Intermittent Fasting Side Effects: What You Need to Know

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Intermittent fasting has become a popular weight loss trend in recent years. It involves fasting for a certain period, usually between 16-24 hours, followed by a period of normal eating. While many people swear by the effectiveness of intermittent fasting for weight loss, people often overlook the potential side effects of this diet. Any diet that requires extreme measures can lead to various negative consequences, and intermittent fasting is no exception.

While weight loss is essential to a healthy lifestyle, it should not come at the cost of one’s well-being. Therefore, sustainable measures promoting overall health and long-term weight management are crucial. The following sections of this article will delve into the side effects of intermittent fasting and why it is vital to pay attention to them. By understanding the potential risks, you can make an informed decision about whether intermittent fasting is the right approach for you or if more sustainable options are available.

Intermittent Fasting: An Overview

Intermittent fasting is a dietary approach that involves alternating between periods of eating and fasting. The concept of intermittent fasting is simple- alternating between eating and fasting, with regular periods of consuming little to no calories. There is a lot of focus on when you eat than on calorie restriction.

The eating window refers to consuming all your meals and snacks, while the fasting window is when you refrain from eating. Water and non caloric beverages like green tea or black coffee are allowed in the fasting window. There are different forms of intermittent fasting, each with its own eating and fasting windows. In addition, the length of the eating and fasting periods can vary depending on the chosen method of intermittent fasting. 

Some of the most popular variations include:

Time-Restricted Eating (TRE)

Time-restricted eating  involves restricting the time when meals are consumed within 24 hours. Typical eating times range from 4–12 hours, without caloric restriction. The most common form of TRE is 16:8, with 8 hours of eating window. It involves eating within a specific time frame every day, typically within an 8-hour window, and fasting for the remaining 16 hours. The eating and fasting windows can suit individual preferences and lifestyles. Other variations of TRE include 18:6 and 20:4 eating schedules. TRE reduces insulin levels, boosts metabolism, and improves overall health.

Alternate Day Fasting (ADF)

ADF involves eating as usual for one day, followed by a day of fasting, and so on. Some people modify ADF by consuming around 25% of their daily calories (about 500 calories) on their fasting days. As a result, ADF can help with weight loss and improve insulin sensitivity.

The 5:2 Diet

The 5:2 diet involves eating normally five days a week and consuming only 500–800 calories on the remaining two days. This form of intermittent fasting shows to be effective for weight loss, improving energy levels, and reducing inflammation.


This intermediate fasting involves fasting for 24 hours once or twice a week.

The Warrior Diet

It involves fasting during the day and eating a larger meal at night.


Intermittent fasting is a dietary approach that involves alternating between periods of eating and fasting. The timing of the food is as important as the quality of food. Different forms of intermittent fasting include time-restricted eating, alternate-day fasting, the 5:2 diet, eat-stop-eat, and the warrior diet. Each has its own eating and fasting windows, with varying lengths. Intermittent fasting can have several health benefits, including weight loss, improved metabolism, and reduced inflammation.

Why do People Choose Intermittent Fasting?

Intermittent fasting has gained popularity as an eating pattern for numerous reasons. Below are some of the most common reasons why people choose intermittent fasting.

Weight Loss

One of the main reasons people choose intermittent fasting is for weight loss. It is a highly effective tool for reducing overall calorie intake, which can lead to weight loss.

Improved Energy and Focus

Fasting gives the body a break from digesting food, which allows it to focus on healing and detoxifying. It can also improve energy levels and mental clarity, improving focus and productivity.

Better Metabolic Health

Research shows intermittent fasting can help regulate hormones and insulin levels, leading to better metabolic health. It can also aid in fat burning, benefiting those looking to lose weight.

Improved Digestion

Intermittent fasting can give the digestive system a break, allowing it to work more efficiently. In addition, it can lead to improved digestion and reduced inflammation.

Reduced Inflammation

Research shows intermittent fasting can reduce inflammation, lowering the risk of certain diseases like heart disease.

Benefits for Those with High Blood Sugar

Intermittent fasting can help regulate insulin levels and improve glucose uptake, making it beneficial for those with high blood sugar. It can also help reduce blood sugar levels and manage appetite.

Benefits for Athletes

Intermittent fasting can optimise muscle recovery and reduce fat burning during exercise, improving sports performance.


