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Home arrow Nutrition arrow Intermittent Fasting arrow Intermittent Fasting Side Effects: 10 Side Effects You Should Know

Intermittent Fasting Side Effects: 10 Side Effects You Should Know

Written by Edibel Quintero, RD
Fact checked by Rosmy Barrios, MD
Last update: October 21, 2023
10 min read 1626 Views 0 Comments
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While intermittent fasting has been associated with weight loss and other health benefits, there are a few side effects as well.

intermittent fasting side effects

Intermittent fasting is one of the hottest diet trends right now. Celebrities and Instagram influencers gush about how intermittent fasting improves their health, focus, and energy and perhaps even helps them with weight loss. 

Although losing weight is the main goal of this meal plan, intermittent fasting is associated with many health benefits, including reduced signs of oxidative stress, lower blood pressure, enhanced insulin sensitivity, reduced heart disease risk factors, and better blood sugar control.

Numerous aspects of intermittent fasting have been studied. However, there hasn’t been enough evidence to support intermittent fasting as a long-term fix for all your problems, whether they relate to your weight, focus, energy, or anything else.

Although intermittent fasting is not harmful, it’s not safe for everyone and might cause minor side effects.

This article discusses 10 of the most common intermittent fasting side effects you should be aware of.

10 Intermittent Fasting Side Effects

  1. Headache and migraine
  2. Extreme hunger
  3. Dizziness
  4. Nausea
  5. Anxiety and poor concentration
  6. Fatigue
  7. Unpleasant breath
  8. Malnutrition and eating disorders
  9. Digestive issues
  10. Sleep disturbances

10 Intermittent Fasting Side Effects You Should Be Aware of

Common intermittent fasting regimens include fasting for 16 hours per day or 24 hours twice weekly. Intermittent fasting is the common method used to manage weight issues due to its extraordinary ability to assist our bodies in losing weight.

However, intermittent fasting has some drawbacks besides its clear and sparkling benefits. Here are some of the side effects.

#1 Headache and migraine

Fasting headaches are common during the first few days of a fasting period. The few potential causes of these headaches range from caffeine withdrawal to dehydration and fluctuations in blood sugar levels.

When your body is dehydrated, your blood vessels contract causing your body tissues, including those in your brain, to constrict or shrink. As your brain tightens, it pulls away from the skull, pressing against your nerves and causing discomfort. Because of this, even slight dehydration can result in a headache.

Lack of water also causes your body to produce less serotonin, a chemical that aids in mood and sleep regulation. Changes in serotonin are thought to trigger migraine and tension headaches.

Hypoglycemia (low blood sugar) is another major cause of headaches during a fast. Oxygen and sugar are derived from food and delivered to the brain via blood. The brain reacts when these sources are too low, such as when you’re fasting. This leaves your body with no glucose to use and can result in headaches, lightheadedness, weakness, and sweating.

Research has shown that fasting headaches frequently originate in the brain’s frontal lobe, and the pain is normally mild to moderate.

#2 Extreme hunger

If you’re not getting enough food, your body will start breaking down fat stores for energy. As a result, you may experience extreme hunger during fasting.

Additionally, because your body is accustomed to having access to food all the time, you will probably feel hungry after a few days when you first start incorporating IF into your regimen. This is due to the decrease in glucose levels. 

Hunger pangs start from the drop in glucose, but after your body is accustomed to intermittent fasting, you won’t feel these variations as much.

#3 Dizziness

When you fast, your body goes into a state known as ketosis. The process occurs when your body uses fat stores instead of glucose for energy. This can lead to weight loss but also cause dizziness if you don’t eat enough healthy fats, protein, or carbohydrates during eating windows.

Dizziness is also caused by dehydration and electrolyte imbalances when you fast. Dehydration occurs when water levels in the body decrease because of excessive sweating or urination; electrolyte imbalances occur when minerals such as sodium are lost through sweat or vomiting, which can also happen when fasting.

Some people may also find that consuming too much water while fasting can cause dizziness because they’re not used to drinking so much.

Fasting also causes your blood sugar levels to dip pretty low from time to time. Your body needs a steady supply of glucose from food or drinks to operate properly. The risk of low blood sugar can cause symptoms like weakness and hunger, headaches, irritability and confusion, dizziness, and shakiness.

