The Potential Side Effects of Intermittent Fasting

While intermittent fasting is generally regarded as safe, it is not entirely without risk. In this tile we will cover what some of those risks are.


While intermittent fasting is generally regarded as safe, it is not entirely without risk. In this tile we will cover what some of those risks are and which groups of people should seek extra advice and information.

As always, this pathway is not medical advice, so please speak with your doctor to see if intermittent fasting is right for you.

Hunger and Cravings

Possibly the most obvious and common side effect related to intermittent fasting is, you’ve guessed it, hunger! After eight hours of fasting your blood glucose begins to dip and you may experience hunger, fatigue, food cravings, and trouble concentrating.

A study including 112 people assigned some participants to an intermittent fasting (referred to as intermittent energy restriction in the study) group. This group of people consumed 400 or 600 calories on 2 non-consecutive days every week for a year. They reported higher hunger scores than those who consumed a low-calorie diet with constant calorie restriction.

Individuals who fast on a regular basis say that hunger is a symptom typically experienced during the first few days of fasting. However, some individuals report reduced hunger after 24 hours.

A 2020 study observed 1422 people who participated in fasts lasting 4–21 days. They reported only experiencing hunger symptoms during the first few days and thereafter the hunger subsided, which does indicate that the hunger pangs might only be a temporary inconvenience.

Headaches and Light-headedness

Headaches are a common side effect of intermittent fasting, which typically occur during the first few days of fasting. However, of the people who experience headaches, most report only experiencing mild ones.

Because your body moves into a metabolic state where your fat reserves are burned for fuel, called ketosis, many individuals that do intermittent fasting also experience what’s known as ‘keto-flu’. Typical symptoms include insomnia, fatigue, nausea or vomiting, endurance issues when exercising, headaches, constipation, and dizziness. Headaches are also a symptom of the detoxifying effect that intermittent fasting has on the body. Researchers have suggested that low blood sugar and caffeine withdrawal may also contribute to headaches during intermittent fasting.

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. What’s more, people who are prone to getting headaches are also more likely to experience headaches during fasting.

Digestive Issues

Another commonly reported issue experienced during intermittent fasting is digestive issues. These include constipation, diarrhea, nausea, and bloating.

Similar to headaches, digestive issues during a fasting period can also be ascribed to the metabolic process, ketosis, which is activated during fasting. These symptoms all form part of the well-known ‘keto-flu’ that is also experienced during the popular fat-centric ketogenic diet.

The reduced food intake that comes along with intermittent fasting can negatively affect your digestion, which could lead to constipation and other side effects. And dietary changes associated with intermittent fasting programs/schedules may also cause bloating and diarrhea.

What’s more, dehydration, another common side effect linked to intermittent fasting, can worsen constipation. This is why it is very important to stay well hydrated when fasting. Adding nutrient-dense foods that are rich in fiber to your diet may help prevent or alleviate constipation.

Irritability and Other Mood Changes

Low blood sugar (hypoglycemia) can occur during periods of calorie restriction or fasting. This may lead to irritability, anxiety, and poor concentration. Plus, dealing with food cravings and caffeine withdrawal could add to a person’s irritability.

If a person enters the metabolic state where the body’s fat reserves are burned for fuel, rather than glucose, toxins stored in the fat reserves get released as the fat is being burned for energy. This could lead to symptoms like irritability among others.

A 2016 study observed 52 women over an 18-hour fast period. Their findings reported that the participants were significantly more irritable during the fasting period than they were during a non-fasting state. Interestingly, the researchers found that, even though the women reported being more irritable, they also experienced a greater sense of achievement, being proud, and self-control at the end of the fasting period than before the fast.

Lethargy and Low Energy

During intermittent fasting, it’s normal to feel groggy, especially if you are new to intermittent fasting. This is because low blood sugar related to intermittent fasting can cause you to feel tired and weak. Because intermittent fasting may lead to sleep disturbances in some people, it may also cause fatigue during the day.

One reason fatigue may be greater earlier in a fast is because your body excretes large amounts of salt and water through urine. This may lead to dehydration and low salt levels.

There is some controversy regarding fatigue and energy levels. Some people practicing various methods of intermittent fasting experience fatigue and low energy levels, while others report having reduced fatigue. According to researchers, fatigue decreases as your body becomes adapted to regular fasting periods. This was confirmed in a study that was aimed at young healthy males and females during Ramadan fasting.

Fatigue can also decrease as your body becomes fat adapted (switching to ketones as fuel) and after the detoxifying effect of fasting subsides.

Bad Breath

Bad breath is an unpleasant side effect that can occur in some people during fasting. Some people report only being made aware by comments of people around them, so take note!

Bad breath from fasting is caused by a lack of salivary flow and the rise of acetone in the breath, a byproduct of ketosis.

This is different from ordinary halitosis or bad breath and has a very distinct taste and smell, which some describe as fruity (like rotting apples) and smell like nail polish remover (acetone).

You can alleviate this by staying hydrated, which is generally beneficial while fasting anyway!

Sleep Disturbances

Some research studies suggest that sleep disturbances might be among the most common side effects related to intermittent fasting. The reason for this is thought to be a response to your body being under the stress of not having enough food. Note it’s the adaptive compensation to this stress that leads to fasting’s health benefits – there’s no free lunch.

In a 2020 study, they observed 1,422 people who participated in fasts lasting 4–21 days. Around 15% of the participants reported sleep disturbances related to their fast and reported this more frequently than other side effects.

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

In a 2021 study, they observed 31 people with obesity who participated in alternate-day fasting, while simultaneously following a low-carb diet for 6 months. The study found that alternate-day fasting did not affect the group’s sleep quality or duration, or insomnia severity. Another study showed similar results.

The results of the different studies are a bit ambiguous and might depend on the nature and duration of your fast.


Dehydration is another common symptom related to intermittent fasting. During the initial days of fasting, our bodies release a large amount of water through our urine – a process called diuresis. Natriuresis, the process of excreting sodium through our urine, also increases.

Without adequate replenishment of fluids and electrolytes, our risk of dehydration can increase.

Some people who practice intermittent fasting may also forget to drink or may not drink enough water, something observed more commonly at the beginning of an intermittent fasting schedule.

To stay properly hydrated, drink water throughout the day and pay attention to the color of your urine. Ideally, it should be a pale lemonade color. Dark-colored yellow or orange urine might be an indication that you’re dehydrated.


Intermittent fasting could lead to malnutrition if not done properly and is one of most important reasons to consult a healthcare professional before fasting.

Compared to other forms of caloric restriction, the risk of malnutrition might be increased with fasting due to its more extreme nature and the fact that fasting can actually get easier over time, masking any potential health consequences.

This is one more reason to make sure the food you do eat during your feeding window is as healthy as possible.

Who Should Avoid Intermittent Fasting?

The general guidance from healthcare professionals is that certain people should avoid intermittent fasting altogether. These include pregnant and breastfeeding women, young children, teens, older adults with pre-existing conditions, and those suffering from immunodeficiencies.

They also advise that individuals with a history of eating disorders, dementia, a history of traumatic brain injury or post-concussive syndrome to stay away from intermittent fasting.

There may be exceptions. For example, healthcare professionals have used fasting to treat epilepsy in children.

Note, not every body is the same, and some otherwise healthy people don’t respond well to fasting. If side effects such as extreme hunger, nausea, irritability, headaches, fatigue, or faintness occur, it may be a sign that it isn’t for you and again you should speak with a healthcare professional.

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