May 7, 2021

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9 Potential Intermittent Fasting Side Effects

7 min read

Intermittent fasting is a term people use to describe patterns of eating that include regular periods of fasting in which they consume very few or no calories (1).

Studies have linked intermittent fasting to a number of health benefits, including (1, 2):

  • weight loss
  • decreased risk factors of heart disease
  • lower blood pressure
  • improved insulin sensitivity
  • reduction in markers of oxidative stress
  • improved blood sugar control

These findings have led to the increased popularity of intermittent fasting regimens like:

  • time-restricted feeding (TRF)
  • alternate-day fasting (ADF)
  • periodic fasting

If you’re interested in trying out intermittent fasting, you’re probably curious to know whether it has side effects.

The short answer: Intermittent fasting is safe for most people. However, studies have shown that intermittent fasting does have some minor side effects. Plus, it’s not the right choice for everyone (3).

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

It may be no surprise 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.

A study including 112 people assigned some participants to an intermittent energy restriction group. They consumed 400 or 600 calories on 2 nonconsecutive days every week for 1 year.

These groups reported higher hunger scores than those who consumed a low calorie diet with continuous calorie restriction (4).

Studies suggest that hunger is a symptom people typically experience during the first days of a fasting regimen.

One 2020 study looked at 1,422 people who participated in fasting regimens lasting 4–21 days. They tended to experience hunger symptoms only during the first few days of the regimens (3).

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

Headaches are a common side effect of intermittent fasting. They typically occur during the first few days of a fasting protocol (5).

A 2020 review 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 (6).

Interestingly, researchers have 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 (7).

What’s more, people who commonly get headaches are more likely to experience headaches during fasting than those who don’t (7).

Researchers have suggested that low blood sugar and caffeine withdrawal may contribute to headaches during intermittent fasting (7).

Digestive issues — including constipation, diarrhea, nausea, and bloating — are symptoms you might experience if you do intermittent fasting (3, 5).

The reduction in food intake that comes along with some intermittent fasting regimens may negatively affect your digestion, causing constipation and other side effects. Plus, changes in diet associated with intermittent fasting programs may cause bloating and diarrhea (8).

Dehydration, another common side effect related to intermittent fasting, can worsen constipation. For this reason, it’s essential to stay properly hydrated while practicing intermittent fasting.

Choosing nutrient-dense foods rich in fiber may also help prevent constipation.

Some people may experience irritability and other mood disturbances when they practice intermittent fasting. When your blood sugar is low, it may cause you to feel irritated (9).

Low blood sugar, or hypoglycemia, can occur during periods of calorie restriction or over periods of fasting. This can lead to irritability, anxiety, and poor concentration (8, 10).

A 2016 study in 52 women found that participants were significantly more irritable during an 18-hour fasting period than they were during a nonfasting period (10).

Interestingly, the researchers found that, although the women were more irritable, they also experienced a higher sense of achievement, pride, and self-control at the end of the fasting period than they reported at the start of fasting (10).

Studies show that some people practicing various methods of intermittent fasting experience fatigue and low energy levels (11, 12).

Low blood sugar related to intermittent fasting 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, some studies show that intermittent fasting can actually reduce fatigue, especially as your body becomes adapted to regular fasting periods (13).

Bad breath is an unpleasant side effect that can occur in some people during intermittent fasting. This is caused by lack of salivary flow and the rise of acetone in the breath (14, 15, 16).

Fasting causes your body to use fat for fuel. Acetone is a by-product of fat metabolism, so it increases in your blood and breath during fasting (17).

What’s more, dehydration — a symptom associated with intermittent fasting — can cause dry mouth, which may lead to bad breath (18).

Some research suggests that sleep disturbances, such as being unable to fall asleep or stay asleep, are among the most common side effects related to intermittent fasting (19).

A 2020 study observed 1,422 people who participated in fasting regimens lasting 4–21 days. The study found that 15% of participants reported sleep disturbances related to fasting. They reported this more frequently than other side effects (3).

Fatigue may be more common in the initial days of an intermittent fasting regimen as your body excretes large amounts of salt and water through the urine. This can lead to dehydration and low salt levels, too (19).

However, other studies have shown that intermittent fasting had no effect on sleep.

A 2021 study looked at 31 people with obesity who participated in an alternate day fasting regimen while also following a low-carb diet for 6 months. The study found that this regimen did not affect sleep quality or duration, or insomnia severity (20).

Another 2021 study had similar results (21).

As mentioned above, during the initial days of fasting, the body releases large amounts of water and salt in the urine. This process is known as natural diuresis or natriuresis of fasting (19).

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

Additionally, people practicing intermittent fasting may forget to drink or may not drink enough. This may 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. A dark-colored urine may indicate you’re dehydrated (22).

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

If a person engages in very long fasting periods and doesn’t replenish their body with enough nutrients, this could result in malnutrition. The same goes for poorly planned continuous energy restriction diets (23).

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 experience malnutrition along with other health complications.

That’s why it’s essential to consume a well-rounded, nutritious diet while practicing 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 smart choice for some people, it’s not appropriate or safe for others.

Some people may be at risk of dangerous side effects if they participate in intermittent fasting.

Healthcare professionals generally advise that the following people avoid intermittent fasting (23):

  • people who are pregnant or breastfeeding/chestfeeding
  • young children and teens
  • older adults who experience weakness
  • people with immunodeficiencies
  • people with current or past eating disorders
  • people with dementia
  • those 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 (24).

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 (23).

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

Studies have linked intermittent fasting to a number of health benefits, including improved heart disease risk factors, weight loss, increased blood sugar control, and more (1, 2).

Though intermittent fasting is generally considered safe, studies show that it may result in side effects including hunger, constipation, irritability, headaches, and bad breath.

Plus, healthcare professionals advise some people to avoid intermittent fasting. These include pregnant and breastfeeding or chestfeeding people and those with eating disorders.

If you’re interested in trying out intermittent fasting, check with a healthcare professional first to ensure it’s a safe and appropriate choice for you.

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