Fasting Basics: Common Side Effects of Fasting

By Jason Fung, MD

Fasting has benefits ranging from weight lost to reduction in medications required for type 2 diabetes and hypertension (high blood pressure), but it does have some short-term side effects.  These unwanted consequences of fasting are due to the body transitioning from sugar burning to fat burning mode.

I always tell patients to think about their body having two different factories: the sugar fuelling factory and the fat fuelling factory.  Many of us have been using our sugar fuelling factory to keep our bodies running for years and maybe even decades.  During that time our fat fuelling factory remains closed.

Now suddenly you switch from eating all the time to forcing your body to fuel on fat stores.  You go from giving your body sugar fuel to using fat fuel for functioning.  This means you must wake-up the fat burning factory and slow down the usage of the sugar burning factory.

Now imagine how many struggles it must take to slow down usage of the factory you’ve primary used for years and starting up the fat factory which has been sitting there collecting dust.  It isn’t going to be a perfectly smooth transition.  There are going to be some inevitable barriers you’ll have to push through in order to get the factory up and running efficiently.

For some people this transition is flawless, and they won’t experience any side effects.  Others aren’t as lucky.  But if you fall into the category that experiences one of the undesired side effects below, don’t fret!  Most side effects go away completely within two to four weeks on sticking to the same routine.

Side effects only tend to linger if people don’t push through and stick to their fasting protocol consistently.  It might be fun, and it could even seem to be hard or near impossible at first, but the more consistent you are with your regimen, the sooner you’ll adapt and the side effect will go away.  It’s the people who don’t stick to it consistently who struggle with the side effects of fasting over the long-term.

You are most likely to experience side effects of fasting if:

  • You’re new to fasting (less common in people who start fasting on a low carb or ketogenic diet)
  • After consuming more carbs and processed foods during a vacation or holiday

You are least likely to experience side effects of fasting if:

  • You are on a low carb of ketogenic diet prior to fasting (your body is already fuelling on dietary fat)
  • You don’t deviate from your diet, i.e. eat lots of junk foods on the weekends
  • You stick with your fasting consistently

Headache, Dizziness, Mental Fog and Lethargy

These group of side effects are usually due to low sodium levels.  Our insulin levels begin to drop significantly when we start fasting.  This drop-in insulin sends a signal to our kidneys to release excess water since insulin causes water retention.  Most people find they urinate quite a lot when they’re new to fasting for this reason.

As our bodies rid themselves of excess water, we also lose electrolytes through our urine.  We’re also not eating as much food and have a dramatic decline in electrolyte consumption.  Our bodies tightly regulate our electrolyte levels, and this sudden change can throw off our body’s homeostasis.

The Solution:

  • Add a pinch of natural salt (Himalayan or Celtic salt are two favourites) under your tongue or in a glass of water a few times throughout the day
  • Drink some bone broth or non-starchy vegetable broth
  • Drink some pickle juice (no sugar)

Expert Tip:  If your sodium levels become too low, you may have to end your fast even if you do try to take some broth later.  We often call this “the point of no return.” To avoid this from happening take a pinch of salt, broth or pickle juice every three hours even if you feel okay.  This will prevent you from feeling unwell in the first place. 

My favourite electrolyte supplement: Fasting Drops


This unpleasant side effect can be extremely problematic and is one of the most common side effects experienced by people who are new to fasting, especially if they’re entering the fast after eating a lot of carbs.  It is also caused by the dramatic drop in insulin levels which signals to the kidneys to excrete excess water, which can result in watery and unwanted bowel movements.

The Solution:

  • Take 1 tablespoon of psyllium husk and stir into 1 cup of water, let it sit for 5-10 minutes before drinking it first thing in the morning
  • Repeat with a second tablespoon if necessary later in the day

Expert Tip: Our bodies can lose electrolytes through bowel movements just as we do through urination.  People who experience diarrhea or watery stools often start to feel symptomatic of low sodium levels.  Make sure you drink an extra cup of broth or pickle juice on days you experience this unwanted side effect.  You can also take an extra pinch or two of salt in your water as well. 


If you’re not eating, should you really expect to have a poop in the first place?  Most of us are used to having one or more bowel movements a day, and sometimes it just feels “weird” or “wrong” not to have one.  If you’re not feeling uncomfortable, then don’t worry.  But if you are feeling uncomfortable, then you should try to troubleshoot the constipation.

Some people have digestive tracts that move a lot slower than others and eating constantly helps things stay moving.  When we start fasting, we’re not consuming anything to help move the previous meals through your system.  After a few days you may start to experience some discomfort.

The Solution:

  • Drink water if you’re thirsty
  • Take magnesium citrate to help hydrate your colon and get things moving along
  • Exercise more

Expert Tip: If all else fails, add some coconut oil or MCT oil to your tea or coffee in the morning.  It’s not a perfect fast but it’ll get things moving and grooving again. 