Intermittent fasting has gained popularity for a variety of reasons. It can aid in weight loss, improve energy levels and focus, benefit metabolic health, aid digestion, reduce inflammation, help those with high blood sugar, and benefit athletes. However, any restrictive diet, including intermittent fasting, can lead to side effects if followed for too long. Therefore, adopting sustainable and long-term measures for weight loss and overall health is crucial. While intermittent fasting may offer benefits, it is important to weigh them against the potential risks and decide whether it is right for you.

10 Unavoidable Side Effects of Intermittent Fasting

Before selecting an intermittent fasting plan, it’s crucial to consult a medical professional about its potential side effects. Although some strategies may cause more side effects than others, knowing the top ten common side effects can assist you in managing them efficiently.

Unhealthy Eating Habits

Healthy eating habits are crucial for maintaining overall health and well-being. Fasting may result in unhealthy food cravings, particularly for sugary, ultra-processed snacks. Instead of fasting to maintain a healthy weight, it is better to establish sustainable eating habits that include nutrient-dense foods. Planning healthy snacks and incorporating them into your diet can help you achieve your health goals without risking your health. Therefore, it is advisable to avoid extreme dieting methods like fasting and focus on developing healthy, long-term habits.

Hunger and Cravings

While intermittent fasting has gained popularity, it is essential to note that it can sometimes increase appetite and cravings for food. In addition, when starting fasting, the body may take time to adjust to the new routine, leading to increased hunger. A study on obese adults showed that the 5:2 diet and calorie restriction resulted in similar weight loss and heart health benefits, but the 5:2 group reported more hunger. 

Paying attention to your body and adjusting your fasting schedule accordingly is essential. For example, shortening the fasting window or eating a limited amount during the fast can help reduce hunger. Remember, choosing a sustainable weight loss approach that works best for you is crucial.


Your body needs time to adjust when you start intermittent fasting, and it’s not uncommon to experience a headache, research. It could be due to a drop in blood glucose levels and caffeine withdrawal during fasting. Fasting can make you more susceptible to both of these things, which can contribute to feelings of dizziness or headaches. It’s also crucial to stay hydrated, as dehydration can worsen headaches. 

These heady feelings usually only happen in the first few days and are typically mild and manageable. However, if you’re prone to headaches and fast for more than 16 hours a day, you may be at a higher risk for fasting headaches.

Regulating caffeine intake and maintaining hydration and sleep patterns are essential to avoid frequent fasting headaches.

Digestive Issues

Studies show intermittent fasting has digestive troubles like bloating, constipation, diarrhoea, nausea, and dehydration. Don’t worry; it’s not uncommon. Your body is adaptable, but changes in your eating routine can throw your gut off balance. When you alter your eating schedule, your gut bugs need to adjust, which can cause tummy troubles. 

Once your body settles in and you are not eating anything new, intermittent fasting can help balance your gut bacteria and reduce inflammation, which may minimise stomach troubles. However, the improvement in gut health over time requires more research.

Irritability and Other Mood Changes

Intermittent fasting can sometimes affect your mood and make you irritable and moody. It is because when your blood sugar level drops, you may experience anxiety and have difficulty concentrating. However, the accomplishment and pride you get when you achieve your fasting goals can make the struggle worthwhile.

Some studies show that intermittent fasting can improve symptoms of anxiety and depression, but that’s not always the case for everyone. As a newbie to fasting, you may feel stressed and crankier initially. However, research shows that many people with obesity experience improvements in energy and mood.

Fatigue and Low Energy

Intermittent fasting may leave you feeling low on energy and fatigued, at least initially. Your blood sugar levels may drop when you skip meals. It makes you feel weak and disrupts your sleep. It’s like that afternoon slump you experience where you only want to close your eyes and nap.

For some people, the fatigue usually subsides once your body gets used to this new routine. In addition, intermittent fasting improves mental health and reduces physical and psychological fatigue in some cases. So, if you do not feel improvement in your mood over time, please consult a certified nutritionist.

