About 21% of adult Scots are now classified as obese

At just 4ft 10ins tall, the retired secretary from Denny knew she was overweight at 12st but, like many of Scots, never considered doing anything about it.

But when doctors told Mrs Whitfield, who suffers from heart defect mitral stenosis, that she was putting a strain on her heart serious enough to require an operation, she decided to take action.

After joining her local Weight Watchers class and following a simple exercise programme of swimming, walking and gardening, Mrs Whitfield slimmed down a healthy 8st 7lbs - a move she believes has prolonged her life.

And she was not surprised by the results of a study carried out by Glasgow University which found that the treatment of obesity-related illnesses in Scotland now costs the NHS more than �171m a year - similar to that associated with smoking.

She said: "Deep down I knew I was overweight but like a lot of people, I'd convinced myself that I wasn't really fat.

"I'd already had an operation when I was 35 to fix the narrowing of a valve in my heart and had been fine for 20 years. Once I started to put on weight however I began to struggle with day to day tasks - my heart just wasn't coping with the extra strain.

"Doctors told me that if I continued as I was, another operation was a certainty. It was then I knew I had to do something about my weight."

Deep down I knew I was overweight but like a lot of people, I'd convinced myself that I wasn't really fat Nan Whitfield

Mrs Whitfield said strenuous exercise was not an option for her.

"My heart wouldn't take kindly to something like aerobics but walking, gardening and swimming all made a difference," she said.

"I wasn't doing lots of exercise but the important thing was I made sure to do something every day and I've avoided the operating theatre as a result."

She added: "The findings by Glasgow University don't surprise me at all. You only have to look around you to see that the number of people overweight or obese in Scotland is rising.

"I don't think many people actually realise just how much losing a few pounds can improve your health but it is a lesson that I have definitely learned.

Death toll

About 21% of adult Scots are now classified as obese. More than 850,000 people have a higher risk of suffering from heart disease, diabetes, disability and some cancers.

It has already been predicted that the death toll from obesity will top that from smoking in the next 10-15 years.

Dr Andrew Walker, of the Robertson Centre for Biostatistics at the University of Glasgow, carried out the study.