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7 Intermittent Fasting Side Effects: What To Know Before Doing The Diet

Mitchelle Morgan

Updated on - Written by
Medically reviewed by Dr G. Michael DiLeo, MD

intermittent fasting side effects

Various diets for better overall health and weight loss have become quite popular. Among the top ones is intermittent fasting.

While this meal plan is used for weight management primarily, that is only one of the purported benefits of intermittent fasting. Intermittent fasting has been related to a range of health advantages, including

Beyond the weight loss results, these additional discoveries above have boosted the popularity of a range of intermittent fasting programs such as

  • feeding on a timetable known as time-restricted feeding (TRF);
  • fasting on alternating days, known as alternate-day fasting (ADF); and
  • fasting regularly–periodic fasting.

On the flip side, you may note that these fasting periods may have some unwanted side effects when you practice intermittent fasting. We shall highlight seven of the most common ones here.

7 Intermittent Fasting Side Effects

The negative effects of intermittent fasting fall into two main categories: short-term and long-term.

The short-term risk factors occur almost immediately after one starts alternating fasting and eating periods. These effects are usually mild health problems, but they could worsen if not addressed appropriately. The most effective way to curb these short-term side effects is to stop fasting and resume your typical eating plan until you can identify the cause.

The long-term adverse effects of practicing intermittent fasting occur after you have been on the meal plan for a few weeks. These side effects can linger after long periods–usually requiring longer than just the time needed to stop the fast.

Short-Term Intermittent Fasting Side Effects

Here are some of the initial side effects when starting the eating plan:

1. Headaches

Intermittent fasting is often associated with headaches[7] that usually happen in the initial few rounds of a fasting regimen.

In a review published in 2020[8], researchers looked at 18 papers involving persons who practiced intermittent fasting. Some individuals in the studies reviewed indicated they had minor intermittent fasting headaches as adverse effects.

Scientists observed[9] that fasting headaches were frequently localized in the frontal brain region, and associated with mild to moderate pain severity. Furthermore, persons who frequently suffer from headaches were more likely to get intermittent fasting headaches when fasting than those who did not.

Low blood sugar and caffeine withdrawal may cause headaches during intermittent fasting.

2. Low energy and tiredness

Low energy during the fasting period, perceived almost immediately, is yet another result of intermittent fasting. According to studies[10], some individuals who practice different intermittent fasting routines attest that they felt weariness[11] and suffered low energy levels.

During intermittent fasting, you may feel tired and exhausted due to low blood sugar. In addition, intermittent fasting may produce sleep disturbances in certain persons, resulting in fatigue during times when you wish to be active.

On the other hand, some studies show that intermittent fasting reduces fatigue, particularly after your body adapts to regular fasting intervals. So, in some instances, reduced fatigue may become a long-term–welcome–health benefit.

Regardless, since our main energy source is usually dependent on the nutrients consumed in our meals, reducing food intake would explain the lower energy supply until you adjust.

3. Food cravings

It is human nature to want to restore a prior comfortable lifestyle you have abruptly changed, so hunger and disordered eating (e.g., craving habits) are common symptoms people experience during the first few days of a fasting program.

You will likely feel hungry by alternating fasting and eating periods since you go long periods on an empty stomach. Worse, without the set structure of a healthy diet during your eating window, you may start craving unhealthy foods with excess sugar content.

As one of the fasting side effects, food hunger and cravings are common since the body seeks to replenish its energy supply. You may experience greater hunger if you cut your calorie intake or go long periods without eating. Also, during an eating window, binge eating may arise.

In another study, 112 people[12] were randomly allocated to an intermittent energy restriction group. For 12 months, they ate 400 (female) or 600 (male) calories on two days of each week. Hunger levels were higher in these subgroups than in individuals who followed a low-calorie diet with constant calorie restriction.

In a 2020 study[13], 1,422 participants took part in fasting regimes that lasted 4–21 days. During the first several days of the eating plan, they suffered hunger pangs. It is likely, however, that such hunger pangs will fade as your body adjusts to regular fasting periods.

4. Bad breath

Bad breath (i.e., halitosis[14]) is an annoying side effect that some individuals on an intermittent fast may experience after a few weeks. The decrease in salivary flow and increased acetone in the mouth cause unpleasant breath. Acetone, a type of ketone, is produced during the ketosis (“ketogenic”) that develops while dieting.

Fasting encourages your body to burn fat as a source of energy, which drives ketosis. Because acetone is a waste product of fat metabolism, it accumulates[15] in your bloodstream and can be perceived from your breath when you fast.

Long-Term Side Effects of Intermittent Fasting

Here are the long-term side effects of intermittent fasting:

5. Mood changes

Individuals who practice intermittent fasting may experience irritation and other mood swings due to elevated body stress levels. When your blood glucose levels are low, which is just one type of body stress, you may become irritable[16].

Hypoglycemia, or low blood sugar, can develop during times of calorie restriction or fasting. Irritability, anxiousness, and poor focus are other possible outcomes related to hypoglycemia.