Bad Breath

If you’re doing intermittent fasting, you might experience bad breath. It is a common symptom called halitosis. A study reveals that when you fast, your body uses fat for energy, producing acetone as a byproduct. This acetone can cause your breath to smell funky, like nail polish remover. Additionally, fasting can lead to dehydration, which can also cause bad breath. And since dehydration can decrease your saliva flow, your mouth can dry out, worsening the bad breath. So if you’re fasting, be aware that bad breath might be one of the side effects you’ll have to deal with.

Poor Sleep

If you’re trying out intermittent fasting, you may experience some trouble with your sleep. It could be because your new meal schedule interferes with your body’s internal clock. Some studies show that sleep disturbances are a common side effect initially, but they tend to decrease over time. However, other research suggests that intermittent fasting may improve sleep quality. 

More research is required to comprehend the relationship between intermittent fasting and sleep fully, but it’s possible that meal timing and size could be factors. Over time, though, intermittent fasting may help reinforce your internal body clock and lead to better sleep.


When you fast, your body loses water and salt through your urine, especially in the first few days. It can result in dehydration, which can cause dizziness, headaches, and fatigue. Drinking more water is essential if you experience these symptoms during a fast. Severe dehydration can even affect your ability to concentrate, remember things, and affect your mood. So, it’s vital to ensure you drink enough water during fasting. If you notice a significant colour change in urination, it is a sign that you need to increase your water intake. It is no secret that staying hydrated is vital for your overall health. But it is even more necessary when fasting.


If you want to try intermittent fasting, ensure you don’t jump into it too quickly, or you could have a nutritional deficiency. Calorie intake is often the focus of fasting, but remember your body’s dietary requirements. Also, remember that fasting only affects when you eat, not what you eat. And prolonged fasting can lead to malnutrition, resulting in unintentional weight loss, fatigue, mood changes, and difficulty concentrating. 

Eating nutrient-dense foods like fruits, vegetables, nuts, seeds, legumes, grains, and fats to stay healthy. To stay on track, consult a dietician and create a nutritious meal plan you can follow during fasting. Avoid restricting calories excessively, as it can affect your body’s functioning. 


Intermittent fasting is a popular weight loss approach that involves fasting and eating windows. Although the benefits are numerous, it is essential to note that there are side effects to be aware of. Ten common side effects include unhealthy eating habits, hunger and cravings, headaches, digestive issues, irritability and mood changes, fatigue and low energy, bad breath, poor sleep, and dehydration. For some, these side effects may be mild and manageable; most people adjust after a few days. However, if symptoms persist or become severe, it is best to consult a medical professional.

Who Should Not Choose Intermittent Fasting?

Intermittent fasting is a widespread practice for improving health and losing weight, but it’s not suitable for everyone. 

Here’s who should avoid it:

If you’re pregnant or breastfeeding, it’s best to avoid intermittent fasting. It could negatively affect your baby’s health and development.Elderly individuals who experience weakness, immunodeficient people, those with dementia, or a history of traumatic brain injury should also avoid intermittent fasting.Those with eating disorders like anorexia nervosa or bulimia nervosa should avoid intermittent fasting, as it could exacerbate their condition.For people with diabetes or on chronic medications, it’s crucial to discuss fasting with a healthcare provider beforehand, as it could adversely affect their condition.Finally, children and teenagers still growing and underweight should not participate in intermittent fasting as it could impede their physical development and overall health.

Remember, intermittent fasting may not be for everyone. Before starting a new dietary regimen, one must always consult a healthcare professional or certified nutritionist.


While intermittent fasting has gained popularity as a way to improve health and lose weight, it’s not suitable for everyone. Pregnant or breastfeeding women, elderly individuals, immunodeficient people, those with dementia or a history of traumatic brain injury, people with eating disorders, and children and teenagers should avoid intermittent fasting. Additionally, those with diabetes or on chronic medications should discuss fasting with a healthcare provider beforehand.

The Final Word

Intermittent fasting can have numerous health benefits, including weight loss, improved metabolism, and reduced inflammation. However, one must be aware of the potential side effects of this diet, and any other diet that requires extreme measures, to make a well-informed decision about whether it is the right approach for you. In addition, adopting sustainable measures promoting overall health and long-term weight management is crucial. Therefore, finding a balanced diet and exercise routine that works for your lifestyle is essential for achieving long-term weight loss goals.

Disclaimer: The purpose of this article is just to disperse knowledge and raise awareness. It does not intend to replace medical advice from professionals. For further information, please contact our certified nutritionists Here.

