10 Potential Intermittent Fasting Side Effects

Written by Aly Owen

Medically reviewed by Josie Porter, RD

Published on February 20, 2023 | Updated on September 24, 2024

Intermittent fasting involves a repeated pattern of eating and fasting that focuses on when you eat rather than what you eat. 

As an eating routine that doesn’t require micro-managing calories, sticking to very specific meal plans, or swearing off your favorite comfort foods (feel free to gaze lovingly at your carton of ice cream here), intermittent fasting is a popular way of promoting healthy habits and achieving certain health goals without turning your whole life upside down.

It’s backed by science, too: extensive research has linked intermittent fasting to a wide range of health benefits, like weight loss,[1] reduced body fat,[2] lower systolic blood pressure,[3] and reduced inflammation.[4] 

Before you ride off into the sunset and live happily ever after with your new eating routine, it’s important to remember that everybody is different: intermittent fasting may not be right for you, and there are some intermittent fasting side effects you might encounter. 

No need to start spiraling or panic googling — we’ve got you. In this article, our experts walk you through the ten most common side effects of intermittent fasting so you know what to watch out for and how to manage them. 

Want to learn more about the basics of intermittent fasting first? Check out our overview of intermittent fasting for beginners. 

10 potential intermittent fasting side effects

Let’s get right to it: what are the side effects of intermittent fasting?

Your body is your body, and the specific fasting diet side effects you experience may vary. Some people experience only one or two, whereas others encounter lots of adjustments. Likewise, some side effects may just be temporary, while others might stick around longer. 

Before you jump into the list below about what you may or may not experience, the first step in any intermittent fasting journey is to decide if it’s the right path for you. To see if intermittent fasting might fit your needs and help you achieve your health goals, take our Simple quiz. 

1. Hunger and cravings

While a fasting-based eating routine may actually reduce your appetite [5] once you adjust to it, it’s natural that you may feel hungrier than usual when you initially reduce your food intake or go for longer periods without eating. The good news is that these hunger pains (or hangry demands) tend to be short-lived; some studies suggest it only takes a few days to recalibrate your appetite.[6]  

2. Headaches and lightheadedness

When you’re fasting, you may be more prone to caffeine withdrawal and low blood sugar,[7] both of which can contribute to headaches or feeling woozy. You also may be more likely to be dehydrated, which can cause or exacerbate headaches, especially if you get them regularly anyway.[8] Typically, these heady feelings (we know, terrible pun) happen only during the first couple of days of your new eating routine, and so far, evidence suggests they’re mostly mild and manageable.[9]

3. Poor sleep

More research is needed to determine the specific relationship between sleep disturbances and intermittent fasting, but so far, some studies suggest that decreased sleep quality [6] and sleep disturbances [10] may be one of the most common side effects from intermittent fasting, especially if you tend to have a big meal in the evening. On the other hand, others suggest insomnia [11] as well as sleep quality and sleep duration [12] may not be significantly affected. Furthermore, over time, intermittent fasting may help to reinforce your circadian rhythms — your internal body clock that helps regulate your sleep-wake cycle — and in turn, improve sleep quality.[13] 

4. Fatigue and low energy

Most of us have been there — the afternoon slump. You know, the one where you want to close your eyes “just for a second,” but gravity seems to be working against you. Just as low blood sugar might lead to headaches and lightheadedness, it can also give you brain fog or make you feel extra tired or weak, especially if you’re not getting your usual quality of sleep. However, under certain conditions and after a certain adjustment period, intermittent fasting may actually reduce fatigue and improve energy levels.[14] 

5. Digestive issues

Your body is super strong, but it can also be super sensitive to changes in routine. Modifying your eating habits can lead to things like constipation, diarrhea, bloating, and nausea since all your gut bugs are adjusting to the new feeding schedule. Plus, water helps keep things moving, so if you’re dehydrated, you might feel more blocked than usual. However, once your body settles — and as long as you’re not eating anything new that may be upsetting your tummy — intermittent fasting may improve the balance of your gut bacteria [15] and reduce inflammation [4] that can contribute to tummy troubles. While more evidence is needed at this stage, this is promising stuff. 

