Doctor issues warning about 72-hour fasting as shocking simulation shows what it does to your body

Warning: This article contains discussion of eating disorders which some readers may find distressing.

A doctor has warned people who plan to adopt his 'extreme' diet that involves fasting for 72 hours straight to make sure their body has one thing in reserve.

Dr Jason Fung has been credited as the inventor of the intermittent fasting diet which he himself has dubbed as being 'very dangerous' if your body is unable to keep up with it - while also claiming it could be a 'godsend' to other.

The trend involves abstaining from eating for a period of 72 hours, but those who are adopting it are still able to drink water, with the hope that longer fasting could induce ketosis and trigger other metabolic benefits.

For those not in the know, ketosis is the metabolic process where the body shifts from using glucose (sugar) as its main fuel source to burning fat.

The intermittent fasting trend sees people go as long as 72 hours without a meal (Getty stock)

"If you don't have any energy to lose, then you're going to start really shutting down your metabolism. Your heart rate is going to slow down; your blood pressure is going to drop, your body temperature is going to drop, because your body is now having to conserve calories," Fund explained to the Daily Mail.

Yes, the doc warned that if you don't have enough calories in your body's fat stores it could be 'very dangerous'.

He continued: "You're going to feel tired, you're going to feel cold, you're going to feel hungry, you're not going to feel well.

"If it continues, then your body is going to have to take those calories by taking functional tissue, which is muscle and other proteins and burn it for fuel... it's not what you want to do.

"You see this with anorexia nervosa. It's not healthy; people die of those conditions."

It comes as a YouTube simulation showed the process your body goes through during the 72-hour fast, explaining what happens to your body the longer you go without eating.

After eight hours without food, the food already in the digestive tract is consumed for fuel and glucagon is released by the pancreas which triggers the liver to release glucose.

When you hit 12 hours of fasting, the human growth hormone (HGH), also known as somatotropin, is released from pituitary gland which is located in the base of the brain. This then stops the breakdown of muscle tissue.

When you eventually arrive at the 36 hour mark, fat will become the body's main source of energy (mental and physical).

For those of you who struggle with the feeling of being hungry, it's apparently not until 48 hours into a fast that the feeling subsides. With this in mind, your mood and alertness improve.

When - or should I say if - you hit 72 hours of fasting, you can start 'gently resuming eating' as your body resets.