Frequently Asked Questions (FAQs)

Q. What are the common side effects of intermittent fasting? 

A. Intermittent fasting can cause hunger, cravings, headaches, digestive problems, irritability, mood swings, fatigue, and low energy. In addition, it also results in bad breath, inadequate sleep, dehydration, and malnutrition. However, these side effects may not affect everyone, but following a sustainable weight-loss measure is crucial to ensure healthy and long-term weight loss.

Q. Is it normal to feel hungry during intermittent fasting? 

A. Yes, intermittent fasting can sometimes make you hungry and crave food. During the start of fasting, your body might endure an adjustment period. It is where you feel hungrier than usual. However, the temporary hunger may not last for long for everyone. But, it is still better to choose sustainable measures.

Q. Can intermittent fasting cause headaches or migraines? 

A. Yes, during intermediate fasting, headache is common. It is because of a drop in blood glucose levels and caffeine withdrawal during fasting. Additionally, dehydration can also result in headaches. However, for some people, it lasts for only a few days of fasting. 

Q. Is it safe to do intermittent fasting if you have diabetes? 

A. However, it is considered that intermediate fasting helps regulate insulin levels and improve glucose uptake. People with diabetes should discuss this with healthcare providers before a fasting regime, which may result in adverse impacts.

Q. Can intermittent fasting lead to nutritional deficiencies? 

A. Yes, intermediate fasting could lead to nutritional deficiencies. Also, prolonged fasting can result in malnutrition, leading to unintentional weight loss, fatigue, and difficulty focusing. Therefore, ensure you should eat nutrient-dense foods during your non-fasting period.

Q. Does intermittent fasting cause muscle loss? 

A. Several studies reveal that intermittent fasting leads to the loss of lean muscle mass. However, if you eat an adequate or high-protein diet, you don’t need to be concerned about losing muscle with short-term intermittent fasting. But it is best to choose more sustainable and healthy measures for those who aim for long-term results.

Q. Can intermittent fasting affect my menstrual cycle? 

A. Yes, intermediate fasting can affect menstrual cycles in women. It is because women’s reproductive systems are sensitive to calorie restrictions. That means prolonged fasting can affect the oestrogen that is vital to the menstruation process. 

Q. Are there any negative impacts of intermittent fasting on sleep quality? 

A. Yes, intermediate fasting can trouble your sleep. It is due to your new dietary schedule, which interferes with your body clock. However, for some, it is a minor side effect of fasting, lasting only a few days. But for some, it may be a long term side effect. Hence, one should look for more sustainable and healthy ways.

Q. Can intermittent fasting lead to increased stress levels? 

A. Yes, intermediate fasting leads to increased stress levels. It is because intermediate fasting can increase cortisol levels, a stress hormone. Therefore, if you are doing intermediate fasting for a long time, it makes you stressed.

Q. Is it safe to exercise while doing intermittent fasting?

A. Yes, it is entirely safe to exercise while doing intermediate fasting. It reduces fat burning during exercise and improves sports performance by optimising muscle recovery. 

Research Sources

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4. Torelli P, Manzoni GC. Fasting headache. Curr Pain Headache Rep. 2010 Aug;14(4):284-91. DoI: 10.1007/s11916-010-0119-5. PMID: 20490742.

5. 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 Jan 2;14(1):e0209353. DoI: 10.1371/journal.pone.0209353. PMID: 30601864; PMCID: PMC6314618.

6. Anic K, Schmidt MW, Furtado L, Weidenbach L, Battista MJ, Schmidt M, Schwab R, Brenner W, Ruckes C, Lotz J, Lackner KJ, Hasenburg A, Hasenburg A. Intermittent Fasting-Short- and Long-Term Quality of Life, Fatigue, and Safety in Healthy Volunteers: A Prospective, Clinical Trial. Nutrients. 2022 Oct 10;14(19):4216. doi: 10.3390/nu14194216. PMID: 36235868; PMCID: PMC9571750.

7. Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects-A Narrative Review of Human and Animal Evidence. Behav Sci (Basel). 2017 Jan 19;7(1):4. DoI: 10.3390/bs7010004. PMID: 28106818; PMCID: PMC5371748.

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