Allergies, faddy diets and veganism are rife in some of our top restaurants - among the cooks, that is. Caroline Roux reports

The following correction was printed in the Guardian's Corrections and clarifications column, Friday March 4 2005

In the article below we say in error that an adrenalin pen, used to counteract anaphylactic shock, is inserted below the knee. The Anaphylaxis Campaign has asked us to point out that it is important the injection is given in the muscle in the outer part of the thigh.

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Inventing new recipes, or changing existing ones, is a chef's primary joy. But for some, it is a challenge. "I have just created John Dory with dill puree, confit of fennel and razor clams," says Shane Osborn of London's Pied à Terre restaurant. "It involves an emulsion of olive oil with razor clam stock. When I taste it, I have to go and spit it out immediately. And then wash my mouth out."

Although Osborn has fished since childhood, fish is one of his favourite foods, both to cook and to eat, and he sees about 90kg (200lbs) a week of the stuff pass through his kitchen (the restaurant is currently closed, but should reopen in July), his allergy to it is so great that an adrenaline pen is always on hand in case of emergency.

"The sous chefs have to be taught how to insert it below my knee, in case I come into too much contact with fish and go into anaphylactic shock," he says, in a way that suggests that, in Osborn's kitchen, the fearless handling of lifesaving medical equipment is no different from the ability to whip up the perfect custard or sweet potato nage.

So much for the time-honoured image of the portly chef in his fluffed hat tasting his way around his kitchen kingdom, ladle in hand (at Pied à Terre, Osborn's sous chef, Roger Ohlffon, who has been with him for five years and knows his palate, does the tasting). Perhaps, in an ideal world, nothing leaves for the dining room untried. But chefs are not immune from allergies or strong food preferences, strict diets or strange eating fads. An increasing number do not eat what they are happy to serve to us. It might come as a surprise that even among those most passionate and educated about food are those vegans, vegetarians and annoying wheat/gluten/lactose-free types who can make ordering in restaurants such an ordeal.

There is one highly-starred Michelin chef, who would prefer to remain anonymous, who just cannot stomach fish, for no other reason than taste. "Can't stand it," he says, pulling a face. Garry Hollihead, who won a star at his first eponymous restaurant and is now head chef at the Embassy in London and the Inn on the Green in Cookham Green, Berkshire, adheres unwaveringly to the blood group diet (also favoured by the actor Martine McCutcheon). He is O positive and therefore, according to the diet, "a hunter".

Osborn, of course, has no choice. "The alarm bells rang in 2001 when I was prepping frogs' legs and my hand swelled. I ate them later that day and my throat swelled closed. It was terrifying. But it still took me a long time to get to the doctors."

He had what was thought to be irritable bowel syndrome. A gastroenterologist also tested him for colon cancer and Crohn's disease. "I said: 'I don't have time to have an illness like that.'"

Osborn was working 17 hours a day, six days a week, and now believes his allergy, which was finally diagnosed in 2002 ("I started itching in 1999"), was a direct result of kitchen life. "I was surviving on coffee and cigarettes until 11.30, then I'd have a couple of bread rolls for lunch and make a risotto or a croque monsieur when I got home at 1am. It's not a healthy lifestyle."

Osborn, pushing himself to the limits for his restaurant, was insisting on doing everything, "including preparing the fish. Red mullet, for example, has little bones all along the dorsal fin. You're being pricked all the time." When he was finally tested for allergies, he registered 3.2 for cod and plaice, out of a possible six - a very serious score.

But not everyone is refusing their own cuisine out of necessity. Ali Cruddas came from South Africa, cooked her way through the smart kitchens of London, and now works as a private chef in Los Angeles. "I just love food," she says. Provided it contains no dairy. And no meat. And absolutely no fish.

So what is her favourite dish to cook? "That would be fish steamed in banana leaf with a kumquat salsa," she says. And would she eat that? "God, no!"

Cruddas, who cooks everything from foie gras to whitebait, is a strict vegan. She won't even taste anything containing meat, fish or dairy, but she has never been turned down for a job. "You always do a trial shift, so I'm obviously doing it right," she says. But how can she know? "A lot of it is down to smell. You just know."

Apart from anchovies ("they really gross me out - the smell, the feel"), she handles everything: filleting a mignon of beef, reducing a whole duck to its individual parts, gutting a fish. "Anything, as long as I don't have to eat it."

Crudass's own peculiarity is perhaps eclipsed by the maladjusted eating of her Los Angeleno clientele. "I have a vegan to cook for who is wheat- and gluten-free and doesn't really like fruit or veg. That's a hard one; that's tofu five billion ways," she says. "For one woman, I make exactly the same salad every day and every day she pays me $60. And she's happy."

But the great and good of the food business aren't so convinced. "I'd have a long conversation with a vegan, or even a vegetarian, about why they wanted to be a chef," says Rose Gray of the River Cafe. "You'd think they'd want to work in a vegetarian restaurant, wouldn't you?" She echoes many of her counterparts when she says: "I'd never send anything out without trying it. You need to make sure you're going to make people happy."

But some have been won over. "I never believed that you could cook something you didn't love eating until I met Jodie Pascali," says Prue Leith, doyenne of the kitchen and her own well-known cookery school. "She is completely passionate about food, and getting ingredients - meat, fish, everything - from the right source. But she's vegetarian."

Pascali is now head chef at the Hoxton Apprentice in east London, the government version of Jamie Oliver's Fifteen, which trains local people for jobs in catering. "I stopped eating meat eight years ago, when I was 20, for health reasons," she explains. Unlike Cruddas, however, Pascali does still taste the food she cooks. "I can't see how you can tell if a chicken liver pate is right if you don't taste it," she says.

At the Hoxton Apprentice, the most popular menu choice is tuna sesame steak, reflecting how many of us are now eating for fitness as well as pleasure. Garry Hollihead's crowd at Embassy, a restaurant connected to a well-known Mayfair nightclub, on the other hand, is not the most health conscious. While his clientele is tucking into roast fillet of beef with wild mushrooms, potato fondant and truffle and foie gras sauce, or lobster ravioli with scallop mousse, Hollihead himself lives under a herbalist, the strict diktats of the blood group diet and a training regime that will allow him to complete the London marathon in around three and a half hours next month.

"I'm allowed fish and meat, though not fatty cuts, but no cabbage and no cooked carrot," he says. He eats no cow dairy, or wheat. "But then no one should eat wheat, it's so mucked about with."

Cost counts against the exclusive use of Hollihead's favoured organic produce, spelt and rye flour on the restaurant menu. So he eats out. "I get my lunch from Fresh and Wild - a nice organic salad and a tin of good-quality tuna," he says. What's good enough for Hollihead, then, does not prevail in the Embassy kitchens. Perhaps that is why you rarely see chef's special on the menu these days.