Jack Mitchell from Sunderland weighed 20st (127kg) at the age of 16 and was being bullied at school because of it.
But instead of doing nothing about his weight, Jack decided to go on a diet.
Two years later he's been named young slimmer of the year after losing more than 7st (44kg).
He's told Newsbeat it was hard being an overweight teenager: "At school it was the worst. That was the worst pain I had.
"I was bullied quite a lot by other pupils, even by a teacher, I was bullied quite severely.
"I wouldn't eat at school. In the end I went in to school for my exams and came home.
"I couldn't cope with the pressure of being in that environment. I was constantly under stress."
Jack Mitchell says he was even called fat while walking down the street.
Jack's meals before diet
Mid-morning snack: Custard creams and glass of cola
Lunch: Turkey burgers and chips
Afternoon snack: Chocolate
Dinner: Takeaways and ready meals
Evening snacks: Sausage rolls and chocolate biscuits
He's told Newsbeat there were two reasons why he finally decided to go on a diet.
He said: "It was after I'd left school, after my GCSEs.
"When people called me fat, it would make me eat more. It didn't spur me on to lose weight.
"I was at a family party and a family friend had lost 2st 7lbs dieting and that spurred me on.
"Also, I was in a clothes store and that was the only store I would go in because I knew they stocked larger sizes.
"Their larger size was too tight. And I thought, 'I'm either going to have to have clothes specially made or I'm going to get stuck in the house constantly and I won't be able to go out and enjoy things with friends.'
"That was the turning point."
Jack Mitchell began attending Slimming World meetings to help him lose weight in July 2006 and switched to a healthy diet.
"The hardest part for me wasn't actually giving up the food, it was admitting I had a problem and going along to the group," he admitted.
"When you're young and when you're a man going to a slimming group, it can be quite a daunting thing.
"You think about TV shows you see and it's all a joke or it's full of older women talking about diets and things you can and can't have.
Jack's meals after diet
Breakfast: Porridge or cereal with fruit and skimmed milk
Lunch: Eggs, bacon, low fat sausage and mushrooms
Dinner: Homemade burgers with red onion and salad, lasagne or spaghetti carbonara
"That's not the case."
Jack didn't do any exercise before he started dieting.
He says he's become far more active since losing the weight.
He said: "You gradually start off following the plan. You fit five minutes of exercise in whenever you like through the day.
"You build that up over time and now I walk five miles a night with my mum."
Jack says his character's changed completely since losing weight and that you have to lose weight for yourself, not someone else.
He said: "My confidence and self-esteem has gone way up. Before, it was nonexistent.
"Now I'll go out. I feel like nothing holds me back. I go on nice holidays. I travel by myself.
"Before I wouldn't have dreamed of going on a plane. Now I like to go abroad quite a bit. I've got friends in Las Vegas and Florida.
"I love seeing new sights. I love shopping. I'm not afraid to go to the cinema with friends.
"I don't think that people are always watching me and judging me."