#4 Nausea

When fasting, your body will try to conserve energy by turning to stored fat and protein in the liver and muscles for energy. This can lead to an increase in the production of ketones in the blood, which can cause nausea and stomach cramps due to the build-up of acids, normally hydrochloric acid (HCL), in your stomach.

During fasting, your stomach experiences an unusual eating pattern where you don’t consume anything solid for long hours. However, the stomach keeps waiting for the food and releasing HCL, leading to acid accumulation. The accumulated acid in your stomach can make you feel queasy now that you’re fasting. 

Additionally, when the sugar content in your body decreases due to fasting, the body activates the sympathetic nervous system to bring the body’s blood sugar level back to normal, which might cause nausea as well.

Certain people may also experience nausea if they drink water on an empty stomach.

#5 Anxiety and poor concentration

Low blood sugar during extended fasting or calorie restriction periods can lead to anxiety, poor concentration, and irritability.

The drop in blood sugar levels causes the body’s stress hormone, cortisol, to increase. This increases the levels of anxiety and stress in an individual. Food restriction can also impact the body’s level of serotonin, also known as the happy hormone.

You might also forgo social interactions due to your dedication to intermittent fasting. As a result, this may cut you off from social connections, which can quickly lead to anxiety, depression, and loneliness.

This is why you should consider simple and shorter fasts. Ensure the fasting window is brief to avoid any negative effects on your mental health.

#6 Fatigue

It’s normal to feel worn out after starting an intermittent fasting program. Your body is functioning on less energy than usual, and since fasting can increase stress levels, it can also disturb your energy levels.

According to studies, some individuals who engage in intermittent fasting feel worn out and overwhelmed.

A dip in blood sugar resulting from intermittent fasting may also cause weakness and fatigue. Additionally, intermittent fasting may produce sleep disruptions in certain individuals, contributing to daytime fatigue.

Because our primary energy source often depends on the nutrients absorbed from our meals, reducing food intake would explain the decreased energy supply until you get used to it. 

#7 Unpleasant breath

Unpleasant breath is caused by a decrease in salivary flow and an increase in acetone in the mouth. Acetone is produced as the body enters a state of ketosis (also known as “ketogenic”) that results from dieting.

Fasting promotes your body to use fat as a fuel source, which kickstarts the ketosis process. Acetone builds up in your system and is detectable from your breath when you fast because it is a waste product of fat metabolism.

#8 Malnutrition and eating disorders

It’s more difficult to get the right amounts of the macro- and micronutrients your body needs when you eat fewer meals and snacks throughout the week. 

Inadequate intake of some nutrients can cause malnutrition, which can cause many health issues, including digestive issues, bone disease, skin problems, or neurological symptoms. 

Stress and an obsession with food and calories can lead to eating disorders or disordered eating behaviors in certain people. Additionally, eating more simply because it’s “time to eat” might result in binging and overeating during the allotted eating window while ignoring any physical signals of fullness.

Your relationship with food may also start to change. You may unintentionally or actively consume less than your body requires, which can negatively affect your physical, mental, and hormonal health, like stopping your period, losing your hair, and having blood sugar problems.

#9 Digestive issues

Intermittent fasting may cause digestive issues such as constipation or diarrhea. That’s because the body’s metabolism slows down during periods of fasting followed by an extended period of eating.

A slowed metabolism means less energy is available for digestion and absorption of nutrients from food and other beverages consumed while fasting. This can lead to digestive distress, such as bloating, cramping, and diarrhea.

In addition, because some people have difficulty breaking down and absorbing certain foods (such as dairy), they may experience gas or bloating when they fast.

Constipation can get worse due to dehydration, another frequent side effect of intermittent fasting. It’s, therefore, important to hydrate well while engaging in intermittent fasting.

Consuming nutrient-dense foods rich in fiber may also help prevent constipation.

#10 Sleep disturbances

Research suggests that prolonged fasting with minimal calorie intake results in a significant increase in the hormone cortisol. 

Cortisol can affect your sleep quality. Eating nutritious meals throughout your non-fasting period and ensuring you consume enough calories to keep you active and mentally sharp are two ways to prevent this surge.

The sleep hormone melatonin can also drop when you fast, which influences your sleep patterns.

Additionally, eating irregularly can cause sleep disturbances and unfavorable changes in mood, alertness, and body temperature. Your body temperature may rise if you eat just before bed, making it difficult to get to sleep and stay asleep.

