RENEE Zellweger is once again facing the perhaps enviable task of piling on the pounds to play the role of plump, lovelorn Bridget Jones. The normally slimline Texan actress is trying to go from a size 6 to a size 14 in just three months for her new film Bridget Jones: The Edge of Reason.

She is hoping to achieve the transformation by adopting the sort of diet most of us are trying to kick. Out goes the Hollywood no-carbs diet and in come burgers, fries, scones, multi-fat milkshakes, doughnuts, peanut butter, pizzas, and potatoes saturated with butter.

Three years ago the 34-year-old star went on a similar regime for Bridget Jones's Diary and managed to gain 20lb in just 12 weeks by implementing a ban on exercise.

But Zellweger is not the only woman in the world who has ever tried to get heavier. Although most of us are constantly fretting about losing weight, some women are too thin and need to get fatter for health reasons.

Being underweight can increase your risk of osteoporosis and heart disease, and being too thin in your earlier years can depress the immune system and cause fertility problems.

Geri Halliwell, who has suffered bulimia since her teens, recently confessed that at the height of her slimming drive her menstrual cycle disappeared. She said: "The thought of not being able to have children was a real wakeup call."

The former Spice Girl has now regained some of the curves that became her trademark back in the 90s by eating three meals a day plus two snacks. She says she is happier and feels "really healthy".

Actress Patsy Kensit is another star who has gained weight but insists she is content with her fuller figure. She piled on the pounds for her latest film role in the comedy The One And Only, and admitted she had found it hard to shift them afterwards.

But for women who want to gain weight, Renee's gut-busting diet, which constitutes 4,000 calories a day - double the recommended number for a woman - is the worst way to go about it, according to the experts.

Suddenly eating a lot of sugary foods can put you at risk of Type 2 diabetes because the pancreas is hard pushed to process all the extra sugar. The digestive system may also be affected, with the liver working overtime, and bloating may result.

Dietician Alison Green of St George's Hospital in London, says: "An average woman should eat 2000 calories a day. To gain weight, you should eat an extra 500-700 calories.

"Anyone who wants to gain weight needs to do it slowly with little changes to their diet.

"It is common sense things like making sure your milk is full-fat, and adding things to food like cheddar cheese to mashed potato; just getting little bits of calories into the diet a person would already have. Little things and more often is probably the best way to go.

"You should eat three meals a day and include two to three snacks as well, such as fruit salad with a little cream, nuts or dried fruit or a scone with jam.

"But you still need to make sure there are plenty of fruit and vegetables in the diet. Although they won't help with the weight gain, they will counterbalance the greater fat content in the diet. They contain antioxidants and vitamins and minerals and keep blood vessels healthy.

"You should still eat protein such as meat, fish and eggs, but too much can put a strain on the kidneys and cause dehydration.

"It's choosing the right fats as well. Saturated fats are bad for cholesterol levels, and a diet of doughnuts, hamburgers and chips should be avoided for this reason. However, healthy fats, such as olive oil and oily fish, are an essential part of the diet.

"Exercise is quite important, because you don't just want to be putting on fat bulk, there should be muscle bulk as well."

According to Green, a healthy weight gain is between one and two pounds a week. But as your weight stabilises, your metabolic rate should increase.

It is also important to carry on exercising to ensure you don't become flabby.

Tim Bean, a personal trainer who runs Total Physique Management, says: "It's important to maintain cardiovascular fitness so the system stays healthy even though the body is carrying the burden of extra calories.

"Short bursts of cardiovascular work would be suitable, but make sure it is not too high intensity so as not to put stress on the heart. Stay well away from strength training and exercise on an empty stomach as much as possible.

"If you put on too much muscle mass, you will increase your metabolic rate, and calories will be used up as fuel rather than going on as weight. You can lose muscle mass by eating less during the day and more at night, but this is often more unhealthy than good."