A 28-year-old man has told of how he 'shrunk' fat on his belly and arms, while building leg muscle by taking on an extreme walking challenge.

British YouTuber Jack Massey Welsh saw changes in his body after just a week of pounding the streets for an astonishing distance.

Mr Welsh walked a total of 250,000 steps in just seven days, covering an average of 35,700 or 17 miles per day, which is more than half a marathon. 

This is about ten-times the 3,000 to 4,000 steps the average Briton is estimated to do each day, and three and a half times the 10,000 daily steps recommended by some experts.

Documenting the experience in a clip that has been viewed more than 1.5million times, Mr Welsh revealed the gruelling impact of the undertaking.

He endured agonisingly painful feet, swollen ankles and a 'black' toenail, which appeared in the days after he completed the 250,000 steps.

But Mr Welsh was most surprised by the physical changes he noticed - which he never thought could be achieved simply by walking.

'What an amazing feeling of accomplishment this was. One quarter-of-a-million steps in one week. For me it was worth it for the experience alone,' he said. 

Before embarking on the challenge, Mr Welsh made no changes to his diet apart from eating a bit more protein in a bid to lose fat instead of muscle as he walked. 

He also weighed himself and measured parts of his body, like the circumference of this torso and legs, so he could compare any differences at the conclusion.    

On the first day Mr Welsh kicked off the challenge with a 15,500 step two-hour morning walk, then a 14,000 step walk before then reaching his daily goal following a small stroll with his dog in the evening.

On his first day he said: 'I am tired, but I am fine. It's just one step in front of the other.'

These words would come back to haunt Mr Welsh on day two as he was in agony from the previous day's effort. 

'I was already reflecting on how overly confident I was on day one. When I confidently said, "It's just putting one step in front of the other isn't it?" because now putting one step in front of the other is no easy task at all,' he said.

'Literally every single step hurts and is a chore.' 

But Mr Welsh noted how he soon got into the rhythm of things and adjusted his routine to try and complete most of the step goal during multi-hour morning walks.

Speaking on day four he noted that the muscle pain in his legs had abated but added he had started experiencing other physical problems.  

'It feels like now the only limiting factor in this challenge is just physical joint pain in my feet,' he said.

'Because even though I in myself have the energy to do 30,000 steps easily today, each step just hurts quite a bit and I am starting to blister up in a few places, so that's kind of what's slowing me down.'

On day five he even needed a painkiller to help him walk due to the pain in his feet, showing viewers how they were covered in blister pads, and he also suffered what later turned out to be a swollen ankle on day six. 

But speaking at the end of the challenge he said he was struck by an 'an amazing feeling of accomplishment' after counting down the final few steps to reach 250,000.

He also detailed how his body had changed in just a week of extreme walking including a change to his general physique. 

Comparing before and after pics he said: 'I seem to be narrower than when I first started, which I didn't really expect. Does this mean I've lost a bit of muscle mass maybe?'.

He also noted he had lost a bit of belly fat in his side profile, which he attributed to dropping 2lbs (0.9kg) over the week. 

Tape measurements revealed most areas, like the circumference of his arms and torso had also shrunk slightly apart from his thighs which had grown 0.5cm.

'I wasn't expecting to be able to see any difference, so this is really surprising,' he said.

While not taking place during the challenge itself Mr Welsh also told viewers how the toenail on the big toe of his right foot had turned black two days after he hit the 250,000 target.

Concluding he said: 'Do I recommend walking 250,000 steps in a week? Absolutely not.'

'My joints just couldn't keep up, it's two days later and I'm still waiting for the swelling around my ankles to reduce.'

However, he urged people try to consider incorporating a more reasonable step count into their day.

'Do I recommend doing somewhere between 8,000 and 20,000 steps per day, depending on your personal circumstances? Absolutely,' he said. 

'I really do feel that most people will feel better if they end up walking more.'

While Brits have been encouraged to do 10,000 steps a day for years multiple studies have shown doing less than this can still have great benefits to your health.

The 10,000 daily step goal, often lauded as the holy grail of regular fitness and the default goal for many fitness apps or devices, is a bit of a myth.

It wasn't based on a forensic study but instead a clever marketing ploy by a Japanese company trying to sell pedometers some 60 years ago with the figure chosen because it was a memorable number.

While some studies have since found some health benefits from doing 10,000 steps many of these are also present at lower daily step counts as well.

Experts have previously told MailOnline people shouldn't get caught up on reaching a 10,000 step goal and that benefits can be seen at as little as 4,000 steps per day.

The NHS itself doesn't recommend a specific step count for walking but instead suggests a brisk 10-minute walk each day as having many health benefits with the activity counting towards the recommended 150 minutes of weekly exercise for adults.