Further corroboration for the effects of hypoglycemia comes from a 2016 study[17] in which an 18-hour fasting phase made participants much more agitated than in a non-fasting phase, according to 52 women.

You shouldn’t underestimate the power of psychology. Remarkably, researchers discovered that while the ladies were irritated towards the completion of the fasting period, they also felt a greater feeling of accomplishment, pride, and self-control than they did at the beginning.

In other studies[17], long-term intermittent fasting also reduced both manic and depressive symptoms.

6. Malnutrition

Intermittent fasting that isn’t balanced or well-planned can cause malnutrition.

Malnutrition can occur when a person fasts for long periods and does not replenish their body with necessary nutrients. The same can be said for arbitrary, long-term calorie restrictions[18] and unhealthy diets in general.

Depending on the eating and fasting window time constraints, you will be okay if you have a balanced diet with healthy fats, proteins, carbs, vitamins, and minerals. However, if you don’t plan or follow your fasting regimen over a lengthy period or purposely restrict calories to an extreme degree, you risk malnutrition and other health problems.

While making sure you’re not restricting your calorie intake too much, it is also critical to eat a well-balanced, healthy diet.

. A healthcare expert familiar with intermittent fasting, e.g., a registered dietitian, can assist you in developing a safe diet that provides the optimum calorie count and nutrients for you.

7. Digestive issues

After a while on an intermittent eating plan, you may also notice some changes in your gut health[7]. Some are positive, while others are not as pleasant.

Some of the unpleasant side effects you may encounter are digestive disorders such as diarrhea, constipation, nausea, and bloating.

The decrease in food intake that various intermittent fasting regimes entail may severely impact your digestion. Your gut bacteria and you work together to navigate the nutrition delivered, and when there is a sudden change to what or how much you eat, the gut microbiome doesn’t have time to adust. As such, dietary modifications connected with intermittent fasting regimens might result in bloating[19] and diarrhea.  

Dehydration can exacerbate constipation, another typical early side effect of intermittent fasting. As a result, it’s critical to stay adequately hydrated while fasting intermittently. You can avoid constipation by eating nutrient-dense, fiber-rich foods and by hydrating throughout the day.

How to Reduce Intermittent Fasting Side Effects

All the above side effects of intermittent fasting can be avoided if you do the following:

Get A Doctor’s Input Before Starting The Eating Plan

Before taking medication or any alternative remedy, the first thing one should do is consult a doctor. Therefore, the same goes with dieting, which is like any other therapy.

By getting a doctor involved, they can set you on the best eating plan to follow. They will help you calibrate the best eating and fasting windows for maximum efficiency of the proposed eating plan.

They will also guide you and do periodic assessments to ensure that your sleep patterns and eating habits are aligned with your eventual goal, be it an immune system boost, weight loss, or controlling or treating another disease.

Adapt To A Healthy Diet

The second most important thing when you join the intermittent fasting bandwagon is to commit to a healthy diet. Whatever you consume impacts your body whenever you are in an eating window.

Stay away from packaged and processed foods with high sugar, salt, and unhealthy fat content. When you indulge in healthy meals, you supplement your body with enough energy supply to take you through the fasting window.

Avoid possible malnutrition from unhealthy eating disorders, and identify bad habits like cravings.

When You Notice A Recurring Adverse Side Effect, Halt The Eating Plan

If you notice that your mental health is deteriorating or your tummy feels uneasy, or it is in utter discomfort, please stop the diet immediately. This is your body’s warning system telling you something is wrong.

Then consult your physician or dietitian to see the way forward. They may help reconstruct the diet or advise you to stop it. Regardless, you will get to the bottom of the issue.

Stopping the diet the moment you notice something is amiss is to avoid the side effect becoming severe, like an intestinal blockage due to constipation.

Who Should Avoid Intermittent Fasting?

If you fall under the following groups of individuals, intermittent fasting is not for you:

  • Pregnant or nursing mothers.
  • People on certain medications.
  • People with current or past eating disorders.
  • Teens and children.
  • People with dementia.
  • People with nutritional deficiencies.

You may, however, find that some people in these groups have been advised to use the diet, such as children with epilepsy[20]. This underscores the importance of a licensed, trained, and certified medical practitioner’s input.


Intermittent fasting is beneficial with its fair share of health benefits like boosting stress adjustment, lowering cardiovascular health risks, and promoting weight loss.

But we cannot be completely blind and take the positives without acknowledging that it also has negatives. The seven side effects of intermittent fasting above are the most common. However, they are manageable, preventable, or treatable.

For example, with issues to do with constipation, simply adding more fiber and drinking more water can ease it. And with malnutrition, adopting a healthy balanced diet can be helpful. The key is to get medical advice before starting so that you are aware of the side effects and how to prevent and manage them.

+ 20 sources

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Mitchelle Morgan

Medically reviewed by:

Michael DiLeo

Mitchelle Morgan is a health and wellness writer with over 10 years of experience. She holds a Master's in Communication. Her mission is to provide readers with information that helps them live a better lifestyle. All her work is backed by scientific evidence to ensure readers get valuable and actionable content.

Medically reviewed by:

Michael DiLeo

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