Side Effects Of Intermittent Fasting

Intermittent fasting can offer a range of benefits from weight loss to improved blood sugar levels.

For many, fasting is more of a lifestyle than it is a diet largely thanks to its flexibility, but there are some side effects that are commonly felt, especially in the short term.

These side effects in response to the periodic reduction of calories are typically due to your brain needing to rewire itself from relying on sugar as energy, to using fat as a fuel source.

Will I Experience Side Effects When Fasting?

When making any significant changes to your diet, you will experience some side effects. This may be as minimal as feeling a little sluggish or having a slight headache. Some may experience more serious side effects such as anxiety or digestive upset particularly with longer fasts such as alternate-day fasting.

In the case of fasting, side effects are usually due to your body adjusting from being a sugar burning machine to an efficient fuel user. When you eat all the time, your body has a constant supply of energy. When you fast, your body is forced to fuel on fat stores.

This transition isn’t necessarily going to occur without any physical and psychological side effects, particularly those who have a high-calorie intake or a highly refined diet. It also can take time before your body adapts and you can start to experience the benefits of intermittent fasting. The type of fast you choose and the duration of the fasting window will also influence if you experience side effects and how severe they will be.

If you are new to fasting, you’re also more likely to experience side effects than people who have tried some form of intermittent fasting in the past. Those who are also low carb or ketogenic dieters will typically experience little to no side effects as their bodies are accustomed to using fat as fuel.

Most Common Side Effects Experienced While Fasting

Below are some of the more common side effects you may experience when trying intermittent fasting for the first time or attempting a longer fasting regime. These aren’t usually long-term effects or of great medical concern.


In the initial days and weeks of fasting, it’s not uncommon to experience cravings or obsessive thoughts about food during both the fasting and eating periods. While some of the times the cravings stem from habitual behaviour, in some cases, this can be a signal that your body is needing a particular nutrient and you need to adjust your diet plan.

For example, if you’re craving oily, fried food, your body may be lacking in omega-3 fatty acids. If you’re craving salt, your body may be experiencing low sodium levels.

However, if you experience sugar cravings and generally have a largely refined or processed diet, this is more likely due to unstable blood sugar levels. It’s not necessarily an indication you need to consume sugar!

To help overcome your cravings, try these options:

  • Salty foods – Add a ⅛ tsp of Himalayan salt to a glass of water several times a day
  • Carbs – Include a small amount of sweet potato, parsnip, butternut pumpkin or swede to your meal in a feeding window
  • Fried foods – Drizzle a little extra virgin olive oil, macadamia oil or coconut oil on your meal. Eating oily fish such as salmon or sardines can also satisfy this craving
  • Sugar hit – Choose low-calorie, low-fructose fruit such as citrus, berries, apple or pineapple. A sprinkle of cinnamon can help with blood sugar balance

Even though you may be ‘hangry’. The key is to listen to your body, show some restraint, especially during your eating window so that you don’t end up binge eating and blowing out your calorie intake.

Headaches, Poor Focus and Fatigue

Headaches, poor cognition and fatigue can be a result of dehydration or low levels of sodium.

For some when they fast, their water intake is reduced or they had insufficient intake to start with. Dehydration can cause feelings of lethargy and is one of the leading causes of headaches.

These side effects can also be due to reduced sodium levels as our kidneys release excess water in response to the quickly declining levels of insulin. The increased urination together with the reduced intake of food can result in a decline in electrolytes, particularly sodium.

These side effects generally subside once your body has adapted to fasting. To help manage these side effects, try:

  • Add a ⅛ tsp of Himalayan salt to a glass of water several times a day
  • Drinking homemade or dehydrated bone broth throughout the day
  • Have a couple of teaspoons of juice from sugar-free fermented veggies

Dizziness and Nausea

Dizziness and nausea can be a result of a change in your blood sugar levels. While this typically subsides quickly, intermittent fasting can cause you to become hypoglycemic. Due to this, diabetics should be cautious before trying any fasting method and consult their health practitioner.

