The Side Effects of Intermittent Fasting

author avatar Dr. Eric Berg 08/31/2023

a black and white drawing that says intermittent fasting on a plate

You’ve heard the buzz about intermittent fasting, right?

There’s no shortage of information about its benefits for your health. If you research fasting online, you’ll find endless amounts of information its positive effects.

And it’s true - fasting is an incredible tool for your health and wellness, especially when you combine it with a healthy diet like keto. I’m a die-hard fasting proponent.

Yet sometimes people experience side effects that are so unpleasant, they give up on fasting and miss the benefits. Which is a shame, because it’s easy to lessen your chances of experiencing them.

I’ll explain the potential side effects just so you’re aware of them.

But first, let’s look at the benefits of fasting.

In this article: -

  1. Benefits of Intermittent Fasting
  2. Potential Side Effects of IF
  3. Just Do Intermittent Fasting Correctly

Intermittent Fasting Has Incredible Benefits

I’ve talked a lot about the benefits of intermittent fasting. They are substantial, and include:

  • Weight loss, especially in your midsection
  • Cognitive improvement
  • Decreased inflammation, including for conditions like arthritis and autoimmune diseases
  • Improved memory
  • Better sleep
  • Decreased hunger
  • Fewer food cravings

These benefits are well-documented. And because I want to be totally transparent, I also want to tell you about the potential side effects of intermittent fasting. You deserve to have the details in case you experience some of these symptoms.


And There Are Potential Side Effects Too

Here are some side effects of fasting that you possibly could experience. They’re more likely to happen during the first three days as your body adapts to fasting: changing your eating periods, entering a fasted state, and possibly reducing your calories.

But if you do get them, they'll subside once you adapt to your new pattern of eating and the good stress you put on your body when you fast.

An increase in uric acid

When you fast, your uric acid level may spike. Uric acid is associated primarily with two conditions: gout, and a certain type of kidney stone.

If you’re predisposed to kidney stones, be aware that they are two types. One is oxalate; the other is uric acid stones. So if you’re intermittent fasting and get a kidney stone, it’s probably going to be the uric acid type.

With gout, your big toe starts to hurt, or you experience pain in joints that comes on when you do intermittent fasting.

But the big question is, why does fasting cause uric acid to increase?

One of the uric acid’s primary functions in the body is as an antioxidant. As your body repairs and detoxifies, it uses uric acid as part of the processes.

If your uric acid increases, that’s a sign your body pH is too acidic. You need to alkalize it. When you eat, make sure you have big salads in order to consume lots of vegetables. Or, you can drink water mixed with an electrolyte powder, or take potassium citrate, which will help increase the alkalinity in your body.

Feeling cold in your hands and feet

This can happen during the time when your body is still adapting to fasting and keto. Your thyroid doesn’t have to work as hard. During this time its functionality will slow somewhat which is why you start to feel cold while fasting. This isn’t a hypothyroid condition. It’s temporary


Moodiness is more common during the first three days of starting fasting, particularly if you jump into longer periods of fasting right away. This is why I recommend starting gradually. Go from three meals per day to two, then to one. But let your body tell you how long you can comfortably go from meal to meal.

If you’re having a hard time going without eating from one meal to the next, it’s a sign you have Fat Storing Hormone resistance. You’ll get moody and display other symptoms of low blood sugar. The way you know you’re in ketosis and everything is working well, is that you can go long periods without eating yet still feel fine.

Don’t worry; the moodiness will go away as long as you stick to your goal of extending your fasting period from one meal to the next while following a Healthy Keto diet.


You may get headaches at first because you’re changing from running your body and brain on sugar. Your brain doesn’t have the capacity to store sugar for fuel and is dependent on the energy source circulating in your bloodstream. Thus, as you change your energy source from sugar to fat, you may get a headache from low blood sugar until your brain is adapted to fat for fuel.

Like the moodiness, this too is temporary and usually will subside after the first three days. Your brain actually prefers burning ketones to sugar. This is why you experience improved memory and concentration.

Increased stomach acid

You may notice an increase in stomach acid; perhaps a little heartburn.

Fasting increases the acid in your stomach. However, most people do fine with increased stomach acid because they don’t have enough of it until they fast! So fasting helps heal your stomach and let it once again produce sufficient acid for proper digestion.

If you do experience heartburn, ease a little more gradually into fasting. That is, take a little longer before you lengthen the periods of fasting between meals.


You may feel nauseated when you eat. And this is interesting, right? You feel good while fasting, then you eat and feel like crap!

Feeling nauseated when eating is simply due to adaptation by your body as it adjusts to the different patterns of eating and not eating. This side effect is usually related to a sluggish gallbladder, and will go away over a couple of weeks.

If you need to, take some apple cider vinegar or bile salts after you eat.

Sometimes when people first start intermittent fasting, they freak themselves out a little bit at the idea that it will be 16 or 24 hours until their next meal. So they stuff themselves in a kind of survival mentality.

Bad idea! You’ll feel very bloated, especially if you’re eating a lot more fat than you’re used to. Simply eat a good-sized meal of healthy foods that satisfy you without overfilling you. Over time, your body will burn its own fat and that will keep you from feeling hungry for longer periods of time.


I know this one sounds frightening. But it really only happens if you try to jump too quickly into prolonged fasting. That is, you get a little carried away and decide right away to go a few days without eating.

Then, you faint.

But this is due to too low levels of sea salt, potassium, or other electrolytes.

Rest assured, you’ll be able to do a prolonged fast if you choose. Just ease into it gradually instead of trying to go too long fasting too early on into your intermittent fasting experience. Take some time to eat healthy and ensure your nutritional base is sound. This will then support you to successfully do a prolonged fast.

As well, when you break a prolonged fast you want to eat just a small amount of food, then wait a short period of time, then eat a little more food, then wait to see how your body responds to this new pattern of not eating combined with a healthy diet.

This is so you don’t overload your system, provoke a quick electrolyte shift, and risk fainting - or other symptoms - because of what’s called refeeding syndrome.

Hair loss

Hair loss may occur if you’re already nutritionally deficient. Perhaps you have a history of not eating healthy food, or you’re not doing Healthy KetoTM and consuming low-quality foods.

Besides hair loss, nutritional deficiencies can cause fatigue, grouchiness, and other unpleasant side effects.

If your hair starts falling out, that’s a sign you need more nutrients - usually B vitamins


Just Do Intermittent Fasting Correctly

a hand-lettered sign saying Not Hungry

I’ve already given you the benefits of fasting that are possible:

  • rapid yet healthy weight loss
  • increased mental clarity when your brain adapts to fat fuel instead of sugar fuel from carbohydrates
  • heightened energy

Yes, there are also potential side effects that are unpleasant. But if you do intermittent fasting correctly, you aren’t likely to experience the side effects. I know it’s tempting to dive into something you’re excited about, but in this case, it pays to be patient and follow my guidelines.

So take your time, eat healthy food, start intermittent fasting gradually, and watch how your health improves!

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Disclaimer: Our educational content is not meant or intended for medical advice or treatment.

Editor’s Note: This post has been updated for quality and relevancy.

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