It's a change that's taking some getting used to. "At school I got constantly teased about my size, so I turned to my school work and made sure I did that well," she recalls. "It meant by the time I was 14 I'd decided I wanted to be a bank manager when I was older. And as soon as I left university I got a bank job and worked my way up over the next 20 years."

She had her son Michael, now 18, with her former husband when she was 28. But as a downside she continued to pile on the pounds after he was born. "I was a single mum and worked long hours. I would eat large sandwiches at my desk and often grab a bag of chips on the way home to keep me going before I had a late dinner."

But leaving work in April 2008 was a shock and, partly just to kill some spare time, Alison went with her mum to a Slimming World class. "I only went to prove it wouldn't work for me. I had tried classes in the past and never stuck with them. I was later told my face was like thunder when I stood on the scales and saw I was 17st 1lb. There was no escaping the truth, I was morbidly obese.

"So I went about tackling my weight like it was a work problem which needed solving. I stuck rigidly to the plan in the first week and found I had lost 6lb."

Alison also got a personal trainer who introduced her to unusual exercises to make losing weight interesting. "I was introduced to spinning. At first I didn't fancy it, but soon I became hooked." Now, in addition to taking five spinning classes a week, Alison has qualified as a personal trainer. "I can't believe I'm now the person at the front showing others how to keep fit. A year ago anyone seeing me - this 17st-plus woman - would have laughed at the thought!" Alison continued to shed weight and after a year and a day, she hit her goal weight of 10st 1lb, losing 7st.

"I feel like a butterfly emerging from a chrysalis. My son can't believe some of the outfits I now wear. And I'm really enjoying my new career path in the world of fitness at the moment."

Alison does triathlons and loves socialising and she admits the weight loss is the key to her new, positive outlook. "I have vowed never to get overweight again - I have had so much fun since losing all that weight!"

Alison's meal makeovers

Before

• Breakfast: Huge bowl of muesli.
• Snack: Cake and a cappuccino.
• Lunch: Two packets of shop-bought sandwiches filled with hard cheese and a packet of crisps.
• Snack: Nibbling through an entire small French stick.
• Evening meal: Large portion of creamy lasagne with ice cream for dessert.
• 

  Snack: Hot chocolate drink.

After

• Breakfast: Porridge with blueberries.
• Snack: An apple and banana.
• Lunch: Vegetable soup. A yoghurt.
• Snack: A biscuit.
• Evening meal: Two pork chops, vegetables, roast parsnip. Fruit.
• Snack: Two plain cream crackers.

How you can do it too

• I stick rigidly to a plan for six days but do have one day off as a treat and go back to it with more energy.
• Be accountable to others, not only yourself - I text my trainer but you could send a message to family or friends saying how you have done each week.
• I learned to eat only when I was hungry, rather than out of habit.
• Try as many different ways of exercise as you can - that was how I discovered my love of spinning!

FAT FILE

Age: 46

Height: 5ft 6in

Top Weight: 17st 1lb

Weight Now: 10st 1lb

Top size: 28

Size Now: 10

Before size: 28

After size: 10

Source: Daily Mirror