Intermittent Fasting Side Effects

Medically Reviewed by Dr. Aman Priya Khanna
Written by Kirti V, last updated on 15 December 2023| min read
 Intermittent Fasting Side Effects

Quick Summary

  • Intermittent fasting is a new trend gaining popularity due to its health benefits.
  • Intermittent fasting side effects, while manageable, can vary from person to person and depend on their fasting routine.
  • Understanding these effects enables you to make informed decisions to ensure a successful intermittent fasting journey.

Intermittent fasting is a new trend gaining popularity due to its health benefits. But not without certain side effects! Intermittent fasting side effects, while manageable, can vary from person to person and depend on their fasting routine. Understanding these effects enables you to make informed decisions to ensure a successful intermittent fasting journey.

So, what are these side effects? Is intermittent fasting the right choice for everyone? The following blog discusses the side effects of fasting and whether it is suitable for you. It also discusses some alternatives to intermittent fasting that offer health benefits without the need to fast. Let’s delve deeper. 

What is Intermittent Fasting?

Intermittent fasting is a dietary plan that alternates between periods of fasting (not eating) and periods of eating. It does not prescribe what to eat but focuses on when to eat. The primary goal is to extend the time between your last meal of one day and your first meal of the next day. 

Intermittent fasting does not involve starving yourself. Rather, it focuses on significantly reducing calorie intake for short periods. Doing so makes your body accustomed to smaller portions and diminishes cravings for unhealthy snacks. 

There are several methods of intermittent fasting, but the most common ones include the following:

  1. Time-Restricted Method: This type of fasting restricts the eating window. It includes the following ways: 
    16/8 method (16 hours of fast with an 8-hour eating window)
    14/10 method (fasting for 14 hours and eating during a 10-hour window)
  2. Modified Fasting or the 5:2 Method: It involves eating normally for five days and significantly reducing calorie intake for two non-consecutive days a week. 
  3. Alternate Day Fasting: In this method, fasting days are alternated with regular days where food and beverages can be consumed without restrictions.
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Side Effects of Intermittent Fasting

In today’s world, intermittent fasting is considered one of the fastest ways to achieve 6-pack abs and a size zero body. It makes people curious whether fasting is safe. While generally considered safe for many people, it can have some potential side effects. Intermittent fasting side effects include:

Hunger and Craving for Food 

Intermittent fasting can sometimes lead to hunger as a side effect. Extended fasting periods with limited calorie intake may trigger a sudden increase in appetite. A study by T M Sundfør et al. was conducted in 2018 on 112 participants (50% men and 50% women) aged 21-70 years with a BMI of 30-45. According to it, intermittent fasting can cause elevated feelings of hunger.

Solution: Stay well-hydrated, consume filling, nutrient-dense foods in eating windows, and keep yourself busy to stay distracted from food. 

Low Energy and Fatigue 

Does intermittent fasting make you weak? The short answer is: Sometimes, but only during the initial days! Intermittent fasting can result in low blood sugar, making you feel weak and tired. 

Here’s the interesting thing: Boya Nugraha et al. conducted a study in 2020 with 34 healthy participants (male and female) older than 18 years. According to the study, intermittent fasting can reduce fatigue in healthy young males and females. It is especially true once your body adapts to the regular fasting regimen. 

Solution: Ensure hydration, get enough rest, and choose nutrient-dense foods to sustain energy during fasting periods. 


Headaches are one of the most common intermittent fasting side effects when starting the fasting routine. A “fasting headache” is a type of headache that causes mild to moderate pain. It is usually experienced in the frontal region of the head. 

But did you know that individuals who frequently experience headaches are more prone to getting fasting headaches than those who rarely do? Plus, headaches usually occur after a minimum of 16 hours of fasting. 

Solution: Fasting headaches tend to subside within 72 hours following the resumption of food intake. To prevent headaches, maintain proper hydration, reduce caffeine consumption, and focus on good nutrition. 


In the initial days of fasting, the body undergoes a natural process called diuresis or natriuresis of fasting. In this condition, the body expels significant amounts of water and salt through urine. Failing to replenish the fluids and electrolytes lost in this process can lead to dehydration. 

Moreover, individuals practising intermittent fasting might sometimes not consume enough fluids, particularly during the start of the fasting routine. You should also pay attention to the colour of your urine. Ideally, your urine should appear pale lemonade in colour. A darker hue may indicate dehydration.

