At the age of 26, Dena Ryness had been big for as long as she could remember. Having been a chubby child and pudgy teenager, Ryness was now a grown-up size 16-18 who had never exercised. She longed to wear trendy clothes but was resigned to being "the fat one" in her family.

Then she read about a local kickboxing class. "I suddenly decided that I needed to sort myself out and that this class could help me." She went along but was so desperately unfit that she couldn't even make it up the 28 stairs to the studio.

Instead of going home for a cry and a cream bun, Ryness struggled up those stairs and literally fought her way to fitness. Eight years later, she is now a super-fit size eight who competes in marathons and runs a nutrition clinic in south Manchester, where she lives with her accountant husband Mark.

Her story is an inspirational one, but, she says, not the story that people want to hear. She explains: "A lot of people ask me, 'How did you lose so much weight?' What they want to hear is one magic thing that required no effort that made my weight fall off instantly. But it doesn't happen like that! It requires hard work, effort and sacrifice."

Ryness's fitness training required intensity and resolve. She recalls: "I was kickboxing three times a week but couldn't keep it up so they stuck me on a treadmill and they told me what to eat."

Dena's tips

Always read food labels. If one item of food has a long list of ingredients, it's full of chemicals.
Friday night dinner can be healthy. Swap oil-laden roast potatoes for lots of veg. Chicken is a good protein but don't eat the skin. Eat smaller meals and snack healthily inbetween.
Don't overdo it at festivals. There's no need to eat five doughnuts at Chanucah - enjoy one slowly.

She stopped being vegetarian, swapping white bread and potatoes for lean proteins like chicken and fish. Breakfast became porridge with berries, with fruit and veg featuring heavily throughout the day, and alcohol was banned. It sounds strict, but as Ryness says: "There is no diet that will enable you to eat anything you want in unlimited quantities."

Slowly, she lost weight and began to run marathons. This feat was made more impressive by the fact that Ryness's job at the time - as a marketing director in the City - involved lots of travel. "It's all about planning. It was difficult but the fact that I could do it shows that anyone can do it."

Other people were sceptical at first.She says: "When I started people were saying, 'you won't be able to keep it up, you won't get below a size 12, you can't lose weight when you hit 30, you're being too obsessive'… They were all wrong. I lost even more weight at 30."

It took Ryness four years to reach her target weight. The hard work continues: she still eats carefully and does regular interval and weight training.

Having discovered a zest for healthy living, she retrained as a nutritionist and now runs Beautiful Active Nourished, a clinic in south Manchester that specialises in weight loss and sports nutrition. "Without my weight loss, this business wouldn't exist. I'm passionate about what I do."

The best thing about losing so much weight? "I can finally wear fashionable clothes! And without sounding like a self-help book, it taught me that you can achieve anything you want: it just takes time and hard work. You can apply it to other areas of your life too."