To help keep your blood sugar levels stable, try:

  • Eating a nutritionally balanced breakfast
  • Consuming protein and dietary fat with every meal during your feeding window.


Slowed bowel movement frequency is a common side effect of fasting. This is largely due to the reduced intake of food, but can also be due to a change in the foods you’re eating.

Ideally, you want to have 1-2 daily bowel movements of a well-formed and easy-to-pass stool. In the initial days of intermittent fasting, this may change or you may have experienced infrequent bowel movements before you commenced the calorie restriction.

If the constipation is new, you’re not uncomfortable and it only lasts a couple of days, it’s probably nothing to worry about. If you have experienced recurrent constipation for more than a week or two and are feeling discomfort, you may want to speak to your health professional such as a nutritionist.

To help normalise your bowel movements and move things along, try these suggestions:

  • Increase your water intake.
  • During your feeding window, consume kiwi fruit or ground flaxseeds regularly.
  • Get moving throughout the day.
  • Try a warm Epsom salt bath.
  • Consider magnesium or probiotic supplementation.


Frequent diarrhoea may be experienced during intermittent fasting. How severe often depends on the length of the fast with longer fasting associated with more explosive diarrhoea due to the high fluid intake including water and coffee.

If you’re accustomed to eating a lot of refined carbohydrates, you may also experience diarrhoea in the initial days due to the reduced levels of insulin and the increased water excretion.

Acute diarrhoea should pass within 24 hours. If required, you may need to increase your electrolyte intake with a rehydrate supplement and broth. If the diarrhoea is persisting, you should speak with your health practitioner to investigate any compounding factors and to encourage gut healing.


Heartburn or acid reflux can occur during intermittent fasting as the body adjusts to this new eating pattern. It can also be associated with overeating during the eating window or choosing aggravating foods.

Generally speaking, those who experience reflux during their fasting days are likely to be long time sufferers or have existing digestive issues. If you are experiencing mild heartburn, there are some things you can try, including:

  • Eating smaller portions at a time during your feeding window.
  • Practising mindful eating by slowing down and focusing on your food.
  • Having a small glass of water with lemon or apple cider vinegar before meals.
  • Consuming water with fresh lemon throughout the day.
  • Avoiding common offenders such as chocolate, caffeine, fried and spicy foods.

If you have a history of reflux, speak to your health care practitioner about measures to take to elevate and manage the discomfort.

Bad Breath

As your body adjusts to a fat-burning machine, it produces ketone bodies which can result in the unfortunate side effect of bad breath. You may also notice an unpleasant acetone taste in your mouth and white coating tongue.

These are all normal symptoms and tend to dissipate once you’ve lost body fat and the loss starts to slow. In the meantime, there are some things you can do help minimise the bad breath and enhance your overall dental health:

  • Drink water regularly throughout the day.
  • Brush more frequently.
  • Use a copper tongue scraper.
  • Try oil pulling three times a week.

Sleep Disturbances

While many fasters report improved sleeping patterns, there is some research to suggest fasting during the day can result in a decrease in rapid-eye-movement (REM) sleep. It’s unclear what is causing this, however, enough REM sleep is associated with many health benefits including improved memory, cognitive function and concentration.

There is some suggestion, an increase in the stress hormone, adrenaline, may be the cause of the sleep disturbances. While this helps us feel energised during the day and increases our metabolic rate, if we’re still feeling the rush at night time, it will affect our ability to fall asleep and remain asleep. For some, sleep disturbances may also be due to increased feelings of hunger.

To ensure your establishing a good sleep cycle, focus on these tips:

  • Establish a bedtime routine that allows you to wind down before sleep.
  • Switch off devices one to two hours before you hit the pillow.
  • Try an Epsom salt bath or applying magnesium oil in the evening.
  • If hunger is in play, eat a protein and healthy fat-rich meal in the evening.
  • Consider reducing the fasting period slightly.

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