Solution: Maintain a consistent daily water intake to ensure proper hydration.

Digestive Issues

Next on the list of intermittent fasting side effects are diarrhoea, constipation, and other digestive issues. Dietary alterations accompanying intermittent fasting programs might result in constipation, bloating and diarrhoea. Dehydration, another prevalent side effect of fasting, can worsen constipation.

Solution: Maintain proper hydration levels when engaging in intermittent fasting. Additionally, you must opt for nutrient-dense foods high in fibre to prevent constipation.

Mood Disturbances

It is common to experience irritability and mood swings during intermittent fasting, at least initially. Calorie restriction during periods of fasting can lead to low blood sugar. It can cause anxiety, irritation, and poor concentration. 

A study conducted by Ellen Watkins and Lucy Serpell in 2016 included 52 women with a mean age of 25. The study revealed that women experienced higher levels of irritability during fasting than when they were not fasting.

Solution: Practice stress-reduction techniques and consume nutrient-dense foods to support stable blood sugar levels.

Sleep Disturbances

In some cases, intermittent fasting can affect sleep patterns, causing difficulty falling or staying asleep. It is one of the most common consequences of fasting regimens. Françoise Wilhelmi de Toledo et al. conducted a study in 2019 with 1422 participants who fasted for 4-21 days. The study revealed that 15% of participants reported sleep disturbances during fasting. Moreover, these were more frequent than other side effects.

However, more research is required to establish the relationship between sleep disturbances and intermittent fasting. 

Solution: Maintain a consistent sleep schedule, manage stress, and avoid larger meals close to bedtime. 


When done abruptly, intermittent fasting can lead to nutritional deficiencies. You must follow extended fasting periods with proper nutrition to ensure a healthy fasting routine. Often, during fasting, the primary focus is on calorie intake while overlooking the body’s nutritional needs. 

Solution: A nutritious diet plan is recommended for a successful and balanced fasting regimen. Seek guidance from a dietician to create a sustainable fasting-period diet plan. 

Hair Loss

One of the important intermittent fasting side effects is hair loss in individuals. It is well-known that a balanced diet plays a crucial role in maintaining healthy hair. Rapid calorie intake and nutritional pattern shifts can trigger a condition called telogen effluvium. It results in temporary hair loss due to the shock experienced by the hair follicles (a tube-like structure in the skin’s outer layer). 

Solution: If you find that you are losing more hair than usual when you wash, you should reconsider the composition of your meals and consult your physician to see if intermittent fasting is truly a good option for you.

Bad Breath 

During fasting, especially extended fasts, the body may enter a state of ketosis. It is a process where the body burns fat for energy instead of carbohydrates. This can lead to the production of acetone, a type of ketone. Acetone has a fruity scent, like nail polish remover, causing bad breath. 

When fasting for extended periods, your mouth can become dry due to less saliva production. Reduced saliva flow can lead to the growth of odour-causing bacteria, resulting in bad breath. Dehydration can also contribute to dry mouth and, subsequently, bad breath.

Solution: Stay well-hydrated, practice oral hygiene, and use mints or sugar-free gum to freshen your breath. 

Is Intermittent Fasting for Everyone?

Intermittent fasting is safe for most people, but it is not for everyone. Its appropriateness can vary depending on individual factors, health conditions, and goals. Some people may experience severe side effects if they participate in intermittent fasting. People who should avoid intermittent fasting include:

  1. Women Who are Pregnant or Breastfeeding: Intermittent fasting can deprive these women and their children of essential nutrients for growth and development. 
  2. Young Children and Teens Under Age 18: The growing bodies of these children require nutrition for proper development. Fasting can cause nutrient deficiencies.
  3. Older Adults With Weakness Issues: Fasting can worsen age-related muscle weakness, impacting overall health. 
  4. People With a History of Eating Disorders: Intermittent fasting can aggravate disordered eating behaviour.
  5. Those With Type 1 Diabetes: Restrictive eating can affect blood sugar control and insulin management.
  6. People With Dementia: This syndrome affects memory, thinking, and social abilities.
  7. Individuals With Immunodeficiencies: It is a state in which the immune system cannot fight off infections or viruses effectively. Fasting can weaken the immune system, increasing the risk of infections. 
  8. History of Traumatic Brain Injury or Post-Concussive Syndrome: It is a condition in which concussion symptoms (headaches, dizziness, concentration issues) persist longer than expected. Fasting might worsen neurological and cognitive symptoms. 

