The Yo-Yo Diet Diary

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To land roles in top Hollywood movies, stars often have to gain and loose weight quickly. Renee Zellweger will once again gain weight for the sequel to Bridget Jones' Diary. She has to go from a size six to a size 14 for the role.

Dietitian Cari Miller says the yo-yo pattern of gaining and loosing weight can be harmful to your body because fat cells are forever. She says during dieting fat cells may shrink, but they won't go away. Miller says even when you loose weight, all the small fat cells are still there. The amount of fat cells may make a person more susceptible to weight gain the second time around.

Zellweger, unlike the rest of us, will receive a large paycheck to adjust her weight. But many people get frustrated by loosing small amounts of weight each week. Fitness instructor Mary Engelland says one pound of body fat is a lot of mass to loose.

Experts say when weight is lost too quickly, lean muscle tissue often goes with it. This then slows down the metabolism and may lead to poor body image.

Renee Zellweger may stay heavier to play the role of Janis Joplin.