What are the side-effects of fasting?


Some people experience side effects in the first few fasts. At this time your body is learning to adapt to using your stored fat as its energy source. If it has been a long time since your body has had to rely on its fat stores, it may be slow to start up the mechanisms needed to use fat as a fuel. This phenomenon, termed ‘metabolic inflexibility’, is often seen in people who are overweight. The side-effects due to metabolic inflexibility occur only during the first few fasts and rapidly improve.

Learn more about fasting and fat burning

The kinds of problems you might experience during your first fasts are:

  • Headaches: make sure you are drinking enough and if they are bad, take a pain-killer

  • Light-headedness or dizziness: a few people have a ‘spaced-out’ or light-headed feeling. Often, all that is needed is a salty drink or food.

  • Feeling very tired, especially mid-afternoon, is because your body has not yet learned to access your fat stores for fuel. A salty drink can help with this too.

  • Becoming short-tempered can be a problem towards the end of the fast, especially as the time to eat approaches. Planning your food ahead of time can help, and being aware that you may be short-tempered can help you to be calm around your friends and family.

  • Insomnia is common on the night of your first few fasts.

  • Constipation/diarrhoea on the day after your fast may occur as your body reacts to the new way of eating.

Longer-term side-effects

Experienced fasters can occasionally have a repeat of these kinds of symptoms. Often it is due to having eaten a lot of carbohydrate in a short period, drinking more alcohol than usual, or having a stressful day just before the fast day. All of these can tip your body out of fat burning and back into sugar burning.

The two most common ‘occasional’ side effects we hear about are:

  • Constipation due to very little food intake on fast days.

  • Poor sleep after the fast day due to increased alertness triggered by fasting.

Read our tips on how to prevent constipation

Read some ideas on how to improve your sleep

In fact most of the other ‘side effects’ our community talks about are a result of losing weight rather than because of fasting. For example:

  • Saggy skin if you lose a lot of weight.

  • Increased hair loss, particularly if you lose weight rapidly.

  • Change in menstrual cycle in women which is likely due to release of oestrogen that is stored in the fat held in your adipose tissue. As the fat shrinks, the oestrogen is liberated.

Read tips on how to deal with these side effects of weight-loss

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