They fast

Paris Hilton caused a stir when she walked into New York restaurant Nobu recently.

When a waiter asked the heiress for her order, he was quietly told: "Mineral water."

In fact, over the course of her two-hour "meal", Paris took sips of water and Red Bull - she didn't eat a single bite.

And she's not alone. Super-slim Desperate Housewives star Marcia Cross was spotted dining at a restaurant a few years back with her now husband Tom Mahoney.

According to one eyewitness: "He ordered sea bass and prawns, but she just sipped fruit juice."

Marcia recently admitted to the pressure to be thin: "Not eating is a constant struggle. It's like they pay me not to eat. It's a living hell."

Then there's the master cleanse, otherwise known as the lemonade diet: water mixed with maple syrup, lemon juice and cayenne pepper.

Beyonce admitted following it for two weeks to drop one-and-a-half stone for the film Dreamgirls, and Jared Leto used it to shed the two stone he'd gained to play Mark Chapman in the film Chapter 27.

Gunnar Peterson calls it the "You're an idiot" diet.

"I had a client who did it and I thought he was ill," says Peterson.

"His skin was grey and his eyes had terrible dark circles under them. He was shuffling when he walked, and this was after only ten days!"

One of Hollywood's dirty little secrets is the "IV diet", in which celebrities check themselves into hospital to get put on an IV so they can avoid eating altogether.

Peterson says of the IV diet: "This is beyond ridiculous. If you're doing this you're not fat, you're crazy!"

Suzanne Peck is the director of programmes at Homefield Grange Retreat, a cleansing spa in the UK that relies on juice-fasting.

She says: "Fasting is designed to rest the digestion, hence allowing clarity of thought, cleansing of toxins and healing of the body in general; not to fit into a size-zero dress."