Intermittent fasting may offer health benefits for some people. But it can also lead to greater hunger, headaches, fatigue, and mood issues. It could also cause malnutrition if you don’t do it correctly.

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Intermittent fasting is several patterns of eating that include regular periods of fasting. Fasting involves eating and drinking very few or no calories.

There have been a number of health benefits linked to intermittent fasting, including weight loss, improved insulin sensitivity and blood sugar, lowered blood pressure, a decreased risk for heart disease, and less stress.

However, intermittent fasting is not for everyone. There are potential side effects of this type of eating style.

This article covers 9 potential side effects related to intermittent fasting.

Research has linked intermittent fasting to several health benefits, including:

These findings have led to an increased popularity of certain types of intermittent fasting, like:

It’s not that surprising that hunger is one of the most common side effects related to intermittent fasting.

When you reduce your calorie intake or go long periods without taking in calories, you may experience increased hunger.

In a 2018 study involving 112 people, researchers assigned some participants to an intermittent energy restriction group. Based on gender, each person consumed 400 or 600 calories on two nonconsecutive days every week for a year.

These groups reported higher hunger scores than the group who consumed a low calorie diet with continuous calorie restriction.

Hunger is a side effect many people may experience during the first days of a fasting regimen.

A 2020 study looked at 1,422 people who participated in fasting regimens lasting 4–21 days. Participants tended to have hunger symptoms only during the first few days of the regimens.

So, side effects like hunger may resolve as your body adapts to regular fasting periods.

Headaches are a possible side effect of any type of fasting. They typically occur during the first few days of a fasting period as your body is getting used to the new eating style.

A 2021 review of studies looked at 18 studies of people undergoing intermittent fasting regimens. In the four studies that reported side effects, some participants said they had mild headaches.

Research from 2010 has also found that “fasting headaches” are usually located in the frontal region of the brain and that the pain is typically mild or moderate in intensity. What’s more, people who commonly get headaches are more likely to experience headaches during fasting than those who don’t.

These researchers also suggest that low blood sugar and caffeine withdrawal may contribute to headaches during intermittent fasting.

Digestive issues — including indigestion, diarrhea, nausea, and bloating — are side effects you might experience if you do intermittent fasting.

During intermittent fasting, reducing the amount of food you eat may negatively affect your digestion, causing certain side effects. Plus, the changes in your diet that occur with intermittent fasting may cause bloating and diarrhea for some people.

Choosing nutrient-dense foods rich in fiber may help prevent these digestive issues.

Some people may experience irritability and other mood disturbances when they practice intermittent fasting as a result of low blood sugar.

Low blood sugar, or hypoglycemia, can occur during periods of calorie restriction or over periods of fasting. This can lead to:

In a 2016 study with 52 women, researchers found that participants were significantly more irritable during an 18-hour fasting period than they were during a nonfasting period.

But while the participants were more irritable, they also had a higher sense of achievement, pride, and self-control at the end of the fasting period than they reported at the start of fasting.

Research suggests that some people practicing various methods of intermittent fasting experience fatigue and low energy levels.

Low blood sugar levels can cause you to feel tired and weak. Plus, intermittent fasting may lead to sleep disturbances in some people, which can cause tiredness during the day.

However, a 2020 study found that intermittent fasting may actually reduce fatigue — especially as your body becomes adjusted to regular fasting periods.

Bad breath is an unpleasant side effect that may occur in some people during intermittent fasting. This is caused by a lack of salivary flow and the rise of acetone in the breath.

Fasting causes your body to use fat for fuel. Acetone is a byproduct of fat metabolism, so it increases in your blood and breath during fasting.

What’s more, dehydration — a symptom associated with intermittent fasting — can cause dry mouth, which may lead to bad breath.

Some believe that sleep disturbances, such as being unable to fall asleep or stay asleep, are among the most common side effects related to intermittent fasting.

This 2019 study observed 1,422 people who did fasting regimens for 4–21 days. Researchers found that 15% of participants reported sleep disturbances related to fasting. Out of all other side effects, this was reported most frequent.

Fatigue may be more common in the initial days of intermittent fasting, as your body releases large amounts of salt and water through urine. This can lead to dehydration and low salt levels, too.

However, other studies suggest that intermittent fasting has no effect on sleep.

In a small 2021 study, researchers looked at 31 people with obesity who participated in an alternate-day fasting regimen. They also followed a low carb diet for 6 months. The researchers found that this regimen did not affect sleep quality, duration, or insomnia severity.

During the initial days of fasting, the body releases large amounts of water and salt in urine. This process is known as natural diuresis or natriuresis of fasting.

If this happens to you and you don’t replace the fluids and electrolytes you lost, you could become dehydrated.

People practicing intermittent fasting may also forget to drink or may not drink enough. This can be especially common when you’re first beginning an intermittent fasting regimen.

To stay properly hydrated, drink water throughout the day and monitor the color of your urine. Ideally, it should be a pale lemonade color. Dark-colored urine may be a sign that you’re dehydrated.

If not done properly, intermittent fasting can lead to malnutrition.

If you do very long fasting periods and don’t replenish your body with enough nutrients, you could become malnourished. The same goes for if you don’t correctly manage a restricted calorie diet.

People are generally able to meet their calorie and nutrient needs on various types of intermittent fasting programs.

However, if you don’t plan or practice your fasting program carefully over a long time period or you deliberately restrict calories to an extreme level, you might become malnourished, along with other health complications.

That’s why it’s important to consume a well-rounded, nutritious diet while intermittent fasting. Make sure you never overly restrict your calorie intake.

A healthcare professional who’s experienced in intermittent fasting can help you come up with a safe plan that provides an appropriate number of calories and the right amounts of nutrients for you.

Although intermittent fasting may be a good choice for some, it’s not appropriate or safe for everyone.

Some people may have a higher chance of developing dangerous side effects if they do intermittent fasting.

Healthcare professionals generally advise that the following people avoid intermittent fasting:

  • people who are pregnant or nursing
  • young children and teens
  • older adults who experience weakness
  • people with immunodeficiencies
  • people with a current or past eating disorder
  • people with dementia
  • anyone with a history of traumatic brain injury or post-concussive syndrome

This list is not exhaustive and there are exceptions. For example, healthcare professionals have used fasting to treat epilepsy in children.

If you have a medical condition or are currently taking medications, it’s important to discuss the benefits and risks of intermittent fasting with a trusted healthcare professional.

Certain people may be more at risk of adverse side effects related to fasting, so it’s important to determine whether intermittent fasting is the safe choice for your specific needs.

Additionally, if you experience prolonged side effects when practicing intermittent fasting, this may be a sign that it isn’t working for your body. Side effects could include:

  • extreme hunger
  • nausea
  • irritability
  • headaches
  • fatigue
  • faintness

Don’t continue intermittent fasting if the program makes you feel miserable.

Even though this way of eating has been tied to health benefits, there are many other things you can do to benefit your health that don’t involve fasting.

Follow a balanced and nutritious diet, get proper sleep, engage in regular physical activity, and manage stress — these are much more important for promoting overall health.

Research has linked intermittent fasting to many health benefits — including weight loss, increased blood sugar control, reduced heart disease risk factors, and more.

But side effects of fasting can include hunger, irritability, headaches, and bad breath.

You may want to reach out to your healthcare team before starting intermittent fasting and overhauling your diet, especially if you’re pregnant, live with an eating disorder, or have any health conditions that affect your eating patterns.