6. Bad breath

Since intermittent fasting may lead to dehydration, decreased salivary flow (which causes dry mouth),[16] and increased acetone production (a byproduct of fat burning that smells like nail polish remover),[17] bad breath can be one of the side effects of fasting. 

7. Irritability and other mood changes

While some studies suggest intermittent fasting may reduce symptoms of anxiety and depression,[18] others also suggest it could increase negative emotions and decrease positive emotions,[19] particularly if you’re a fasting newbie.[20] When you initially start fasting, you may also be more likely to feel crankier or more stressed. 

8. Malnutrition

Intermittent fasting helps bring more conscious awareness to your eating habits and what you eat. While this attentive focus can be a helpful motivator for people looking to introduce more nutrient-dense foods into their routines, it also means you have to be even more mindful about what you’re eating to ensure you’re still getting everything your body needs in a shorter eating window. This focus on meal planning can also then lead to a preoccupation with food or exacerbate disordered eating tendencies,[21] which is why we always emphasize that fasting is not suitable for anyone with a history of disordered eating.

9. Low blood sugar

One of the benefits of intermittent fasting is that it can decrease both cholesterol and blood sugar and improve sugar and fat metabolism — all of which are factors that generally improve your metabolic health.[22] However, when your body is transitioning into a fasting state, your blood sugar may drop and lead to headaches, dizziness, and fatigue. Likewise, if you already have a condition that involves insulin or thyroid problems, low blood sugar can be dangerous. 

10. Unhealthy eating habits

Let’s get this straight: no food is inherently “good” or “bad” — our plate is not some weird interpretation of the Garden of Eden. That said, there are some foods that are less nutrient-dense and less health-promoting than others, and since fasting restricts the time in which you’re allowed to eat, you may find your inner munchies monster creeping in to whisper sweet nothings of all the sugary, ultra-processed foods you should binge-eat during your eating window to make up for lost time.  

Luckily, fasting is generally a safe process overall, and there are fairly simple ways to limit these side effects or avoid them altogether. We’ll cover those hacks for healthy fasting shortly. 

Are some intermittent fasting schedules more prone to such side effects? 

One of the most important aspects of any intermittent fasting schedule is figuring out which one works best for you. 

That means not only knowing why you want to adopt an intermittent fasting routine but also factoring in your unique body and circumstances — like your budget, schedule, food preferences, nutritional needs, and other lifestyle considerations. 

Our team at Simple all agree, the best eating routine is the one that’s right for you — and one that you can stick to. 

That said, if you’re dipping your toes into the intermittent fasting pool and want to limit your risk of capsizing from fasting side effects, some schedules may be better than others for keeping your body stable.

Any eating routine that involves fasting for certain periods can introduce potential side effects — after all, any change in eating habits can rock your body’s boat — but if you think of the fasting schedule as waves, a good rule of thumb is that the bigger the wave, the bigger the potential (positive and/or negative) impact. 

Time-restricted eating schedules that involve alternating between hours of fasting and hours of eating (like the intermittent fasting 16:8 schedule, the 12:12 schedule, or the 14:10 schedule) involve less of a change to your typical eating habits, as they simply extend the natural fasting period that happens during sleep. So, your body will still need several weeks to adjust, but the side effects from fasting may be more limited in scope and severity. 

Alternatively, fasting routines that involve either longer fasting windows (like the Warrior Diet and the Eat Stop Eat schedule) or days of more significant calorie restriction (like the 5:2 diet or alternate-day fasting) may cause more of an upset to your body’s natural rhythms and introduce more challenges around ensuring your body still gets the nutrients it needs. That means you need more careful planning and monitoring to ensure your body stays the course.

If you’re looking to test the waters, we always recommend starting small with as few changes as possible and trying out different approaches to see what feels best for your body. While some side effects are to be expected in any approach, if you’re experiencing them consistently or severely, stop and consult a medical professional immediately. Otherwise, just focus on the shore you’re trying to reach. Hunger, hanger, fatigue, or any of those other side effects won’t cling to you like a barnacle and last forever — you just have to ride the waves out to smoother sailing! 