Individuals with kidney stones, gastroesophageal reflux, or other health conditions should talk to their doctor before starting intermittent fasting. 

When to Consult a Doctor?

If you encounter persistent side effects during intermittent fasting, this approach may not suit your body well. In such a situation, you should discontinue intermittent fasting and consult a doctor. Side effects that may require doctor consultation include:

  1. Excessive hunger
  2. Anxiety
  3. Nausea
  4. Headaches
  5. Irritability or mood changes
  6. Fatigue
  7. Faintness


Intermittent fasting can offer various health benefits like weight management, improved metabolic health, etc. However, you must know the potential side effects, including hunger, fatigue, and digestive issues. While intermittent fasting side effects are typically manageable, it is crucial to develop a well-balanced plan. Consult a doctor to ensure a safe and effective fasting experience.

Experts at HexaHealth can provide valuable insights into your health and wellness through personalised recommendations. We can help you find the right doctors and hospitals for all your medical concerns. With HexaHealth, you can take proactive steps towards a healthier and more informed life.|

Suggested Reads

Right Way to Break Intermittent Fasting - Expert's Guide
7 Days Indian Diet Plan for Intermittent Fasting
Intermittent Fasting 14/10: Meal Plan, Benefits & Results
15:9 Intermittent Fasting: How-to, Meal Plan and Results

Frequently Asked Questions

Intermittent fasting is an eating plan that alternates between periods of fasting and eating on a regular schedule. It focuses on when you eat rather than what you eat. It can offer health benefits like weight management and improved heart health.

Common intermittent fasting mistakes include:

  1. Choosing an incorrect intermittent fasting plan or starting too aggressively

  2. Not considering individual health conditions like diabetes, eating disorders, etc.

  3. Not eating enough or overeating during eating windows

  4. Consuming calorie-dense food or those that contain sugar, fat, or salt during the eating window 

  5. Ignoring hydration

  6. Not exercising and sleeping enough 

  7. Being impatient

The primary rule of intermittent fasting is to adhere to your designated fasting and eating windows. Do not consume any calories during the fasting window because calorie consumption can disrupt the state of ketosis (burning of fat for energy). 

Intermittent fasting side effects are usually minor. However, they can sometimes lead to serious complications like malnutrition and dehydration if not done properly.

Side effects of intermittent fasting include:

  1. Hunger

  2. Headaches

  3. Low energy and fatigue

  4. Irritability and mood changes

  5. Digestive issues like bloating, diarrhoea, or constipation

  6. Dehydration

  7. Malnutrition

  8. Sleep disturbances

  9. Bad breath

Intermittent fasting may offer benefits like:

  1. Helps lose weight and visceral (hidden) fat 

  2. Eliminates brain fog 

  3. Better blood sugar control

  4. Promotes heart health 

  5. Reduces inflammation

  6. Improves sleep 

However, results can vary depending on individual factors and adherence to the fasting regimen.

Yes, there are certain drawbacks of 16/8 intermittent fasting. These may include:

  1. Overeating and weight gain

  2. Hormonal changes affecting menstrual cycles

  3. Short-term side effects like hunger, weakness, and fatigue

Yes, intermittent fasting can make you feel weak. However, this is usually during the initial days when the body tries to adjust to fasting. However, once your body adapts to the fasting periods, intermittent fasting can reduce fatigue and weakness.

Intermittent fasting side effects for hair loss result from nutritional deficiencies and stress. Drastic calorie restriction can cause insufficient nutrients for hair health, leading to temporary hair shedding.

Typically, fasting alone does not trigger diarrhoea. You are more prone to experience diarrhoea when you break your fast than during fasting. It is because the bowel’s functioning decreases when it is not used. Intermittent fasting side effects of diarrhoea can sometimes occur due to an oversecretion of water and salts in the GI tract. It can happen due to drinking liquids like tea and coffee.

The safety of intermittent fasting depends on the specific fasting regimen and individual factors. Short-term intermittent fasting, such as 16/8, is safe for most healthy individuals. However, fasting for two to three days at a time can be risky. It should be discussed with a specialist.

The long-term consequences of intermittent fasting or other highly calorie-restrictive diets remain uncertain. To achieve lasting weight loss, adopting healthy eating and exercise habits is advisable.