In addition to acid reflux, nausea, and gas, heavy meals can upset the stomach. Unnatural eating patterns can also affect your body’s circadian rhythm, messing up your internal clock and making you feel awake at night and sleepy during the day.

What Does Intermittent Fasting Mean?

Intermittent fasting is a diet plan that involves periods of eating and fasting. The idea behind intermittent fasting is to burn fat and lose weight. You do this by cutting calories for a specific time, then eating normally again.

Various types of intermittent fasting start with deciding on regular fasting and eating periods. There are, for example, alternate-day fasting and modified alternate-day fasting.

You may try the 16:8 pattern – eating only for 8 hours each day and fasting the other 16. The 5:2 pattern has also become popular. This is where you eat five days a week and restrict calorie intake to 500–600 for two days.

Is Intermittent Fasting Bad for You?

Even though some people use intermittent fasting to lose weight and others to treat long-term problems like irritable bowel syndrome, arthritis, or high cholesterol, intermittent fasting isn’t right for everyone.

Do not attempt intermittent fasting if you are pregnant or nursing. IF is not recommended if you are underweight (have a low BMI) or have a history of eating disorders. Additionally, if you are older than 70, you shouldn’t try fasting because it can cause muscle loss, which is important to preserve as you age.

Also, consult your doctor beforehand if you have diabetes, as this could not be safe for you. Skipping a meal might cause a low blood sugar level, resulting in lightheadedness, fainting, and falls. You should also get in touch with your doctor if you are taking any medications that must be taken with food and at a specific time.

What Can You Have During Intermittent Fasting?

When you’re on intermittent fasting, you’re not allowed to eat any food, but you can take drinks such as zero-calorie beverages, coffee, tea, and water. 

Some types of intermittent fasting permit small amounts of low-calorie foods during the fasting window. Taking supplements during fasting is generally acceptable as long as they contain no calories.

Stay away from soda or diet soda while doing intermittent fasting, even if you’re not following any diet.

Regular sodas often include high sugar levels and calories without any nutritional benefit. While there isn’t enough data and research to say whether diet soda is safe to drink during IF, studies suggest that consuming too many artificial sweeteners, which diet sodas frequently contain, can increase cravings and hunger and encourage weight gain and fat storage.

A Word From Our Nutritionist

Intermittent fasting has become the most popular health trend. Some people firmly believe in the eating pattern, discovering that it supports optimum health while managing their appetite and weight.

Many health benefits of intermittent fasting are linked to daily fasting intervals of at least 12 hours, although some experts suggest that a minimum of 16 hours of fasting may be necessary.

Most people who practice IF don’t experience the major effects of intermittent fasting. However, mild side effects are likely to occur, especially when first starting.

Severe or incapacitating side effects are uncommon and signify that you should immediately break your fast. You may also switch from alternate-day to periodic fasting, allowing you to eat daily within a given period.

It’s also crucial to prepare in advance and know how to respond in case you experience adverse effects during fasting. Having a plan will increase your chance of succeeding more often.


Any healthy lifestyle can benefit from intermittent fasting programs, and many people can fast safely and successfully in their daily lives. Before beginning a fast, ensure you are aware of possible side effects and prepare for your response. 

Remember that not everyone should practice intermittent fasting. Children, pregnant women, nursing mothers, and those with diabetes shouldn’t engage in intermittent fasting. You should always consult your doctor before beginning a new diet or eating regimen if you are managing any chronic illness. Finally, refrain from fasting if you have a history of disordered eating or are at risk of acquiring them.

If you experience any side effects of intermittent fasting or don’t feel your best, consult your doctor or nutritionist immediately. Always be cautious and pay attention to your body.

Written by Edibel Quintero, RD
Edibel Quintero is a medical doctor who graduated in 2013 from the University of Zulia and has been working in her profession since then. She specializes in obesity and nutrition, physical rehabilitation, sports massage and post-operative rehabilitation. Edibel’s goal is to help people live healthier lives by educating them about food, exercise, mental wellness and other lifestyle choices that can improve their quality of life.
The article was fact checked by Rosmy Barrios, MD
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Written by Edibel Quintero, RD
Fact checked by Rosmy Barrios, MD
Last update: October 21, 2023
10 min read 1626 Views 0 Comments

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