Want to figure out which schedule might be your best fit? Our Simple quiz is here to help you find your match! 

What about the long-term side effects of intermittent fasting? 

While there is significant evidence to support the short-term benefits and risks associated with intermittent fasting, more research is needed with respect to the long-term side effects of intermittent fasting. The majority of clinical trials so far have only studied participants for less than a year, and more long-term observation across different populations would be needed to understand how intermittent fasting might affect the body over time.  

So far, what we do know is that some of the side effects we mentioned in our list above could become more long-term effects of intermittent fasting if left unchecked, particularly malnutrition and unhealthy eating habits. These side effects can then, in turn, open the door to additional side effects that stem from being underweight or undernourished, like changes to your immune system, digestive system, muscle function, and menstrual cycle. 

It’s also possible that a fasting-based eating routine can modify certain hormone levels in assigned-at-birth women and men.[23] While some of these adjustments may be helpful (like reducing androgen in women with polycystic ovarian syndrome), those same adjustments may not be beneficial to others. However, existing research is very limited and fails to explore the effects of fasting specifically on trans men and women, nonbinary people, and individuals currently on hormone replacement therapy.

How to prevent intermittent fasting side effects 

So, now you know the “what” of “What are the side effects of fasting?” but the real question is how do you prevent them? 

Here’s the good news: preventing intermittent fasting side effects — or mitigating them if you’re caught in the middle — is fairly straightforward. It may not be a magic wand for fixing all your ailments, but it can make a massive difference in your fasting experience and set you up for a successfully smooth journey. 

And the key lies in your pantry. 

Drinking enough water during your fasting windows and focusing on health-promoting, nutrient-dense foods like whole grains, lean protein, colorful fruits and veggies, calcium-rich foods, and healthy fats during your eating windows are the building blocks to keeping your superhero body fit and ready for action. 

Think of these nutrients as your power-ups — making sure you’re hydrated and filled with the fuel you need will help banish and ward off hunger pains, cravings, mood swings, low energy, and other potential fasting baddies. Remember, fasting isn’t starvation — it’s helping your body rest, digest, and rejuvenate in between satisfying, fulfilling meals. Not sure where to start? We’ve got your back with our guides on what to eat during intermittent fasting and what you can drink while fasting.

For some side effects, like sleep disturbances and mood shifts, techniques that can combat stress and promote relaxation — like yoga, gratitude journaling, mindfulness meditation, and low-intensity physical activity — may also be useful. These are all great ways to boost your overall mental health, too, which never hurts!

What are the benefits of intermittent fasting? 

As with potential side effects, intermittent fasting benefits can vary. While everybody experiences fasting differently, and certain benefits may not be a guarantee for your unique body, some of the science-backed pros of intermittent fasting include: 

Since cutting calories typically slows metabolism — so our body can preserve its energy stores for as long as possible — we often get asked questions like, “Does intermittent fasting slow metabolism, too?” While intermittent fasting can lead to a natural reduction in calorie intake, it may actually support your metabolism by preserving muscle mass, especially if you eat adequate amounts of protein and build strength training into your routine.  

So, who should avoid intermittent fasting? 

The first question to ask yourself before beginning any fasting-based food routine is whether intermittent fasting is right for you. 

We recommend speaking with your doctor before starting any fasting method since it’s not right for everyone and can be dangerous in certain situations. You should be particularly cautious about intermittent fasting if you: 

• are pregnant, breastfeeding, or trying to conceive; 
• have a history of disordered eating;
• have diabetes or another chronic medical condition;
• have poor or irregular digestion;  
• have a BMI <18.5
• are extremely active
• are under the age of 18, or 80 years old or more; or
• are prescribed medication (for example, anti-hypertensives, diabetes medications, or any that need to be taken with food).

Written by Aly Owen

Medically reviewed by Josie Porter, RD

Published on | Updated on September 24, 2024