Fast weight loss commonly occurs when you start a ketogenic diet and severely restrict carbohydrates.

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

As you progress further into a ketogenic diet, you will start to burn fat and ketones as the main fuel sources.

The most reliable and accurate method of measuring ketosis is to measure your blood ketone levels using a specialized meter.

It 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 ketogenic diet, nutritional ketosis is defined as blood ketones ranging from 0.5–3.0 mmol/L.

Measuring ketones in your blood is the most accurate way of testing and is used in most research studies. However, the main downside is that it requires a small pinprick to draw blood from your finger ().

What’s more, a test kit costs around $30–$40, plus an additional $5 per test. For this reason, most people will just perform one test per week or every other week.

Summary Testing blood ketone levels with a monitor is the most accurate way to determine whether you’re in ketosis.

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

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

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

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

Another good technique is to measure the presence of ketones in your urine on a daily basis with special indicator strips.

These also measure ketone excretion through the urine and can be a quick and cheap method to assess your ketone levels each day. However, they’re not considered very reliable.

Summary You can measure your ketone levels with a breath analyzer or urine strips. However, they’re not as accurate as a blood monitor.

Many people report decreased hunger while following a ketogenic diet.

The reasons why this happens are still being investigated.

However, it's been suggested that this hunger reduction may be due to an increased protein and vegetable intake, along with alterations to your body's hunger hormones ().

The ketones themselves may also affect your brain to reduce appetite ().

Summary A ketogenic diet can significantly reduce appetite and hunger. If you feel full and don't need to eat as often as before, then you may be in ketosis.

People often report brain fog, tiredness and feeling sick when first starting a very low-carb diet. This is termed the "low carb flu" or "keto flu." However, long-term ketogenic 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.

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 (, , ).

Therefore, it comes as no surprise that long-term ketogenic dieters often report increased clarity and improved brain function (, ).

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

Summary Many long-term ketogenic dieters report improved brain function and more stable energy levels, likely due to the rise in ketones and more stable blood sugar levels.

The initial switch to a ketogenic diet can be one of the biggest issues for new dieters. Its well-known side effects can include weakness and fatigue.

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. It generally requires 7–30 days before you are in full ketosis.

To reduce fatigue during this switch, you may want to take electrolyte supplements.

Electrolytes are often lost because of the rapid reduction in your body's water content and the elimination of processed foods that may contain added salt.

When adding these supplements, try to get 2,000–4,000 mg of sodium, 1,000 mg of potassium and 300 mg of magnesium per day.

Summary Initially, you may suffer from tiredness and low energy. This will pass once your body becomes adapted to running on fat and ketones.

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, which provide the main and most efficient fuel source for all forms of high-intensity exercise.

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

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

One famous study found that athletes who had switched to a ketogenic diet burned as much as 230% more fat when they exercised, compared to athletes who were not following this diet ().

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

Summary Short-term decreases in performance can occur. However, they tend to improve again after the initial adaptation phase is over.

A ketogenic diet generally involves a major change in the types of foods you eat.

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

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 are low in carbs but still contain plenty of fiber.

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

To help plan your diet, you may want to check out 16 Foods to Eat on a Ketogenic Diet.

Summary You might experience digestive issues, such as constipation or diarrhea, when you first switch to a ketogenic diet.

One big issue for many ketogenic dieters is sleep, especially when they first change their diet.

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 ketogenic dieters claim that they sleep better than before after adapting to the diet.

Summary Poor sleep and insomnia are common symptoms during the initial stages of ketosis. This usually improves after a few weeks.

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

Ultimately, if you’re following the guidelines of a ketogenic diet and stay consistent, you should be in some form of ketosis.

If you want a more accurate assessment, monitor ketone levels in your blood, urine or breath on a weekly basis.

That being said, if you’re losing weight, enjoying your ketogenic diet and feeling healthier, there is no need to obsess over your ketone levels.