Kicking off the mini series is 15-year-old Ieuan Rosser, who weighs 17 stone, and 13-year-old Jess, who weighs six and a half stone.

It's not an easy ride for Ieuan who is used to consuming a a shocking 4000 calories a day. Nor for his mum Kay who's forced to face up to why she hasn't been able to stop her son from putting on weight.

Meanwhile super fussy Jess's inability to try new healthier foods is put to the test. But can laidback dad Dave finally do what he should have done years ago and put his foot down when it comes to his daughter's unhealthy diet?

But it's not just Mums and Dads who need to take responsibility for their children's diets, and so Dr Jessen's on a mission to get the nation's kids to take charge of what they put in their mouths too. Using actual samples of human bodies, he delivers the first of four hard-hitting lectures to a group of junk-loving school kids on the strain that eating too much or too little puts on the heart.

The Supersizers get a glimpse into what the future might hold as they watch personal video messages from America. Fifteen-year-old Hayli Cohen from New York state weighs in at a hefty nineteen stone. Allowed to eat whatever she wanted after her parents divorced when she was three years old, Hayli's weight soared. She's even resorted to having a gastric band fitted - something Ieuan is also considering. But will her experience and words of warning give him and Mum Kay the push they need to sort out his diet for good?

Also the harrowing tale of 16-year-old Erin Wade, whose eating disorder spiralled out of control after she became addicted to pro-anorexia websites.