The best intermittent fasting rule depends on individual preferences and goals. The 16/8 method is popular for its simplicity and flexibility. In this method, individuals fast for 16 hours and eat during an 8-hour window. Other popular methods include:

  1. Eat-Stop-Eat: It includes fasting for 24 hours once or twice a week. 

  2. 5:2 Diet: Individuals must consume only 500-600 calories for two non-consecutive days of the week. They can eat normally for the remaining five days. 

The first meal after intermittent fasting should include gentle foods. Examples of food to eat after fasting include:

  1. Smoothies

  2. Soups (with protein and easily digestible carbs)

  3. Dried fruits like dates, apricots, and raisins

  4. Cooked, starchy vegetables

  5. Healthy fats like eggs and avocados

  6. Fermented foods like unsweetened yoghurt

The following are the tips to maximise intermittent fasting results:

  1. Ensuring consistency in fasting and eating windows

  2. Prioritising nutrient-dense foods

  3. Drinking water throughout the day to stay well-hydrated

  4. Practicing mindful eating during the eating window

  5. Exercising before or during the eating window

  6. Trying meditation during the fasting period to avoid stress and hunger pangs


All the articles on HexaHealth are supported by verified medically-recognized sources such as; peer-reviewed academic research papers, research institutions, and medical journals. Our medical reviewers also check references of the articles to prioritize accuracy and relevance. Refer to our detailed editorial policy for more information.

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  2. Intermittent fasting: Facts vs myths [Internet]. 2022. link
  3. 9 Potential Intermittent Fasting Side Effects [Internet]. Healthline. 2021. link
  4. Bradley S. 10 Intermittent Fasting Side Effects That Might Mean It’s Not A Great Fit For You [Internet]. Women’s Health. Women’s Health; 2019. link
  5. Johns Hopkins Medicine. Intermittent fasting: What is it, and how does it work? [Internet]. Johns Hopkins Medicine. 2021. link
  6. Borzì A, Biondi A, Basile F, Luca S, Vicari E, Vacante M. Olive Oil Effects on Colorectal Cancer. Nutrients [Internet]. 2018 Dec 23;11(1):32. link
  7. pubmeddev. PubMed - NCBI [Internet]. link
  8. 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. Ruscica M, editor. PLOS ONE [Internet]. 2019 Jan 2;14(1):e0209353. link
  9. Nugraha B, Riat A, Ghashang SK, Eljurnazi L, Gutenbrunner C. A Prospective Clinical Trial of Prolonged Fasting in Healthy Young Males and Females—Effect on Fatigue, Sleepiness, Mood and Body Composition. Nutrients [Internet]. 2020 Jul 30;12(8). link
  10. Health benefits of intermittent fasting (and tips for making it work) [Internet]. link
  11. 16/8 Intermittent Fasting: A Beginner’s Guide [Internet]. Healthline. 2018. link
  12. Diarrhea During Fasting: When to Stop Fasting and Treatment Options [Internet]. Healthline. 2019. link
  13. Harris K. Intermittent fasting does it work and is it safe? [Internet]. OSF HealthCare Blog. 2021 [cited 2023 Oct 26]. link
  14. Mayo Clinic Q and A: Long-term benefits and risks of intermittent fasting aren’t yet known [Internet]. link
  15. Gunnars K. Intermittent Fasting 101 — The Ultimate Beginner’s Guide [Internet]. Healthline. Healthline Media; 2018. link
  16. What Breaks a Fast? Foods, Drinks, and Supplements [Internet]. Healthline. 2019. link
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Updated on : 15 December 2023

Disclaimer: The information provided here is for educational and learning purposes only. It doesn't cover every medical condition and might not be relevant to your personal situation. This information isn't medical advice, isn't meant for diagnosing any condition, and shouldn't replace talking to a certified medical or healthcare professional.


Dr. Aman Priya Khanna

Dr. Aman Priya Khanna

MBBS, DNB General Surgery, Fellowship in Minimal Access Surgery, FIAGES

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Dr Aman Priya Khanna is a well-known General Surgeon, Proctologist and Bariatric Surgeon currently associated with HealthFort Clinic, Health First Multispecialty Clinic in Delhi. He has 12 years of experience in General Surgery and worke...View More


Kirti V

Kirti V

B.A. English | M.A. English ( Magadh University, Bihar)

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With 3 years of full-time experience as an SEO content writer, she has honed her skills to deliver captivating and persuasive writing that leaves a lasting impact. She is always ready to learn new things and expand...View More

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