Notoriously cagey about her weight, this Olsen twin checked into rehab in Utah four years ago suffering from anorexia. Weighing just 51/2st during her six-week stay, Mary-Kate managed to put on around 10lb, but recent pictures show her looking worryingly skinny again.

VICTORIA BECKHAM

Age: 34

Height: 5ft 5in

Real Weight: 6st 8lbs

It’s a diet of steamed fish, seaweed protein shakes and vegetables that keeps Posh so slender. But recent reports suggest she wants to gain weight to help her get pregnant again. Luckily pal Eva Longoria Parker has been on hand to give Victoria her calorific cheese enchiladas to help her pile on a few pounds.

RENÉE ZELLWEGER

Age: 39

Height: 5ft 5in

Real Weight: 7st 7lbs

Famous for bulking up to a size 14 for the two Bridget Jones films, Renée is now a size 6. She follows a macrobiotic diet of lean fish, rice and raw vegetables, and endures a daily two-hour workout with her personal trainer. She says it’s not “a fitness regime, just a lifestyle”.

JENNIFER LOPEZ

Age: 38

Height: 5ft 6in

Real Weight: 8st 7lbs

After giving birth to twins Max and Emme in February, Jennifer started 4am exercise sessions including Pilates, boxing and exercise bike workouts to get her back into pre-baby shape. But she’ll never be a size zero. “I couldn’t get down to that size and still be healthy,” she says.

LILY ALLEN

Age: 23

Height: 5ft 2in

Real Weight: 10st

After being diagnosed with a heart murmur, Lily hired a hypnotist and personal trainer to help her lose weight. She dropped two dress sizes in six weeks by working out for 90 minutes three times a week. Lily claims the stress of work is to blame for her recent weight gain.

BETH DITTO

Age: 27

Height: 5ft 2in

Real Weight: 14st 9lbs

Despite being constantly cross-examined about her weight, the singer claims she is happy with her size-20 figure. “In my late teens, I decided I was either going to spend the rest of my life trying to change myself or accept myself as I was,” she says. “It works for me.”

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Photography: PA, Rex, Capital, Goffinf.com, UK Press