The Side Effects Of Intermittent Fasting


8 Min Read

Intermittent Fasting offers both negative and positive results. While it reduces risk of diabetes and rejuvenates cells, it can also cause bloating, diarrhea, caffeine-dependency, reduced energy, hunger pangs, heartburn, brain fog and headaches. It also reduces athletic performance and may have a negative effect on a woman's hormones. Avoid during pregnancy.

Every other article about Intermittent Fasting talks about its benefits and how amazing it is. But no one talks about the challenges one faces when fasting. After all, Intermittent Fasting (or rather IF) is an extreme way of dieting. Humans have been eating several meals a day for hundreds of years. We can’t just switch to caveman-eating and expect everything to be normal.

‘Intermittent Fasting’ diehards are probably in defense mode right now. But you should relax – I’m not trying to convince anyone to quit Intermittent Fasting. I just want to show you the other side of it that no one talks about. It’s entirely up to you to decide if Intermittent Fasting is for you or not. No single approach works for everyone.

Benefits Of Intermittent Fasting

Before I show you the side effects, here are some popular benefits:

  • Reduces insulin resistance and lowers risk of type II diabetes
  • Rejuvenates cells
  • Can help you live longer

Frankly, I tried Intermittent Fasting for only a few weeks and realized it wasn’t for me. So the side effects that I’ll be sharing in this article are not from my limited personal experience but rather from the experiences of Intermittent Fasting practitioners. Note that not every Intermittent Fasting practitioner experiences these side effects. And some of the side effects are short term. But every Intermittent Fasting beginner should be aware of them.

But before we go further, let me answer a question that’s probably lingering on your mind.

Does Intermittent Fasting Enhance Weight Loss?

I didn’t mention weight loss as one of the benefits of Intermittent Fasting and here’s why – it’s the total number of calories you consume in a day that determines weight loss, not when you eat. Intermittent Fasting can help reduce the total number of calories you consume in a day. Most Intermittent Fasting methods give you a small feeding window to eat all your daily calories. And it’s hard to overeat if you only have a few hours to eat. Subjects in this study were able to reduce their calorie intake by up to 8% when practicing Intermittent Fasting.

Note that Intermittent Fasting can help you lose weight only if you maintain a calorie deficit. Besides that, it has no weight loss benefits. So eating one or two meals a day won’t help you lose weight if you overeat. A group of researchers from Netherlands conducted a study to find out if Intermittent Fasting has weight loss benefits. They divided their subjects into two groups. One group was on a regular diet and another on an IF diet – they all ate the same number of calories and had the same diet composition. And the researchers found no difference in body fat and lean body mass of the two groups after the study was complete.

While Intermittent Fasting may have its benefits (for some), it does have its side effects as well.

Side Effects Of Intermittent Fasting

Being Uncomfortably Full After Eating

Most of us are used to eating several meals a day. This makes it easy to consume enough food without being overly full. But with a small feeding window, you have to eat one or two big meals. These big meals can leave you feeling uncomfortable and constipated. Furthermore, eating big meals at night can reduce the quality of sleep.

Most times you’ll be very hungry when the fasting period ends. So you’ll eat a lot of food and also eat again after a few hours before you start fasting again. With IF, you have to learn to handle big meals and you should be comfortable with your stomach being stretched out for a few hours. Also note that digesting a big meal can put stress on the body.

Obsession With Fasting And Feeding Window

Some IF dieters obsess over when they’ll eat. They go to the extent of counting down hours and minutes to the feeding window. This leads to an obsession over food and folks start thinking about food all the time. Other dieters stop following IF dieting protocols and extend the fast-period in order to lose weight faster.

Unfortunately, this leads to unhealthy weight loss and regain of weight in future. These obsessions don’t last long so the dieters eventually go back to old (unhealthy) eating habits. Surprisingly, some dieters like IF because it reduces the obsession of eating after every few hours. They prefer the simplicity of one or two meals a day.

Over-Reliance On Coffee

Many Intermittent Fasting plans allow dieters to drink coffee to control hunger and stay energized. But drinking coffee regularly can lead to addiction. And as you may know, drinking caffeine can lead to poor sleeping habits, stress and anxiety – causing weight gain in return.

Hunger And Food Cravings

Hunger and cravings are usually a challenge for anyone trying to lose weight. If you can’t control hunger and curb your cravings, you won’t lose weight. Some people experience hunger pangs even when they eat 6 meals a day. So fasting makes their hunger much more intense. But realize it’s natural to feel hungry at the start of an IF diet and your body may get used to hunger with time.

Reduces Athletic Performance

It’s okay to train moderately when fasting. But doing intense workouts like power-lifting, cross-fit or high intensity interval training can hurt you. Research shows that athletic performance usually diminishes when fasting. In this study, Senegalese researchers tested the athletic performance of runners during Ramadhan and during non-fasting time.

And they found that the athletic performance of the athletes reduced significantly during the month of Ramadhan. Other studies also show increase of fatigue after intense workouts when fasting.


Many people experience heartburn when practicing IF. Sometimes the heartburn rectifies itself after 5-6 weeks, but if they don’t, then you should see a doctor. The heartburn happens because the body is used to your old eating pattern so it releases acids at certain times. And when you change the eating pattern, it’ll try to stick to previous eating behaviors.


Headaches are a common occurrence during fasting. Most folks complain of mild-headaches every now and then while others experience them throughout. Drinking enough water helps relieve the headaches in some cases.

Negatively Affects Pregnant Women

Some studies show that fasting during pregnancy does not affect the fetus or the mother. But you may want to avoid IF during pregnancy. Let hunger dictate what you eat during this period. Don’t starve yourself or the fetus in the name of Intermittent Fasting. After all, fasting may not be beneficial during pregnancy. In fact, the Intermittent Fasting side effects on this list can make life harder for you during pregnancy.

Frequent Diarrhea

Many IF PR practitioners experience diarrhea after fasting. But how severe it is depends on how long the fast was. Longer fasting duration causes explosive diarrheas. This is caused by high fluid intake – drinking a lot of coffee and water.

Brain Fog

Brain fog and feeling sluggish are also common with many IF dieters. But the brain fog usually disappears after some time. In fact, some studies show that long term fasting improves brain function.

Negatively Affects Women’s Hormones

Some women who try Intermittent Fasting claim to experience missed periods, metabolic disturbances and early-onset menopause. Women’s reproductive hormones are highly sensitive to energy intake. So going for long hours without food affects the functionality of these hormones. Other negative effects include: adult acne and obsession over body image.

Low Energy

Low energy is one of the biggest concerns for folks who want to try Intermittent Fasting. And it’s a genuine concern. Fasting results to low energy and weakness. This can stop you from exercising or staying physically active.

Final Word

I hope this article gave you an insight on the challenges of Intermittent Fasting. Now, you’ll be more prepared if you choose to try it. Note that these side effects are not specific to any method of Intermittent Fasting. So you may or may not experience them, depending on the approach you take. And others only last for a while and disappear.

Don’t feel the need to practice Intermittent Fasting if it doesn’t work for you. You can lose weight even when eating several meals a day.

Have you tried Intermittent Fasting before? What side effects did you experience?

Brian Syuki

Brian Syuki is a fitness expert who teaches people how to get leaner and healthier using bodyweight exercises and proper nutrition.

Brian Syuki

Brian Syuki is a fitness expert who teaches people how to get leaner and healthier using bodyweight exercises and proper nutrition.