What Are the Risks and Side Effects of Intermittent Fasting?

What Are the Risks and Side Effects of Intermittent Fasting?

Intermittent fasting has trended for years, and everyone from stay-at-home moms, Silicon Valley CEOs and fitness influencers swear by its efficiency at shredding fat off your body.

If you’re reading this article, you probably know how intermittent fasting works and why you may want to do it. You have a small window of time each day, for example 8 hours, when you eat and the remaining 16 hours you don’t.

This helps you in several ways.

It’s easier to control how many calories you eat since it’s harder to snack and nibble on cookies all day. It helps you become more insulin sensitive, which prevents storing energy as body fat.

In short, you want to do intermittent fasting when you want to lose fat. But are there any possible side effects of intermittent fasting?

Yes, there is. Keep reading to get the complete, honest and nuanced picture:

Is Intermittent Fasting Dangerous?

Online content mills love to scare you with clickbait titles about fitness trends and diets being “dangerous”. Same goes for intermittent fasting, and throughout the years I’ve heard claims that intermittent fasting could possibly cause heart attacks or cardiac arrest.

But what’s the truth?

The truth is that the claims that intermittent fasting is dangerous only apply to persons with pre-existing medical conditions or in high-risk groups.

No, you should not start intermittent fasting without consulting your doctor if you’re diabetic.

No, if you’re anorexic you should not start intermittent fasting.

If you have a medical condition you should always consult your doctor before you even walk outside your house. But let’s be clear: Intermittent fasting is safe for every normal, moderately healthy person.

But the fact that intermittent fasting is safe doesn’t mean it’s good in all instances.

There may be some cases where it’s counter-productive:

If this applies to you… DON’T do intermittent fasting…

The sole goal of intermittent fasting is to lose fat (and weight). That means if you do it successfully, you will be in a calorie deficit — which means your body burns more energy than you eat.

And if you and I think about it logically, we can think of situations where we don’t want to follow intermittent fasting:

You don’t want to do it if you’re underweight.

You don’t want to do it if your goal is to gain weight.

You don’t want to do it if you’re a teen and your body is still growing.

You don’t want to do it if you’re pregnant or breastfeed and have to feed another living creature.

Got that? Great. If nothing if this applies to you, you’re ready to get into intermittent fasting. But even though it may be a good choice for you, this style of eating will still have a few side effects that you need to be aware of from the start.

Here we go:

You’ll be hungry (but do this and you won’t be).

It’s hard to understand, but hear me out:

If you want to burn fat with intermittent fasting, you need to consume less calories than your body needs to maintain the same amount of mass. There’s no way to get around that.

And when you are on a calorie deficit, you will almost always feel a bit hungry. Thus, you’ll also feel hungry when intermittent fasting,

When you first start fasting, you will feel grumbles in your stomach. Especially if you’re used to breakfast and constant snacks. But don’t despair… Your body will get used to this new way of eating — it’s just a matter of sticking to the plan — and you can use a few tricks to keep your hunger under control:

Rather fast during the morning and daytime than later in the day.

In the morning, you will sleep, wake up and get to school or work. During the day, you’ll hopefully be busy with work that occupies your mind.

But later in the day, when you get home and kick off… your mind tends to wander away to the thoughts of delicious meals and tasty snacks.

You will be much more prone to eat at the end of a day, so it’s naturally smarter to place your eating window during that period.

Eat satiating foods that keep your belly full.

Salad-greens like lettuce, spinach, and cucumber allow you to eat a whole fricking bowl and only ingest 50 kcal. One factor that determines how satisfied or hungry you are is how full your stomach is. Fiber-rich greens like beforementioned are great for this, as they take up a high volume with little calories.

The eye also satisfies you — seeing a huge bowl of food, and then eating it, will trick your mind into feeling full no matter how many calories you actually ate.

They also contain quite an amount of fiber, which takes long to ingest and keeps you satisfied longer. Fiber is overrated and overconsumption leads to bloating and constipation, but a little bit can be helpful during your diet.

Other tips to stave off hunger during your fast:

  • Drink a lot. When you don’t eat, you’ll miss water you’d otherwise get from food — which means you have to drink it instead.
  • Drink coffee. It’s a good appetite suppressant, and the caffeine in coffee has an impressive effect on how many calories your body burns in a day.
  • Stay focused on something else than food. Use your fasting period to be extra productive and work more efficiently. Your mind will be more clear anyway.
  • You may want to try a weaker stimulant like Synephrine to further increase the number of calories you burn in a way, and encourage fat burning in stubborn areas such as the belly…
  • … Or stronger ones like Yohimbine. Personally, I only stick to small amounts of weak stimulants for my own well-being.

You can feel tired.

When you’re new to intermittent fasting, it’s normal to feel a lack of energy. Almost as if you just woke up.

You’re on a calorie deficit and go long periods without any food at all, which is bound to cause lower energy levels. And if you’ve ever done a really hard diet before, you know eating below maintenance messes up your sleep — which also applies when you do IF.

With time, you’ll start to get used to the fasting. And eventually, you will probably find that fasting gives you higher energy levels than before, since your body doesn’t have to focus on digesting all that food!

Note: I recommend you to NOT work out fasted. For some people, it’s hard to time their workouts and fasts in their schedule — but try to at least have an EAA or protein shake, or a small bite of meat before your workout (if you haven’t had a meal before it during the day). This is to counter muscle breakdown during your workout.

If you decide to train fasted anyways, you’ll be fine except that you won’t be in an optimal to maintain or grow muscle. But does it matter in practice? Not really. You may feel a bit shaky at first, but drink enough water and ease into to workout and you’ll be alright.

You can feel irritable

I’m going to tell you a little secret:

Sometimes… I become a real bitch when I don’t eat.

There. I said it. I’ve been intermittent fasting for 5 years, and I still become a bit irritable when I’m hungry. It’s normal when fasting or being in a calorie deficit since your body produces all these extra stress hormones that make you anxious and depressed.

You can’t do much about this — if you happen to feel the same way — except exercising a bit of self-control. Acknowledge that you’re irritable because you are hungry, and what feels like a pain at the moment probably doesn’t matter at all.

You have a hard time building extreme amounts of muscle

Okay, I’ve got to be clear with you:

Intermittent fasting is mainly for you who want to lose fat. When you want to lose fat, it’s really hard to build muscle at the same time.

Fasting also works if you’re trying to maintain a ripped physique all year round, like Martin Berkhan:

But if you’re looking to become a bodybuilder… someone with extreme amounts of muscle mass…

Intermittent fasting is just not going to cut it. You will need more frequent meals to keep your protein synthesis in a constantly elevated state. At least if you want to maintain muscle mass when dieting.


Intermittent fasting is great for dieting and health. But you will experience side effects, so you need to know if intermittent fasting is the right choice for you — and how to combat the few pain points you experience.

Until next time,

— Alexander Contrarian