On the last leg of my journey to the Fresh Festival in Weston-super-mare ("the world's most educational and inspiring raw food event") I confide to my taxi driver that for today and the rest of the week I'll be consuming nothing but raw food. He shudders. "All these faddists - they're often the most anaemic specimens you'll ever see. I don't believe in it myself," he notes in his broad Somerset accent. "I'll eat anything." He pauses. "I've eaten swan."

"Erm," I shift in my seat, "isn't that illegal?"

"Oh no, I didn't kill it," he clarifies. 'I noticed it fly into some electrical cables and I don't believe in wasting nothing. I've eaten badger. That was good."

"Like chicken?"

"No," he muses, "quite gamy. I tried fox - can't say I enjoyed it, but apparently the thing to do is to leave it in a stream for a few days and let the water run over it. Cleans the flesh."

"Used to be a pig farm, this," he continues, gesturing towards a low-slung modern building: the festival venue. I had had visions of a sprawling country house, stalls of ripened vegetables dotting the lawn, dewy-eyed individuals flitting between them, powered by nothing but those carrot sticks so fresh that beads of moisture briefly blind you as you snap them in half. Outside it's grey and deserted. We pull into the car park, I hand over my fare, get out, and shiver.

I have been challenged by my editor to eat entirely raw for a week, and reading up online has left me veering between mild enthusiasm and intense scepticism. Proponents attribute massive benefits to eating only raw foods, including increased energy, weight loss, emotional balance, and even the prevention of cancer and heart disease. They claim that raw, unprocessed food contains enzymes essential for digestion which are destroyed in the course of cooking - many even believe that cooked food is actively toxic.

Which is all well and good. Like many people, I could cope with losing a stone or four, increased energy would certainly be a bonus, and - while I don't consider myself emotionally unbalanced - no one could conscionably call me serene.

I also have my suspicions, though. One concerns the argument advanced by many raw-foodists that their diet is best for us because the human digestive system was founded on a raw vegan diet. This is disputable - many prehistoric communities (including the Inuit) are thought to have lived primarily on meat and fish, while archaeologists have traced cooking back more than 1.5m years. Even without such evidence, though, these ultra-Luddite arguments annoy me. Many prehistoric humans also lived in caves, never brushed their hair and had a perilously low life expectancy, and I don't fancy that much, either.

I make my way into the conference room, where about a hundred people are gathered for a demonstration of raw "gourmet" cooking. The chef is making raw "pasta" - slivers of carrot submerged in oils to give them the consistency of linguine. It looks pretty good. He then moves on to the subject of "nut cheese" (nuts ground into a paste) and I can't help sniggering. He said "nut cheese"! The woman in front of me turns around, glowering.

Meeting Karen Knowler, who runs the Fresh Network, I am newly enthused. Knowler - also known as "The Raw Food Coach" - is slim, straight-talking and doesn't smell of incense. I ask her what I will be allowed to eat on this diet - I'm not vegetarian, so presumably I can have sushi? She shakes her head. She suggests that I could eat raw goat's cheese if I want some dairy - but the ultimate aim is to stick to raw vegan foods. Can I have wine? "Wine is considered raw," she agrees, "but our emphasis is on health, and as nice as wine tastes, it's terrible for your health." She counsels moderation.

"There are over 20 different raw food groups and I've listed them all in an e-book I'm bringing out," she continues, "so I'll give you a copy and you can see the range and diversity of food." I perk up considerably.

Knowler takes me through to the festival shop, where cooking equipment - industrial-looking blenders, food processors and juicers - vies for attention with an array of food. I'm shocked by the prices, but don't have time to note them down - logging on to the Fresh Network website later, though, I confirm that it really is true. 1.13kg of raw cacao powder will set you back £52.99, 1.13kg of goji berries - a "superfood" that many raw foodists swear by - goes for a whopping £54.

"It can take as long to get cooked foods out of your system as you've been eating them for," warns the man behind the counter, "so if you've been eating them for 30 years, it could take that long again for you to really feel the benefits of the diet." Great.

My excitement plummets further when I'm accosted in the hallway by two very friendly raw-foodists. Elliott has been raw for just over two years, and has lost five and a half stone. "I went vegetarian, then gave up dairy, became vegan, went to a raw ashram at Glastonbury, went raw, and then went on the natural hygiene diet [an even more extreme and regimented raw food programme]." He's now preparing for a six-week water fast. "Why?' I splutter. "Well," he ponders, "it stops being about food, you see, and becomes more of a spiritual journey."

Shirley has been eating raw on and off for 30 years, and has always been a vegan.

She has been binge-eating since January, in which time she has gained three and a half stone (according to nutritionists this is a common reaction to such a restrictive diet - the body's cravings give rise to bingeing). They both warn me that I could feel dreadful as I embark on the diet, due to all the built-up toxins fleeing my body. "You could have blinding headaches, diarrhoea, you could start to smell ..."

