Barry Austin, who is believed to be the fattest man in Britain, has decided to turn his life around; he has cut his daily intake from 29,000 calories to 1,500 in an effort to lose some of his 50-stone bulk.

It will be a challenge but, he says, 'I put the weight on, I can lose it.'

What has caused such a dramatic shift of attitude in a man who would once eat nine curries and down up to 40 pints of lager in one sitting? A television programme. Tempted by the offer of a full medical check-up, Austin volunteered to appear in a programme that would examine the science of obesity. Inside Britain's Fattest Man, to be screened next Monday on Sky One, examines the effects on the body of being morbidly obese and compares Barry's organs to average people.

Austin said that, although his GP practice was 'fantastic', it was not used to seeing people his size and so he seized the chance of having such thorough health tests. Body scans, blood tests, body-mass index assessments, brain scans, cardiovascular assessments, kidney, liver and heart tests were used to create animated copies of his organs. 'I wanted to do it to show other people the dangers of obesity,' he said. The results were shocking and can, to a lesser extent, be applied to the one in five people who suffer from obesity: 'They said I would have five years to live if I didn't change. That motivated me to alter my ways.'

The tests showed:

· Heart: Austin's heart does not rest even when relaxing. It pumps harder than an average man's heart when walking up a hill and the smallest exertion can cause a massive increase in heart rate.

· Blood: His blood was full of glucose molecules from fizzy drinks and junk food. He had a third more fat in his bloodstream than an average man's.

· Liver: An average-sized person's liver weighs about 3lb; Austin's weighs 8lb and is black and covered in fat.

· Diabetes: Austin was diagnosed as having Type II diabetes, linked to his weight.

· Sleeping: Someone who is obese has difficulty breathing when sleeping because of the pressure put on his lungs when lying down. A normal sized man could take in six litres of air in a deep breath, Austin manages three.

A specialist nurse, Wendy Waller, who saw Austin through the tests, said that, although he was an extreme case, one in five adults was obese and would suffer some or all of the complications. 'There are also the mental problems,' said Waller, who helps prepare patients for obesity surgery. 'Three out of five of the patients I see have suffered some form of neglect, abuse or trauma in their lives. Barry was depressed about his size and what it meant he couldn't do.'

Austin admitted that 'life would be much easier' if he was thin. 'I can't go swimming, I have to think what supermarket I have to go to because of turnstiles, and it is difficult to fly because I need to pay for three seats.' He said he was not embarrassed about his weight because he had brought it on himself, but said he suffered from discrimination in the workplace. 'They think I am perfect on the phone, then when I go in they try everything to get out of [giving me employment].'

His weight problem arose because of years entering eating and drinking competitions to win prizes for him and his friends. Until recently he would eat a fry-up for breakfast with 1lb of bacon and sausage, six to eight eggs and fried bread, have fish and chips for lunch and curry for dinner, while snacking on up to 20 packets of crisps a day. Since the programme he has cut down to a bacon sandwich - two pieces of bread, two pieces of bacon and one egg - a chicken salad sandwich for lunch, a lean dinner and litres of water.

He said he hopes to appear on television again when he has lost the weight to show people what can be done. He said he chose to take part in the programme to highlight that it was a disease and that people should seek help.

'When I went to a psychologist once they asked me to draw myself and I drew a thin person. That is what I used to see. The psychologist said that people with bulimia would draw a fat person. It is the same thing, but it is harder to find support when you are obese.'

Dr Andrew Hill, a psychologist at Leeds University medical school and former chair of the Association for the Study of Obesity, said there was a belief that overeating was within people's self-control and so obesity was a failure. 'The degree to which we have control over our weight is overestimated,' he said. 'You only need to eat 2 or 3 per cent more than another person over a period of time and you will gain weight. We live in a society where we are surrounded by the marketing of delicious high-caloried food and that encourages obesity.'

Hill said there were questions to be asked about Sky putting on such a show. He said: 'There does seem to be a move by broadcasters to focus on people who are distressed in rather a bleak way. I wonder if the people highlighted realise the consequence of their actions.'

But Mark Carter, Sky producer and director, said: 'Barry is happy with the programme. It is entertaining but hammers home the message of what indulgences do.'