The transformation from world's most celebrated chubby singleton into super- svelte Hollywood glamourpuss has become her trademark.

But, second time around, Renee Zellweger's rapid post-Bridget Jones weight loss was anything but easy.

After filming on Bridget Jones: The Edge of Reason ended in April, the 35-year-old booked herself into a £ 2 , 500 - a-week weight-loss spa - where daily 12-mile hikes are mandatory - and embarked on a high-protein diet to shed nearly three stone.

The 5ft 5in actress went from her usual 7st 7lb to more than 10st when she first agreed to play Bridget in 2001, fuelled by doughnuts, chocolate and cheese-laden pizzas.

Although she succeeded in piling on the pounds - rapidly losing them on the Atkins Diet - the huge amounts of sugar she was consuming, together with a sedentary lifestyle, left her sluggish.

So when, a year ago, the time came to put on weight for the sequel, Miss Zellweger employed a nutritionist who put her on a 4,700-calorie per day diet.

This time she ate healthier foods to gain bulk, including lasagne, fish and chips, stir-fried chicken, avocados and weightgain shakes, made up of flaxseed oil and protein powder.

'I ate healthy foods with a high, hidden fat content - it's the easiest way to do it,' she said.

On her return home to Los Angeles, Miss Zellweger began visiting a gym for daily, two-hour workouts. Her meals were mainly tuna or cold meat coupled with dressing- free salads and raw vegetables.

In June, she checked into the £2,500-a-week Ashram Spa in Calabasas, California. Visitors share rooms with a stranger and do a 12-mile hike every day.

Owner Catharina Hedberg told Heat magazine: 'Renee is a lovely, sweet girl who was willing to work hard when she came here.'

Miss Zellweger has spoken of her concern that her yo-yo dieting could affect her fertility.

'I know there's been some research into losing weight quickly and the effects that has,' she told Closer magazine. 'Of course, it's concerning. I'd love to be a mum one day. I'm looking forward to being pregnant and putting on weight naturally.'

l.benjamin@dailymail.co.uk