"What?!" Blinding headaches are one thing - but I really don't want to smell.

"Well ..." They seem genuinely surprised that I'm upset by this prospect.

On the way home, I snack on nuts and fruit I've picked up (I've been eating raw all day and am starting to feel ravenous) and turn excitedly to Knowler's list of food groups. Scanning the list, I feel cheated. Vegetables seem to make up at least six distinct groups - vegetables, salad vegetables, leafy green vegetables, indoor greens, sea vegetables and herbs, and wild greens (how can herbs be a separate category?). And that's assuming that algae and edible flowers - two of the other specific food groups -aren't classed as vegetables. Onion and garlic also come into another category - stimulants. I feel myself wilting. I sense that I'm going to need some stimulants myself, and it's unlikely that onion will cut it.

Arriving home, I have my first proper meal of the day - a salad of rocket, chickpeas, olives, avocado and pumpkin seeds. It tastes good, but I'm still hungry when I finish.

I'm still hungry, too, when I wake up on Monday morning. With a blistering headache, I munch some fruit, before heading into work. On arrival, I notice myself sweating, and try to work out whether I'm starting to smell - and the more I worry, the more I sweat, and the more the worry rises and rises and rises. OH MY GOD! I stomp about in the kind of rage that requires counselling, feeling so hungry that I fantasise about eating my own arms. However many nuts I consume, it seems that nothing will stem the hunger - and I am reminded of Knowler's story about a truck driver she had met who experienced some sort of kidney problem after deciding to eat raw and consuming nothing but nuts. I step away from the almonds.

Come the evening, I make up some guacamole and hummus, and have them with carrot sticks. It is, I decide grudgingly, OK.

And the next day, I actually feel pretty good - a weird, light-headed euphoria. I'm having to eat all the time to keep my energy up - when your diet is essentially limited to fruit, vegetables, nuts, seeds and pulses, the sheer volume you have to consume is incredible.

I'm not hungry any more then, but on Wednesday I feel so weak that it's difficult to get out of bed. When I do, it's to rush to the bathroom to vomit. I spend the rest of the day intermittently weeping and sleeping, until my boyfriend returns from work and insists I eat some "proper food". He feeds me a steak, medium rare. I feel much better.

On Thursday, I am determined to stick to the diet - particularly since my editor for this piece has promised me lunch at a vegetarian restaurant in London that apparently serves raw food. We arrive there, salivating (she has agreed to have a raw lunch in an act of solidarity), to find that there is nothing suitable on the menu - we end up having the closest thing we can find, flatbread with baba ganoush and hummus dips. (Knowler had told me that when she dines out she sometimes calls ahead, tells them she's a raw-foodist and asks them what they can do for her, but I hadn't thought that necessary in this case.)

Nutritionists naturally praise the idea of eating a diet rich in fruit and vegetables, but are wary of the raw food approach. "If people wanted to try it for a couple of days," says nutrition scientist, Bridget Aisbitt, "it's not going to do them any harm. As long-term diets go, though, if you cut out major food groups then you're at risk of becoming deficient in particular nutrients."

I ask Aisbitt about the enzyme argument that's used to justify the raw food diet. "I've heard this thing about enzymes before," she confirms, "but the fact is that any enzymes you eat are broken down in your gut before they're digested and what you actually absorb are amino acids anyway ... we specifically evolved our own enzymes to fulfil the individual processes that take place in the human body, and an enzyme in a sprouting chickpea is unlikely to be able to fulfil that role."

"To be honest," she continues, "I think that if you were eating completely vegan and raw foods, you'd have to take supplements, because you just can't get enough of things like iron, B-vitamins and zinc from your diet. That just seems unnecessary. It's expensive, and you could be enjoying eating something nice, rather than simply popping a pill."

I stumble on with my raw food diet on Friday, but give up a day early, when I experience the worst period pains I've had since I was a teenager - searing cramps that no quantity of Nurofen can quell. I realise that many of the symptoms I experienced were a result of detoxifying, and I don't doubt that it is possible to be healthy on a raw food diet, but the level of planning, shopping and actually eating that's necessary would probably put the average person off. Knowler argues that eating raw food frees people from worrying about their diets, and, while that might well be true in her case (she does very occasionally eat cooked food, she notes, as she doesn't want to be ruled by her diet), for some people I can only imagine that this is a major form of displacement activity. By putting so much time, thought and energy into their diet, they make themselves feel special and superior (raw-foodists I spoke to repeatedly described this as a diet for "high-achievers") and distance themselves from normal human interaction - after all, even just having a meal out with friends becomes a genuine challenge.

Beset with relief, then, I return to eating a varied, unrestricted diet. Well, I say unrestricted - even I'm not ready for roadkill.

· Some names have been changed. For more information go to fresh-network.com