What happens in your body when you fast?

Intermittent fasting is a popular lifestyle practice with many potential benefits:

• Weight loss
• Anti-inflammatory
• Gut health
• Anti-ageing
• Autophagy + anti-cancer
• Mental alertness
• Sports performance

It’s helpful to understand what happens in your body when you fast so you can benefit to the max.

After your last meal, absorbed glucose is circulating in your blood.  This stimulates insulin release from your pancreas which pushes the glucose from the blood into your body cells where it can be used for energy or stored.  Insulin keeps your blood glucose level within the normal range.

As time goes by, your body cells metabolize the glucose, and your blood glucose level decreases which means you no longer need insulin, and your insulin level will start to decrease.

When your blood glucose level drops sufficiently, your body will release stored glucose from your muscle cells and liver.  You have a short-term glucose supply in these tissues which lasts about 1 hour if you are exercising and up to 8 hours if you are more sedentary.

When your short-term glucose stores are completely depleted, you have to get energy from other sources and maintain a normal blood glucose level to keep your brain fueled and working well.

At this point, you enter fat-burning mode.  You will start breaking down fat into fatty acids and ketones, which your brain can use as a fuel source.  When ketones appear in your blood, you are in ketosis and using fat as your energy source.

For most people, ketosis happens about 12 hours after their last meal.

Now, there is no need for insulin, so blood insulin levels are very low.  Your blood sugar level will remain within the normal range because your body will make glucose from the fat molecules that are broken down (gluconeogenesis).  Usually, when you are in ketosis your appetite will be lower than usual because insulin is low.

As you continue to fast, human growth hormone is released which is responsible for healing and regeneration of old or damaged tissues.  This has an anti-inflammatory effect.

And, by 18 hours of fasting a process known as autophagy is triggered.  Autophagy has powerful anti-ageing and rejuvenation effects because old cells are literally engulfed by immune cells and recycled into new molecules for healing.

After 20 hours of fasting, autophagy continues to increase throughout your body and by 36 hours it is well established.  Autophagy is responsible for the anti-cancer effect of fasting.  Autophagy peaks at around 72 hours of fasting.

It takes some time to adapt to fasting.  If you are used to eating 3 meals and 2 snacks a day and suddenly stop eating for 3 days, you will probably feel awful.  It is better to gradually increase your ‘fasting window’ over a few weeks so your brain can get used to using ketones for fuel.

That way, you will be able to benefit from fasting without experiencing side-effects.