Dolly Parton: Cabbage Soup Diet / Getty

Behind most stars is a mad, fad diet. Writer Rebecca Harrington tried them all, she tells Kate Wills

When 18th-century French gastronomer Jean Brillat-Savarin said "You are what you eat", he could easily have had Beyoncé's maple syrup cleanse or Gwyneth's macrobiotic quinoa in mind, so synonymous are certain celebrities with what they do (or don't) consume. Elizabeth Taylor mixed cottage cheese and sour cream. Marilyn Monroe drank raw eggs whipped with warm milk. Greta Garbo swore by celery loaves and boiled grapefruit. Where there's a Hollywood star, there's often a seriously deranged diet and writer Rebecca Harrington, 29, tried them all for her new book I'll Have What She's Having.

"It was a bit like Confessions of an English Opium Eater," she tells me from New York, fresh from finishing the Taylor Swift diet (basically, drinking pumpkin spice lattes while wearing bowler hats). "The whole time [Thomas] de Quincey's like, 'I ate opium and it's awful', but you can tell that he's deeply enjoying it. I was actually having fun in a warped way."

Having read her catalogue of face rashes, fainting spells and fears of salmonella, Harrington is clearly a masochist if she found this fun, but what first inspired her quest to eat famously (other than having one eye on writing a book about it)? "About two years ago, someone sent me an online article about William Howard Taft, who was America's fattest president," she explains. "It had his daily diet from 1905 and it was totally disgusting – boiled fish, mutton and glutinous biscuits for snacks. I thought it would be funny to try it, and then my friends suggested that I try some diets of real celebrities who people were actually interested in. My first was Liz Taylor's."

Liz-approved recipes: steak with peanut butter, dry toast every morning and swordfish with lime juice

Using Elizabeth Takes Off, Taylor's diet book from 1987 (now out of print) as her guide, Harrington embarked on 15 days of eating only Liz-approved recipes, such as steak with peanut butter, dry toast every morning and swordfish with lime juice, which tasted like "a lime-flavoured old shoe on the ground". During one particularly low point, Harrington took one of Liz's blue cheese and sour cream dips into her office at The Huffington Post. "It smelt repugnant. My colleagues loved that one." But stinky dip was a picnic compared to Greta Garbo's celery loaf. "When I was cooking it, it smelled like a rotting body and it tasted like baked vomit. I didn't know what to do with it so it was just sitting in my oven for seven days. I'd go past the oven and retch. I was living in a tiny studio apartment at the time so there was nowhere to hide."

But it wasn't all retch-inducing. Sophia Loren famously quipped "Everything you see, I owe to spaghetti", and wrote two pasta cookbooks, cue Harrington scoffing spaghetti carbonara for every meal. "That was actually the hardest one, in a way, because the portions Sophia ate were so small and the best things are terrible in small packages." So not quite the dolce vita, then.

Were any of these diets remotely enjoyable? "The best moment was when I made Gwyneth's fish tacos. They were delicious, but they used so many expensive ingredients! If I was a millionaire, it would be the best diet in the world. As would Jackie Kennedy's baked potatoes with Beluga caviar."

Channelling hungry celebrities has given Harrington a new-found empathy for the rich and famous. "I felt sorry for all of the women whose diets I tried. They have immense privilege, but to have people scrutinising you to such a level that you feel like you have to eat in this regimented way would be awful."

But from Taft to Taylor Swift, our appetite for celebrity diets doesn't seem to be slowing down. "We think that if we eat like this famous person then we might be able to become them," says Rebecca. "Or maybe it's because celebrities are so managed now that their personality inadvertently gets conveyed through these diets and cookbooks."

So which of these bizarre meal plans was the most successful? "I lost a bunch of weight on Dolly Parton's Cabbage Soup Diet, but I also lost something more important – the ability to be in my kitchen for any length of time without smelling like cabbage."

'I'll Have What She's Having: My Adventures in Celebrity Dieting' by Rebecca Harrington (Virago, £8.99) is out now