Can Intermittent Fasting Cause Side Effects

Intermittent fasting is a popular diet trend that has been gaining traction in recent years. It involves periods of fasting followed by periods of eating. While there are some potential benefits to this approach, there are also some drawbacks that should be considered.

One of the main concerns with intermittent fasting is that it can be difficult to stick to. If you’re used to eating three meals a day, going 16 hours without food can be tough. There’s also the risk of overeating when you do finally sit down to eat. After all, it’s hard to resist temptation when you’re famished. Another concern is that intermittent fasting can lead to food cravings. When your body is deprived of food for long periods of time, it can start to crave high-calorie, high-fat foods. This can also lead to overeating and sabotage your weight-loss efforts.


A common complaint when people are fasting is that they feel “hangry”. Or in other words, they feel irritable due to being hungry. But why does this happen? There are a few theories. One possibility is that when we’re fasting, our blood sugar levels drop. This can lead to feelings of irritability and low energy, which can in turn cause cravings for sugary or high-carbohydrate foods. Another theory is that when we’re hungry, our bodies produce more of a hormone called ghrelin. Ghrelin sends a message from the stomach to the brain telling us we need food. Higher levels of ghrelin have been linked to feelings of irritability and agitation (1). Whatever the cause, feeling “hangry” is definitely a downside of intermittent fasting.

If you’re struggling with this side effect, there are a few things you can do to help mitigate it. Make sure you’re drinking plenty of water and staying well-hydrated. This sends the signal to the brain that the stomach is full which reduces appetite and reduces hunger (2). When you do eat a meal, make sure it includes protein, healthy fats and fibre. These take longer to digest than other foods, which helps keep hunger at bay. And if all else fails, remember that this is only temporary – once you break your fast, your hunger and anger will quickly dissipate!

Low blood sugar

Another potential side effect of intermittent fasting is low blood sugar. When you fast, your body is forced to break down stored glycogen for energy. Glycogen is a compound that consists of glucose molecules bonded together with water. It is stored in the liver and muscles. During a fast, your glycogen stores can become depleted, leading to low blood sugar levels (3). This can cause side effects like dizziness, light-headedness, headaches, and fatigue (4). If you’re experiencing any of these symptoms while fasting, it is important to eat meals containing lean meat, fish and vegetables to maintain your blood sugar levels. Alternatively, consider fasting for less time until your body adapts to this new regime.

Another option is to spot if these symptoms are happening at a certain time of day. If you wake up feeling light-headed or with a headache, maybe skipping breakfast is not the best idea. However, if the evening is when you are grumpy and tired, having a nutritious meal at that time may help. With that said, do not eat within two hours of bedtime. If you go to bed on a full stomach, it can cause indigestion and heartburn, leading to disturbed sleep (5), which may be the reason you are waking up feeling sluggish. With a little planning and preparation, you can minimize the risk of low blood sugar and enjoy all the benefits of intermittent fasting.

Tiredness and fatigue

Another side effect of intermittent fasting is feeling really tired. This is linked to having low blood sugar and eating the ‘wrong’ foods when you are eating. The wrong foods are processed foods that are digested quickly. These give a short, sharp rise in blood sugar and hence energy. To mitigate the tiredness, make sure you are eating plenty of foods that contain B vitamins. These vitamins are vital to help release energy from food. The best sources are eggs, dairy, fortified cereals and nutritional yeast; leafy vegetables such as cabbage and spinach; organ meats, seafood and legumes (6).


Intermittent fasting can also cause constipation (7). This is because fasting can slow down the digestive process, leading to hard stools that are difficult to pass. When you eat three meals a day, each meal effectively pushes the next through the gut. Think of it like a conveyor belt. If you are only eating one or two meals a day movement through the gut automatically slows down.  In addition, when you do eat, you may be more likely to overeat, which can further contribute to constipation.

To avoid this problem, it is important to make sure that you drink plenty of fluids to soften the stools and flush food through the gut. Eating high fibre foods, such as leafy, green vegetables also helps keep food moving through the gut (8). If you are not keen on green vegetables, eating foods high in water such as melon, cucumber, celery and tomatoes will also do the trick. 


While some people may experience digestive issues like constipation, others may experience nausea. When you are hungry, having not eaten for some time, stomach acid with no food to break down can build up. This can lead to feeling nauseous. Another explanation for experiencing nausea when intermittent fasting, is dehydration (9). It is vital to drink plenty of water, especially when fasting. Every single part of our body needs water in order to function properly (10). If when you feel nauseous you also feel light-headed, tired or dizzy, a glass of water could be all you need to feel better.

It interferes with sleep

Another potential downside of intermittent fasting is that it can disrupt the body’s natural circadian rhythm. The circadian rhythm is an internal 24-hour clock that regulates many important functions, including sleep, hormone release, and metabolism (11). Intermittent fasting can actually help to reset the body’s circadian rhythm when it is out of sync (12). However, when it is used excessively or incorrectly, it can have the opposite effect and disrupt the body’s natural sleep-wake cycle.

This happens especially when meals are eaten when it is dark. When darkness falls, melatonin, the sleep hormone is produced. This signals to the liver (13) and digestive system that it is time to slow down (14). Therefore, if you have eaten a big meal in the evening, especially in winter when it gets dark earlier, it won’t be digested efficiently. This can lead to indigestion which will disturb your sleep (15).

While there are a few potential drawbacks to intermittent fasting, they can be easily overcome. As with any change in the way you eat, or what you are eating, the body needs time to adapt. However, once it has adapted you can benefit from the many advantages of intermittent fasting. If you hit a roadblock with this way of eating there is lots of help online or you can reach out and get some advice from a medical professional.

Finally, it’s important to remember that intermittent fasting is not a magic bullet. It requires making healthy food choices and being mindful of the reasons you started in the first place.


  1. Mood disorders: A potential link between ghrelin and leptin on human body?
  2. Effect of excessive water intake on body weight, body mass index, body fat, and appetite of overweight female participants
  3. Hypoglycemia
  4. Low blood sugar (hypoglycaemia)
  5. Is Late-Night Snacking Really So Wrong?
  6. B vitamins and folic acid
  7. Health Effects of Alternate-Day Fasting in Adults: A Systematic Review and Meta-Analysis
  8. Concerned About Constipation?
  9. Does Hunger Cause Nausea?
  10. 7 Health Benefits of Water Backed by Scientific Research
  11. Effects of Intermittent Fasting on the Circulating Levels and Circadian Rhythms of Hormones!po=79.5455
  12. Resetting your circadian clock to minimize jet lag
  13. Intermittent Fasting: Pros and Cons According to NUNM
  14. Aspects of sleep effects on the digestive tract
  15. Relationships among dietary nutrients and subjective sleep, objective sleep, and napping in women

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