The metabolic effects of fasting

BY Added Health Editorial Team | 13 December 2022

Intermittent fasting can have a positive effect on your health and wellbeing.

By burning fat and repairing diseased cells we can help to protect our bodies from some cancers, as well as prevent weight gain and premature ageing.

Fasting has gained popularity recently due to its health benefits, but religions and cultures have widely practiced it as far back as the 5th century.

Western diet is linked to disease

The Western Diet is spreading across the world. It is high in red meat, refined sugar and saturated fats but has very little fibre. The bad stuff includes junk food, ready meals, sugary drinks and biscuits, but also added sugar is sneakily hidden in cereals, smoothies, yoghurts and so-called health bars. We are feeding our bodies a constant supply of fuel, but we aren’t burning it off. Why not? Because we sit at our desks all day then watch Netflix in the evening. Our sedentary lives and snacking means the engine is overloaded.

Digesting all that food uses up energy. 30% of the energy you get from food is then taken up digesting it. If you’re grazing and snacking during the day, there’s no time for maintenance or repair. Fasting gives the body that chance to service the engine. Even if you exercise a lot and burn off the excess calories, it’s still a good idea to build fasting into your life.

Fasting will reduce:

  • body fat
  • fasting blood glucose levels
  • insulin resistance
  • inflammation
  • blood pressure
  • And a decreased risk of cardiovascular disease

So let’s encourage our body to do the metabolic switch.

Do the metabolic switch and start to burn fat stores

The metabolic switch is when our bodies start to burn stored fat instead of glucose. This is called “Ketosis”.

Once you have eaten a meal, your digestive system starts to break it down. Blood sugar levels rise, and insulin levels rise as cells absorb glucose for energy. Any leftover glucose goes to the liver to become glycogen, a backup energy reserve.

3–5 hours after you have eaten, your blood glucose and insulin levels drop. The liver converts stored glycogen back to glucose and release it into the bloodstream as fuel for our cells. The liver can store enough fuel to last 12–24 hours.

The digestive system starts to empty; the stomach into the small intestine, then along to the large bowel. Like a sensor on the petrol tank, the brain signals you to eat some more with tummy rumbles and a heightened sense of smell.

In the third phase, the liver is out of fuel, and so the body burns its own fat for fuel (called lipolysis). This is the metabolic switch. It burns ketones, a fatty acid, instead of sugars (from glycogen).

How fasting and ketosis prevent disease by “spring cleaning”

When we start using ketones for fuel, it is called ketogenesis. During this phase, a few interesting things start to happen.

One is a process called autophagy. This Greek word stems from auto meaning “self”, and “phagy” meaning “to eat”. The body starts to recycle old or damaged cells to make way for new ones. These old, damaged cells cause disease, so this spring cleaning process has many health benefits. It can even clear away harmful proteins that have built up in the brain and even stimulate growth in new brain cells. This reduces the risk of neurological diseases like Alzheimer’s or Parkinson’s.

The longer you fast, the more the body can start to repair and reset itself. Studies show that fasting for three or more days reduces fasting insulin and blood glucose levels by a third. Fasting also reduces a hormone called Insulin-Like Growth Factor (IGF-1), which is linked to the degeneration of cells, ageing and cancer. There is evidence to suggest that fasting may protect us from developing cancer by reducing cellular and DNA damage.

Prolonged or repetitive fasting can prevent metabolic diseases and cut down belly fat. It resets hunger hormones like leptin reducing sugar cravings. There is also evidence to suggest that it can help to reduce inflammation, particularly in arthritic conditions.

Fasting may help you live longer, and although more studies are required, intermittent fasting has been linked with the slowing of ageing and disease processes in humans.

The pros and cons of fasting

Choose a fasting regimen that works for you, perhaps:  

  • twice weekly or even daily intermittent fasting (IF)
  • a weekly 24-hour fast or  
  • 36–48 hours monthly or  
  • a 48–72 hour quarterly fast.  

Feel free to mix it up using a blend of IF together with a 36-hour fast every three months, for example. Between fasting, eat balanced meals with plenty of good quality fats, lots of vegetables and a little protein. This regimen will very likely prolong your health span! 

 Are there any negative sides to fasting?

Some people should avoid fasting: pregnant women, those with underlying health conditions like type 2 diabetes or people who are underweight. Also, people with mental health issues like eating disorders. Because it involves skipping meals, you should also ensure that you get enough nutrients through food or stop if you feel unwell. So it’s important to eat a balanced diet in between fasting and not to binge when the fast ends. In general, we are designed to be able to fast on a regular basis, but it may not be for everyone.

What if fasting isn’t for you?

If you don’t want to fast, think carefully about the carbohydrate content of your meals. If you eat a lot of bread, pasta, rice and potatoes, your blood glucose levels will shoot up and down. This leads to a constant hunger/snacking cycle, which can contribute to insulin resistance, obesity and many other metabolic diseases.

A lower carbohydrate diet ensures you regularly switch between burning glucose (sugar) and ketones (fatty acids). It won’t trigger autophagy and the cellular spring-cleaning systems initiated by fasting.

Our strong recommendation is to fast regularly — even if a little — unless, it’s against medical advice.

Take-home message from Added Health

Our bodies were not designed for the high calorie/low activity lifestyle that has sadly become more common in the western world. When done safely and with the appropriate medical advice, cycling between fasting and non-fasting over a defined period of time can help us maintain a healthier weight and may even help us live longer too.


Dong, T.A., Sandesara, P.B., Dhindsa, D.S., Mehta, A., Arneson, L.C., Dollar, A.L., Taub, P.R. and Sperling, L.S., 2020. Intermittent fasting: a heart healthy dietary pattern? The American Journal of Medicine133(8), pp.901-907.

López‐Lluch, G. and Navas, P., 2016. Calorie restriction as an intervention in ageing. The Journal of Physiology594(8), pp.2043-2060.