Everything that happens to your body when you start fasting

Fasting, favoured by celebrities such as Vanessa Hudgens and Halle Berry, and experts like oncologist Umberto Veronesi, is the process of restricting eating to a certain number of hours a day.

While some embark on this gruelling task for weight loss purposes, others are committed for religious reasons, such as Yom Kippur, which was observed this year on the evening of Sunday 20 to Monday 21 September.

Some popular methods include 16:8 (fasting for 16 hours and eating all meals within an eight-hour window), the 5:2 method (five normal eating days and two days of 500-calorie fasting), and the OMAD variant (a 23:1 pattern where only one meal is consumed each day).

There is also circadian fasting, a ‘form of time-restricted eating that aligns food intake with the body's internal clock, rather than an arbitrary fasting window or time-restricted calorie intake like the 5:2 diet’, Hannah Alderson, a hormone-specialised nutritionist, told British Vogue.

Fasting is favoured by celebrities and health experts alike (Getty Stock Image)

When followed diligently, any type of intermittent fasting can be a balanced approach to eating healthy—but experts warn it could have some dire consequences.

What happens to your body when you fast?

Healthline reports that it often takes your body around six hours to register that you are fasting.

Once your stomach and intestines are empty, the body will begin to siphon energy from its stores.

Eight hours of saying no to your favourite meals will cause the body to begin using up glycogen, another form of stored glucose, for energy.

After eight hours, the body enters mini-ketosis mode. It’s likely that your insulin levels will have hit rock bottom.

As your body is burning ketone bodies, it’s likely that you will have less of an appetite and may feel fatigued.

After 16 hours, autophagy - a process where the body breaks down and recycles unnecessary cells - begins.

The Cleveland Clinic said that this gets rid of nonfunctional cell parts and bacteria, adding that it 'isn’t a clear-cut harmful or beneficial process concerning disease prevention or treatment'.

Some people use fasting as a weight loss method (Getty Stock Image)

Once you reach the 24-hour mark, the body targets fat as a source of fuel.

Healthline reports that inflammation in the body will decrease at this time while sensitivity to insulin continues.

Can fasting help with weight loss?

Dr Eric Berg, who is known for his work on intermittent fasting, wrote on his website that fasting can help you lose weight—an idea recently challenged by the Cochrane Database of Systematic Reviews.

In a study, experts examined health and nutrition data from nearly 2,000 adults and suggested that the 16:8 fasting method had ‘no clinically meaningful impact on weight loss’.

“Compared to regular dietary advice, intermittent fasting may result in little to no difference in percentage from baseline weight loss,” the researchers wrote in the conclusion.

The study was backed up by Dr Deepali Misra-Sharp, NHS GP and menopause specialist, who told Marie Claire: “Intermittent fasting has often been promoted as though it has a unique metabolic advantage, but when researchers compare it with more traditional approaches to weight loss, the results tend to be very similar.”

Other health benefits of fasting

Berg argued that eating within a certain window may help to improve your metabolic and digestive health.

The expert also wrote that by abstaining from eating for a certain amount of time, you may be able to lower your risk of coronary artery disease, heart attack, and stroke.

Studies suggest fasting may lower your risk of coronary artery disease (Getty Stock Image)

“Many things happen during intermittent fasting that can protect organs against chronic diseases like type 2 diabetes, heart disease, age-related neurodegenerative disorders, even inflammatory bowel disease and many cancers,” added Mark Mattson, Johns Hopkins neuroscientist.

Research published in the New England Journal of Medicine also suggested fasting may trigger cellular repair processes linked to ageing.

Fasting comes with major health risks

However, there are some serious risks involved when it comes to fasting, with Dr Berg warning of dehydration, lack of nutritional reserves and a risk of overeating.

He also said that fasting without drinking water can lead to serious kidney problems as well as cause electrolyte imbalances in the body.

Those who suffer from electrolyte disturbances are likely to experience paralytic ileus and cardiac arrhythmias, among other substances, as per The Pharmaceutical Journal.

A study published in the journal Molecular Metabolism saw experts warning that prolonged fasting can also cause dizziness, muscle loss, and complications for people with underlying health conditions.

"While fasting may be beneficial for treating some conditions, often times, fasting won't be an option to patients suffering from ill health,” reported Maik Pietzner, Health Data Chair of PHURI and co-lead of the Computational Medicine Group at the Berlin Institute of Health at Charité.