YouTube star Mr Beast completes brutal 14-day fast and is floored by the 'depressing' impact on his body

A social media star who undertook a two week fast has revealed the brutal impact the challenge had on his body.

Jimmy Donaldson, known online as 'Mr Beast', has earned a 404 million strong YouTube following by taking on increasingly elaborate challenges, including spending 100 hours inside the Pyramids and seven days stranded in a cave.

But months ago the now 27-year-old embarked on a new feat, drinking only water and not consuming food for 14 days.

While not unfit, the 6ft 5in influencer and entrepreneur weighed around 220lbs (110kg) — giving him a BMI of roughly 26, overweight.

Now, in a recent YouTube video, which was been viewed more than 3 million times, he told comedian Theo Von how the challenge had a 'depressing' effect on his body.

Despite swathes of studies suggesting intermittent fasting — which shot to prominence in the early 2010s — does work, experts remain divided over its effectiveness and the potential long term health impacts.

Some argue that fasters usually end up consuming a relatively large amount of food in one go, meaning they don't cut back on their calories — a known way of beating the bulge.

They even warn that it may raise the risk of strokes, heart attacks or early death.

Scientists have also urged caution over fasts that last more than a day because of their 'short-lived' effectiveness, advising that they could trigger chronic health issues like digestive conditions.

Mr Donaldson said: 'I did 14 days, only water, I lost 20 something pounds,'

'After day five or six you're pretty drained, you have no energy.

'I did a dexa scan before I did my extended fast, where they measure the fat and muscle, I lost like six pounds of muscle, it was pretty depressing. So I lost 13 lbs (5.9kg) of fat.

'Even after you start eating, the muscle didn't just come back.'

'It was intense not eating, I was still filming and working throughout it, it was really messing with me.

He added: 'Standing up was brutal, and walking around, because you have no energy.

'I never got that super human clarity, I was just f**king tired all the time, I was too tired to have clarity.'

Tracking his progress over the course of the two-week challenge, daily weigh ins showed by day three Mr Donaldson had already shed almost 4lbs, at 216.4lbs (98.2kg).

By day six his weight had fallen to 210.1lbs (95.3kg) and day 10, 206.2lbs (93.5kg).

But at day 12, weighing just 204.3lbs (92.7kg), he noted: 'I was so nauseous that I couldn't film for more than 20 minutes without sitting down for a break.

'By the end of the day I felt like I was going to pass out.'

It was only after filming a cooking video with celebrity chef Gordon Ramsey on day 14, that he decided to break his fast and eat a breakfast sandwich the 58-year-old had prepared.

He did, however, immediately spit the sandwich out over fears if refeeding syndrome—where a sudden increase in calories causes an imbalance of fluids and salts, which can affect heart function.

This means people require constant monitoring.

At a final check on day 14, Mr Donaldson weighed just 202lbs (91.6kg), putting his BMI in the healthy category.

However, he cautioned he had a team of doctors around him throughout the two weeks to regularly monitor his vital signs, such as heart rate and blood pressure to ensure he didn't 'have a heart attack or worse'.

'Obviously as I stated in the video multiple times don't try this at home without medical supervision like I had,' he added.

'Also for more context, I have Crohn's disease and I wanted to give it a break from food to see if that would help reduce inflammation because Crohn's sucks.

'I learned a lot about how I can use fasting to reduce my inflammation and I'm glad I did this challenge.'

Roughly half-a-million Britons suffer with Crohn's disease, which causes agonising pain, diarrhoea, exhaustion and extreme weight loss.

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Around a third of patients living with the condition, where the gut lining becomes inflamed, will require surgery.

Many patients take several months to recover from the invasive procedure, while others are left with a stoma—where the bowel is diverted out of the body and replaced with a bag.

It comes as an illuminating video posted online last month garnered thousands of views for its alarming display of exactly what happens to the body —hour by hour— when fasting.

The four-minute gut-wrenching simulation claimed the body stops digesting food after just four hours, entering what is known as the catabolic phase—when muscle and fat is used for energy.

By 12 hours, it has hit a 'fasted state' depleted of blood sugar. Here, the liver begins to break down stored fat into fatty acids called ketones to use as fuel.

But ketones can be dangerous—high levels of them in the blood can lead to a condition called ketoacidosis, when blood becomes too acidic. If not treated promptly it can be life-threatening.

Studies have also found that prolonged exposure to ketones can be detrimental to the heart, increasing the risk of heart attacks and strokes.

Fasting up to 16 hours can then stimulate autophagy— where cells break down and remove damaged or unnecessary components like old proteins—it is claimed.

Research has long warned, however, that excessive or prolonged autophagy can lead to cell death, potentially harming organs.