The Science Of Fasting

Lucy Bell-Young

by Lucy Bell-Young

9th December 2020

Fasting is now a commercially promoted diet, hailed by health gurus and influencers as a healthy way of cleansing your body of toxic metabolites and improving your overall health. But do these claims have a factual basis? And if so, what is the science behind fasting?

Alarm clock and plate with cutlery . Concept of intermittent fasting

What Is Fasting?

Fasting is the intentional restriction of food over a set period of time, whether for religious reasons, as a political protest, or, more recently, as means of losing weight and even health therapy. Whether for short or long periods of time or even intermittently (more on that below), the practice of abstaining from food can be traced as far back as ancient history.

The History Of Fasting

Fasting is virtually ubiquitous in all societies, though it’s rooted most strongly in religious traditions, where it’s practised either as a form of penance or as a cleansing ritual. In fact, intentional fasting has been part of many religious traditions for thousands of years: it’s not only seen as a form of sacrifice, but also a way to purify both body and soul. 

One of the oldest recorded practices of fasting dates back to Ancient Greece – around 500 BCE, to be exact – when the mathematician and philosopher Pythagoras (580-500 BCE) popularised the concept. While modern day societies know Pythagorus best for his geometric theorem, in Ancient Greece, he was well-known for his ascetic practices, including endurance fasting. Following his example, Pythagoras’ acolytes believed that this form of fasting improved mental perception and creativity.

A contemporary of Pythagoras and Father of Medicine himself, Hippocrates (460-357 BCE), elevated fasting as a medical means of restoring health. He reasoned that because sick people tend to lose their appetites, their eventual recovery was in some way tied to this involuntary fasting: since giving food to a sick person seemed to cause more harm than good, it was assumed that restricting food intake would restore health.

But the Greeks weren’t the only ancient civilisation to believe in the benefits of fasting. The peoples of Ancient India, China, and Israel also practised fasting for religious or spiritual reasons. According to the New Testament, for example, Jesus fasted in the desert for 40 days, and so fasting quickly became a part of Christianity. This is still celebrated by modern day Christians in the form of Lent, when people abstain from something for 40 days as a test of endurance, sacrifice, and self-betterment.

Similarly, fasting is believed by Muslims to be one of the Five Pillars of Islam. Ramadan involves fasting for around 30 days, depending on the lunar cycles. During this holy month, Muslims fast from sunrise to sunset, though they may eat a pre-fast meal known as suhur everyday before sunrise, and can also break their fast after sundown with a meal called iftar

Fasting is thus seen by many cultures and religions as a way of practising godliness and purifying the soul. Many also believe that fasting in this way facilitates spiritual breakthroughs and life epiphanies.

Statue of Pythagoras
Pythagoras popularised the concept of endurance fasting

Is Fasting Good For You?

The question of whether fasting is good for you is widely contested: while fasting has proven to have some health benefits, such as fighting inflammation and increasing growth hormone secretion, it also puts your body into a calorie deficit which, if maintained for a long period of time, can result in slower metabolism, nutrient deficiency, and bone weakness, among other things.

Adults typically need between 1600-3000 calories of food per day, and consuming less calories than your body needs is considered partial fasting, or a calorie deficit. This can, in fact, have some health benefits, depending on a few factors, such as:

  • The type of fasting
  • The duration of fasting
  • The intervals between fasting
  • Your overall health status
  • Your age, level of activity, and body build 

If you’re in good overall health, fasting can be an effective way to lose weight, manage your metabolism, and even boost your brain function. Medical fasting is also used to reduce insulin resistance in diabetic patients because it can promote better blood sugar control. In this context, intermittent fasting or cyclical fasting is effective at normalising fasting blood sugar (FBS) levels.

Calorie reduction is also argued to be effective at prolonging the human lifespan, given its proven ability to extend life expectancy in animals, as detailed in Clive McCay’s 1935 report  of rats having a 33% longer lifespan when subjected to a calorie restrictive diet. The effectiveness of calorie restriction at extending human life has been widely contested, however, with many scientists concerned with the long-lasting effects fasting can have on the body. 

