Common side effects of fasting

Fasting has benefits ranging from weight loss to reduction in medications required for type 2 diabetes and hypertension (high blood pressure), but it does have some short-term side effects.1 These unwanted consequences of fasting can be due to the body transitioning from sugar burning to fat burning mode.
Disclaimer: This guide is mainly based on the clinical experience of the author and her co-workers. It does not meet the Diet Doctor criteria for being evidence based, but may still provide helpful clinical suggestions for those experiencing side effects with intermittent fasting. Full disclaimer

/ Bret Scher, MD FACC

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.

Suddenly switching from eating all the time to fasting forces your body to fuel on fat stores.  You go from giving your body sugar fuel to using fat for fuel.  This means you must wake up the fat-burning factory and slow down the usage of the sugar-burning factory.

Imagine how many struggles it might take to slow down usage of the factory you’ve primarily used for years and start 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. 2  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 a few weeks of sticking to the same routine.

We have noticed that side effects more frequently linger if people don’t stick to their fasting protocol consistently.3  It might be challenging, but the more consistent you are with your regimen, the sooner you’ll adapt and the side effects will go away. 

You are most likely to experience side effects of fasting if:4

  • 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 side effects can be 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 increases 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 sodium through our urine. We’re also not eating as much food so we can have a decline in electrolyte (sodium, potassium, and magnesium) 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 added)

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

My favorite electrolyte supplement: Fasting Drops5


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 may also be related to the dramatic drop in insulin levels that signals to the kidneys to excrete excess water.


The solution
  • Take 1 tablespoon of psyllium husk and stir into a minimum of 1 cup of water, and let it sit for 5-10 minutes before drinking it first thing in the morning (It’s important to drink plenty of fluid when taking psyllium husk in order to achieve its desired effects)
  • 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 experience symptoms of low sodium or potassium 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 more water, especially when 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 a couple of teaspoons of 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 can be 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.6 People who suffer from anxiety start to worry that the fasting makes it worse, but it probably doesn’t. It’s just your body producing more adrenaline that is making the jitters worse than usual, and your body will adapt with consistent fasting.


The solution
  • Practice proper bedtime etiquette such as turning off electronics 90 minutes before bed or 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 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 or even disappear once your body has 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!7

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

acid reflux

The solution
  • Add 1-3 tablespoons of lemon juice to your water throughout the day
  • You can add 1-3 tablespoons 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 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. no more than 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

Great things in life often come with some sort of negative consequence. One of the consequences of losing weight can be bad breath.  We often call this keto breath.  When we’re experiencing keto breath, our tongues may become white and we have the taste of acetone in our mouth, because acetone is a by-product of fatty acid metabolism.

Some peoples spouses refuse to kiss them and even make them sleep in the guest room because their 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 likely improve. Your tongue will go back to being pink and you’ll be welcome back in your bedroom.

bad breath

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

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Intermittent fasting

  1. Many of the studies regarding intermittent fasting and weight loss investigate restricting calories to very low levels for a limited time, like one or more days per week (e.g. 500 calories per day). Actual fasting means restricting calories to close to zero for a time, so it may or may not be even more effective.

    JBI Database of Systematic Reviews and Implementation Reports 2018: Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis [strong evidence]

    Obesity Reviews 2017: Short‐term intermittent energy restriction interventions for weight management: a systematic review and meta‐analysis [strong evidence]

    Obesity (Silver Spring) 2016: A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity [moderate evidence]

    The American Journal of Clinical Nutrition 2015: Health effects of intermittent fasting: hormesis or harm? A systematic review [moderate evidence]

    Intermittent fasting and type 2 diabetes:

    JAMA Network Open 2018: Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: a randomized noninferiority trial [moderate evidence]

    Diabetes Research and Clinical Practice 2016: The effects of intermittent compared to continuous energy restriction on glycaemic control in type 2 diabetes; a pragmatic pilot trial [moderate evidence]

    BMJ Case Reports 2018: Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin [case reports, very weak evidence]

  2. This is based on consistent clinical experience of low-carb practitioners. [weak evidence]

  3. This is based on consistent clinical experience of low-carb practitioners. [weak evidence]

  4. This is based on consistent clinical experience of low-carb practitioners. [weak evidence]

  5. Diet Doctor will not benefit from your purchases. We do not show ads, use any affiliate links, sell products or take money from industry. Instead we’re funded by the people, via our optional membership. Learn more

  6. This is based on consistent clinical experience of low-carb practitioners. [weak evidence]

  7. This is based on consistent clinical experience of low-carb practitioners. [weak evidence]