Insomnia and Feeling Anxious

These side effects are due to the production of the counter regulatory hormone adrenaline that is produced when we start fasting.  Usually, it’s a good thing.  It boosts our metabolic rate and helps us feel energized.  But it can make us feel energetic at certain times of the day when we’d rather be sleeping.

It can also make us feel jittery.  Most people who don’t suffer from anxiety report that they feel like they’ve had too much coffee when they first start fasting.  People who suffer from anxiety start to worry that the fasting makes it worse – it doesn’t.  It’s just your body producing more adrenaline that is making the jitters worse than usual.

Humans are a highly adaptable species.  That is one of the many reasons why we’ve been so successful.  And our bodies will adapt to having these higher adrenaline levels as we continue to fast consistently.

The Solution:

  • Practice proper bedtime etiquette by turning off electronics 90-minutes before bed and wearing blue light glasses in the evening
  • Take Epsom salt baths to help your body relax
  • Lather yourself in magnesium oil or gel in the evening
  • Take magnesium bis-glycinate or malate 4-6 hours before bed
  • Scale back on the fasting, i.e. if you’re doing a 36 hour fast, then maybe do a 24 hour fast and work your way up to doing a 36 hour fast after a few weeks of successful fasting

Expert Tip: Even pro fasters can experience these side effects if they try to mix in a prolonged period of fasting into their regimen.  If you’re looking to dramatically increase the duration of your fast, pick a slower time in your life (i.e. career, family, social functions, etc) where you can afford to be tired for a few days.

Acid Reflux

We aren’t exactly sure why people experience acid reflux when they start fasting.  From my many years of clinical experience, this side effect only tends to occur in people who have a long-standing history of reflux.  Rarely does someone experience reflux for the first time when they’re new to fasting.
There is some good news!  If you do suffer from reflux, your reflux will likely improve dramatically or even disappear once your body as adapted to fasting, especially if you’re following a low carb diet in conjunction with the fasting.  Just like so many things in life it goes from bad to worse to better!

People who have a history of reflux should take some preventive measures to prevent it from occurring or becoming worse.

The Solution:

  • Add 1-3 TBSP of lemon juice to your water throughout the day
  • You can add 1-3 TBSP of raw, unfiltered apple cider vinegar to your water too
  • Avoid broth and pickle juice

Expert Tip: Avoid spearmint of peppermint teas as they may make your symptoms worse.  Instead opt for licorice tea, which may help prevent against reflux.


Like acid reflux, we’re not quite sure why people develop gout while fasting.  Also, people rarely develop gout from fasting if they don’t have a history of gout attacks.  I’ve had less than three cases in nearly a decade where an individual with no history of gout developed gout from doing a fast, especially intermittent fasting.

The Solution:

  • Stick to intermittent fasting when you start, i.e. 24, 36 or 42 hours, three times a week
  • Add 1-3 tablespoons of lime juice to your water
  • Take cherry root extract – this won’t disrupt your fast

Expert Tip: It’s best for people with a history of gout to start off slowly with fasting.  Go from three meals a day to two meals to one over the course of one to two months.  If you start to experience gout pains, then scale back.  The remedies listed above are best used together as apposed to using one or the other. 

Bad Breath

All great things in life often come with some sort of negative consequence.  Take motherhood for example.  Having a baby is a beautiful thing.  Not sleeping for 12 months sucks and takes a toll on our systems regardless of how much you love being a new mom.

The same is true for weight loss.  One of the consequences of losing weight is bad breath.  We often call this keto breath.  When we’re experiencing keto breath, our tongues become white and we have the taste of acetone in our mouth, because acetone is a biproduct of fatty acid metabolism.

Many people immediately freak out about their white tongues and assume they have some sort of horrendous nutrient deficiency.  Others have spouses that refuse to kiss them and even make them sleep in the guest room because they’re morning breath is so unbearable.

Don’t worry!  You’re just burning fat, and it’s a good thing.  As fat loss starts to slow down, your breath will improve.  Your tongue will go back to being pink and you’ll be welcome back in your bedroom.

The Solution:

  • Oil pull with coconut oil two to three times a week
  • Brush your teeth more frequently throughout the day
  • Use a tongue scraper
  • Drink more water

Expert Tip: You may want to scale back a little bit on your protein intake during the first month of fasting.  If you’re looking to lose weight and maintain muscle mass, then you should be consuming 0.6 g of protein per kilogram of total body weight. 

Our next post in this series will talk about some of the less common side effects of fasting that often worry people.

Read my previous post on in this series on How to Get Started Fasting.

By The Fasting Method

For many health reasons, losing weight is important. It can improve your blood sugars, blood pressure and metabolic health, lowering your risk of heart disease, stroke and cancer. But it’s not easy. That’s where we can help.

Jason Fung, MD

By Jason Fung, MD

Jason Fung, M.D., is a Toronto-based nephrologist (kidney specialist) and a world leading expert in intermittent fasting and low-carb diets.

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