Is Fasting Healthy?

The bottom line is that fasting isn’t good for everyone, particularly those with certain types of medical conditions such as ulceric hyperacidity, liver or kidney problems, or a compromised immune system. Fasting is also not recommended for pregnant women because of the risk it poses to the foetus and the mother: more nutrients and calories are needed during pregnancy, not less.

If you’re healthy, fasting can be a good option if you want to lose weight. However, it’s important to remember that most of the weight lost during fasting is actually fluid loss, which can happen very rapidly. As well as this, if you’re not eating healthily in the first place, fasting could be very detrimental to your overall health by depriving you of even more essential nutrients. 

You should always consult a qualified physician or dietician before you try fasting diets; vitamins and minerals are essential to our health, and if you don’t fast safely, these can very quickly fall into a deficit and have dangerous knock-on effects. Remember that the healthiest way to control your weight is by sticking to a nutrient-rich diet and making sure you get some exercise! 

Intermittent fasting health benefit info graphic vector
If you’re in good health, fasting can have many benefits, but always consult a doctor first

What Is Intermittent Fasting?

Intermittent fasting involves dividing a day or week between fasting and normal eating periods. If you want to try a fasting diet but can’t or don’t want to commit to a long-standing calorie reduction, you may find that intermittent fasting is a better option for you. Here are some methods you can try:

  • The 16/8 method: Also known as Leangains protocol, this method divides a day between an 8-hour period for eating and a 16-hour period of fasting. It’s up to you how you divide your day between these two states.
  • The eat-stop-eat method: This basically involves fasting for 24 hours straight once or twice per week. For instance, if you eat breakfast on Monday, the next meal that you should eat would be breakfast on Tuesday. This means you only end up skipping lunch and dinner.
  • The 5:2 diet: This involves consuming only 500-600 calories on two consecutive days per week. The remaining five day are for normal eating. You might want to consider fasting on the days you don’t need to work or do strenuous activities.

What Are The Benefits Of Fasting?

The main benefit of fasting is weight loss and the various health benefits that follow this. If you’re overweight, for example, your heart has to pump harder, making you susceptible to heart disease. It’s also common for people who are overweight to have high cholesterol and FBS levels.

Fasting, also known as cutting, has a few more benefits in addition to weight loss:

  • It can help lower your heart rate and dilate your blood vessels
  • If you have hypertension, fasting can help lower your blood pressure by thinning your blood
  • Fasting reduces insulin resistance, making it easier for the body to transport glucose 
  • It can reduce blood pressure and high cholesterol, leading to a healthier heart
  • You can also naturally improve your human growth hormone (HGH) through fasting diets
  • By decreasing your metabolism, fasting also reduces the free radicals and toxins produced by this process, which can accumulate and cause diseases, such as various types of cancers

It’s important, however, to take these with a pinch of salt: it’s easy to conflate the health benefits of weight loss with a fasting diet. For example, weight loss in general, no matter how it’s achieved, will reduce blood pressure and heart health – you don’t necessarily have to fast to do it. In reality, many of the benefits experienced during fasting are a result of weight loss itself, not necessarily calorie restriction as a whole.  

The 16/8 method diagram
The 16/8 method divides a day into an 8-hour eating period and a 16-hour fasting period

Fat Burners And Fasting

Fat burners are food supplements that accelerate your metabolism. So, when these are combined with fasting, the result is very rapid weight loss. The concern here is that fat burners used during fasting can reduce muscle mass, so you should always consult a physician before you combine the two.

In general, it’s okay to combine fat burners with fasting diets, but you should always be cautious and monitor your progress. Additionally, no matter how much of your calorie intake you’re restricting, you should always have sufficient macro and micronutrients in your diet to maintain good health.


All content published on the blog is for information only. The blog, its authors, and affiliates cannot be held responsible for any accident, injury or damage caused in part or directly from using the information provided. Additionally, we do not recommend using any chemical without reading the Material Safety Data Sheet (MSDS), which can be obtained from the manufacturer. You should also follow any safety advice and precautions listed on the product label. If you have health and safety related questions, visit