What are the side effects of intermittent fasting?

Intermittent fasting focuses on when you eat rather than what you eat. 

It involves setting specific windows for eating and fasting. The different types of intermittent fasting vary based on how long you fast and what counts as a fast.

Intermittent fasting has become popular fairly recently, so the research is still evolving.

So far, evidence suggests that it may provide health benefits, such as better blood sugar control and improved heart, gut, and brain health. It may also support weight loss. 

But it’s not without risks. Knowing what side effects to expect can help you approach intermittent fasting safely and effectively.

With the ZOE at-home test, you’ll learn how your unique body responds to food. We’ll provide personalized nutrition advice to help you achieve your long-term health goals.

Take our free quiz to get started.

What is intermittent fasting?

There are different types of intermittent fasting. The differences depend on how long you fast and what you can still consume during a fast.

Time-restricted eating (TRE) is a common type. It involves eating within a limited time frame every day and fasting for the rest of the day. 

Examples include 16/8, 18/6, and 20/4 eating schedules. The first number is how many hours you fast, and the second is your eating window.

So, on a 16/8 schedule, you only eat within an 8-hour window, say from 10 a.m. to 6 p.m. 

Alternate day fasting (ADF) involves eating as usual for one day, fasting the next day, and so on.

Some people have a modified ADF schedule. This involves still eating around 25% of your typical daily calories (around 500 calories) on your fasting days. 

The 5:2 diet involves eating as usual on 5 days a week and having only 500–800 calories on the other 2 days. 

It’s normal to have side effects when you start intermittent fasting. Below, we look at common side effects and other risks to consider. 

1. Hunger

People typically feel hungry during their fasting periods, but intense hunger and cravings can be disruptive. 

Very little research has compared long-term hunger in people with different intermittent fasting schedules.

Generally, you can expect to be hungrier than usual at first. Some studies report that this goes away in time, but it isn't always the case.

Some researchers have compared hunger levels between people doing intermittent fasting and people doing traditional calorie restriction. 

A 2022 study included 34 participants who followed either a 5:2 diet or calorie restriction and also did resistance training. After 12 weeks, both groups had low hunger levels, and most participants had stuck to their plans. 

Another study looked at 112 adults with obesity who followed either a modified 5:2 diet or calorie restriction for 1 year. The weight loss and heart health benefits for both groups were similar, but the participants with the 5:2 diet felt hungrier, on average.

Ultimately, it's important to pay attention to your body. 

If you find that your hunger is disruptive, you can adjust the intermittent fasting schedule to suit you. This may mean having a shorter fasting window or eating a limited amount during your fast. 

2. Headaches

During a fast, you may be more likely to have a headache. This risk is higher for people who are prone to headaches and fast for more than 16 hours a day.

A fasting headache is its own type of headache. The pain is usually mild to moderate, and you’re likely to feel it either throughout your head or only in the front.

Scientists aren't sure what triggers it, but it may involve:

  • caffeine withdrawal

  • dehydration

  • low blood sugar

  • lack of sleep

If fasting headaches are frequent, it can help to look at your caffeine habits, water intake, and sleep hygiene.

If this or any other symptom is concerning you, ask a healthcare professional for advice.

3. Fatigue and mood changes

When your body has less fuel, the result can be tiredness, difficulty concentrating, irritability, and aggression.

However, one clinical trial saw significant improvements in mental health and physical and mental fatigue after 3 months. The participants were following a 16/8 TRE schedule for at least 5 days a week.

And one review found that in some studies, many participants with overweight or obesity reported improvements in their energy and mood. 

So far, research suggests that intermittent fasting can improve mood and fatigue. But it's important to pay attention to your body to decide what works best for you.

4. Dehydration

When you fast, you lose salt and water in your pee, especially during the first 2–4 days. Losing too much of both can cause dehydration. 

Some early signs are dizziness, fatigue, and headaches. If you’re feeling any of these during a fast, it may be a sign to increase your water intake.

More severe dehydration can also lead to problems with attention, memory, and mood.

Make sure you’re drinking enough during fasting. It can help to check your pee — if it's dark, you may be dehydrated.

5. Sleep problems

Starting intermittent fasting may cause sleep difficulties. This may be because your new meal times interfere with your circadian rhythm.

One review looking at the influence of intermittent fasting on sleep found no overall negative effects. Some of the studies indicated that this type of eating might actually improve sleep quality. 

So, if you’re having sleep problems, it’s likely that your body is adjusting to the new schedule, and the problems should improve with time. 

6. Malnutrition

Intermittent fasting only affects when you eat, not what you eat. And fasting for long periods can increase your risk of not getting enough nutrients.

Possible symptoms of malnutrition include:

  • unintentional weight loss

  • feeling cold

  • tiredness or weakness

  • frequent illnesses

  • difficult concentrating

  • mood changes

While intermittent fasting can have multiple health benefits, it isn't a silver bullet. It's important to pay attention to what you’re eating.

Aim to eat a diverse variety of nutrient-dense fruits, vegetables, nuts, seeds, legumes, and grains, as well as sources of healthy fats, like extra virgin olive oil. 

At ZOE, we recognize that nutrition is personal. There's no one-size-fits-all way of eating.

With the ZOE at-home test, you can learn about your unique blood sugar and blood fat responses, as well as which “good” and “bad” bugs you have in your gut. Then, we can provide you with nutrition advice tailored to your body.

Take our free quiz to get started.

Who should avoid intermittent fasting?

Intermittent fasting isn’t for everyone. People who should avoid it include:

  • pregnant women

  • anyone with an eating disorder or a history of one

  • older adults

  • people with type 1 diabetes

  • anyone with type 2 diabetes who takes insulin, sulfonylureas, and meglitinides

If you need to take medication with food, ask a healthcare professional to recommend a fasting routine that works with your medication schedule.

And if you have a chronic health condition, it’s best to speak with a healthcare professional before you make any big changes to your diet.


Intermittent fasting may have health benefits, like improving your gut and heart health and blood sugar control, as well as supporting weight loss. But it isn’t for everyone. 

If you try it, you might experience some mild side effects, so it can help to know what to expect. These side effects include headaches, fatigue, hunger, dehydration, and sleep problems. 

It’s very likely that most side effects will go away in time. You can also reduce them by making small changes, like increasing your water intake.

If your side effects last, you may need to change your intermittent fasting schedule. If you have any concerns, speak with a healthcare professional.

Intermittent fasting only involves changing when you eat, not what you eat. To reduce any risk of malnutrition, make sure you’re eating a diverse range of nutrient-dense, whole plant foods when you’re not fasting.

Some people shouldn’t try intermittent fasting. This includes pregnant women, older adults, people with type 1 diabetes, people with type 2 diabetes who take certain medications, and anyone with a history of disordered eating.

If you’re in one of these groups, speak with a healthcare professional about your health aims and the best ways to reach them.


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