How to Handle 10 Potential Side Effects of Intermittent Fasting

Although intermittent fasting is very effective in losing weight, adequate caution is advised because of the side effects that could arise.

The good news is these side effects are usually not serious and can be corrected or prevented. Here are a few of them and how you can manage them:

1. Headaches

Headaches are a common side effect, especially in the first few days for a beginner. Research shows that people complain of mild to moderate headaches during fasting periods. This may be due to the reduced glucose levels in the blood.1 

Intermittent fasting could also cause a drop in the sodium levels in the blood, and this may cause headaches. Adding some sea salt to your water would help to prevent this.

Also, people who typically get headaches more than usual are at a greater risk of getting headaches during periods of intermittent fasting.2

The headaches are usually mild and resolve with rest or drinking water with sea salt. However, if the headaches become constant and get worse, you need to end the fasting and seek help. 

2. Dizziness

You may feel dizzy when you are on an intermittent fasting regimen. This happens because of the reduction in blood flow to your brain and low blood sugar. It is typically short-lived and happens in the beginning stages of intermittent fasting, but some people may have it worse than others. 

It is relieved by rest. Hence, you should avoid taking part in serious labour that requires full concentration and strength when you’re on an intermittent fast.

3. Nausea

Reduced intake of food causes acid to build up in your stomach, and this causes reflux that can make you feel nauseous and bloated. When you begin to feel this way, you should consider putting a stop to your fasting, especially if you have an underlying sickness before starting the fast. 

Nausea can be an early non-specific side effect from intermittent fasting that should be taken quite seriously. Avoid lying on your back for too long, and don’t go on prolonged fasting periods to prevent the worsening of the symptoms.

4. Fatigue/Poor Concentration

This may be due to having low blood sugar from not eating for a long time. If you begin to show symptoms of this by trembling fingers, faster heart rate or feeling faint. Your body gets adapted to the fatigue that comes with intermittent fasting after some time, although this is not always the case.

Hypoglycemia can become a very serious adverse effect that becomes an emergency. When you begin to feel faint, immediately call for help and stop your fasting.

To prevent hypoglycemia from occurring during fasting, it’s crucial to eat enough nutritious food during the feeding window. It will help build glycogen stores which can then be converted to glucose during fasting to reduce the severity of low blood sugar.

Poor concentration could happen during intermittent fasting. If you experience that, you may want to switch to more relaxed fasting methods like crescendo, where you fast only 3 non-consecutive days a week. It will help you to maximize the positive effects and reduce the likelihood of complications. 

5. Sleep Disturbances

Difficulty with maintaining a normal sleep pattern is associated with intermittent fasting in some individuals.3 In fasting regimens that last up to 21 days, up to 15% of people that participated in a study reported that they were having more trouble with sleeping.4

There has also been research that showed that diurnal intermittent fasting can cause reduced REM sleep. Rapid-Eye-Movement sleep is important for maintaining good memory, cognition and concentration.5

Other people who have been on intermittent fasting regimens for long don’t suffer from insomnia. The inability to sleep well may be a result of extreme fatigue or hunger that comes with fasting. 

You can overcome this by creating a more suitable regimen for your fasting and ensuring adequate calorie intake.

6. Digestive Problems

You may begin to experience problems with food digestion when you begin intermittent fasting. Heartburn is the least common side effect, which happens when there is a flow of acid upwards from your stomach into your esophagus, causing irritation. You can reduce this effect by taking in more water and sitting up to prevent the backward flow of your stomach contents. 

Constipation also occurs as a side effect of intermittent fasting, especially when you’re not well hydrated. Bloating and abdominal discomfort may be associated with the inability to pass feces. You can also avoid this by including fibres in your diet and ensuring you take in enough water.6

7. Sensitivity to Cold

During intermittent fasting, some people report that they have an increase in sensitivity to cold. This happens because the blood flow to the fat under the skin is increased in a process that dilates the vessels. This process typically results in cold clammy body parts, especially the hands and feet.

The increased blood flow happens to help burn the fat stores in the body, but can quickly result in you feeling colder than usual during these times. You can prevent this by wearing more layers of clothing and avoiding triggers like staying out in the cold for long and taking cold drinks.7

8. Dehydration

In the initial stages of intermittent fasting, you may feel dehydrated because of the natural process of losing salt and water that happens during fasting. The body needs a balance of electrolytes and fluid during this time to reduce the side effect of dehydration which is seen as dry mouth, dry skin and reduced urine production after some time. 

You may forget to drink water as a beginner to intermittent fasting, and this is one of the most common causes of dehydration. Ensure you take in enough water during your fasting periods to avoid this.8

9. Muscle Cramps

You can get very serious muscle cramps from prolonged intermittent fasting. This happens because of reduced creatinine levels in your blood.

You can manage muscle cramps by taking more protein and creatinine supplements to support your muscles. You should also try to avoid a very drastic reduction in your total calorie intake. 8

10. Bad Breath

Getting bad breath from fasting is an unpleasant side effect that occurs in some cases. This happens due to the increase in the levels of acetone in the blood as a result of the increased breakdown of fat into ketones.

A dry mouth that results from dehydration can also cause foul-smelling breath.10 You can prevent this by ensuring you maintain good oral hygiene during your intermittent fasting, and make sure your mouth doesn’t get dry by taking in enough water at all times.


Despite these potential side effects, it is clear that the benefits of intermittent fasting greatly outweigh its side effects. Taking the steps above during intermittent fasting would help to prevent and relieve these side effects and improve your overall fasting experience.

  1. Cui Y, Cai T, Zhou Z, et al. Health Effects of Alternate-Day Fasting in Adults: A Systematic Review and Meta-Analysis. Front Nutr. 2020;7:586036. Published 2020 Nov 24. doi:10.3389/fnut.2020.586036. Link
  2. Torelli P, Manzoni GC. Fasting headache. Curr Pain Headache Rep. 2010;14(4):284-291. doi:10.1007/s11916-010-0119-5. Link
  3. Phillips MCL. Fasting as a Therapy in Neurological Disease. Nutrients. 2019;11(10):2501. Published 2019 Oct 17. doi:10.3390/nu11102501. Link
  4. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. Published 2019 Jan 2. doi:10.1371/journal.pone.0209353. Link
  5. Almeneessier AS, BaHammam AS. How does diurnal intermittent fasting impact sleep, daytime sleepiness, and markers of the biological clock? Current insights. Nat Sci Sleep. 2018;10:439-452. Published 2018 Dec 7. doi:10.2147/NSS.S165637. Link
  6. Side effects of intermittent fasting. Popsugar Website. Link
  7. Funada J, Dennis AL, Roberts R, Karpe F, Frayn KN. Regulation of subcutaneous adipose tissue blood flow is related to measures of vascular and autonomic function. Clin Sci (Lond). 2010;119(8):313-322. Published 2010 Jul 6. doi:10.1042/CS20100066. Link
  8. Side effects of intermittent fasting. Healthline Website. Link
  9. Anderson JC. Measuring breath acetone for monitoring fat loss: Review. Obesity (Silver Spring). 2015;23(12):2327-2334. doi:10.1002/oby.21242. Link
  10. Kapoor U, Sharma G, Juneja M, Nagpal A. Halitosis: Current concepts on etiology, diagnosis and management. Eur J Dent. 2016;10(2):292-300. doi:10.4103/1305-7456.178294. Link