Last week, after 12 years of strict vegetarianism, I tucked into an oozing pink slab of sirloin steak. By the time the meat reached my plate, I hadn't eaten a morsel of meat during my entire adult life.

The idea of going vegetarian had hit me - literally - one morning during my first year at Oxford. On my way to a lecture, I was bashed in the face by a blood-caked dead pig that was dangling outside a butcher's shop. Already feeling hung-over and fragile, I promptly threw up and vowed never to eat meat again.

That evening, while my friends dined on beef Wellington at Formal Hall, I nibbled smugly on sautéed spinach with a side dish of broccoli. I had become revolted by meat when I was 10 and a classmate at school informed me that sausages (which I loved then) were made of pig testicles and wrapped in slivers of intestine.

Despite my horror at this, I continued eating meat and simply made a huge effort not to associate sausages with pigs or hamburgers with cows. But once I'd left home (where everybody ate meat and lots of it, at every meal) and started university, I felt keen to assert my newfound independence.

I became a vegan overnight - and found this surprisingly easy, mainly because I naturally hate the foods that veganism forbids: eggs and dairy products. Then I moved to New York, home of the obscure food fad, and I immediately upgraded from veganism to macrobiotics - the brown rice and seaweed diet that reputedly keeps Gwyneth Paltrow and Madonna slim and serene.

Sadly, macrobiotics didn't really work for me. I ended up looking pale and feeling immensely hungry. I decided that the reason I felt washed out was because my diet, despite being organic and meat-free - wasn't healthy enough. So I cut out sugar, white bread, white rice, pasta, dairy products and wheat and, after reading that fashion designer Donna Karan was following a "raw food" diet - I stopped cooking my vegetables.

Karan had apparently lost a stone and a half and gained loads of energy by eating raw vegetables and "sprouted" grains. I tried it and gained half a stone and felt so extraordinarily sleepy that I could barely stagger to the local organic supermarket to buy my vegetables. Why, I wondered, did my friends who lived on pizza, pasta and hamburgers look so much healthier than me?

By this time, vegetarianism had become a huge part of my identity. I frequented health food shops, secretly looked down on people who ate meat, thinking them unenlightened and Neanderthal. I fantasised that I was doing wonderful things for my health by rejecting meat.

The truth was that with each slightly more extreme variation of vegetarianism I tried, I grew slightly weaker, more lethargic, more depressed and - worse still - slightly fatter.

Every time I had a late night, I seemed immediately to get a cold afterwards. Often, I'd faint during the first day of my period each month. I felt ravenously hungry immediately after I'd eaten.

No amount of chickpeas, tofu, vegetables, fruit or lentils seemed potent enough to fill me up. When I looked at my reflection in the bathroom mirror I would be disgusted by how pale I looked. Even my tongue and lips looked pale.

I consulted my GP, who did a blood test that confirmed I was anaemic. She suggested that I gave up vegetarianism, because, she claimed, red meat is the only easily assimilated source of dietary iron. In being anaemic, I was far from alone. According to the Centers for Disease Control and Prevention, 11 per cent of women of childbearing age in America are iron deficient.

As well as making you feel weak and tired, anaemia apparently gradually undermines your intellectual performance. "Women especially need to know this is actually affecting their brain and the way they're thinking," says Laura Murray-Kolb, a postdoctoral fellow at Penn State University.

I decided to persevere with vegetarianism anyway (although I did buy iron tablets, which didn't seem to do much good). Then, a few months ago, something strange happened. While shopping for my groceries at the local deli, I started lingering by the organic meat counter, stealing surreptitious glances at slabs of marbled beef and chunky lamb chops. I was like a little girl eyeing the glamorous clothes in her mother's wardrobe.

Around the same time, I read that Elizabeth Hurley was following something called the Blood Type Diet, started by a naturopath called Dr D'Adamo who claimed that there was no such thing as a one-size-fits-all healthy diet. In short, vegetarianism simply couldn't work for everybody.

D'Adamo claims that it all boils down to your blood group: people whose blood group is A can thrive as vegetarians and shouldn't eat any meat, while those of blood group O can't thrive without red meat. My blood group is O. I suddenly started to feel that meat, the only thing I hadn't tried eating in recent years, was the missing link to good health.

"There are certain people who simply need meat in order to thrive," says Meg Richichi, a New York doctor of Oriental Medicine. "I can look at people and tell that vegetarianism simply won't work for them. Chinese medicine dictates everything in moderation, including red meat."

And so, last Monday, while shopping for organic vegetables at Dean & Deluca in Manhattan, I decided to buy a steak. Looking at the vast bloody array of tenderloins, sirloins and ribeyes, I felt dizzy and overwhelmed.

Finally, I pointed at any old steak, said: "Give me half a pound of that" (nobody in New York ever says "please") and scuttled to the checkout, feeling slightly sordid.

When I got my parcel of steak home, I put it on the kitchen table, unwrapped it carefully and stared at it. I felt as though I had just brought home something dangerous, something that I had no idea how to use: like a gun.

My steak (medium rare and served with organic asparagus) was delicious. I felt daring and grown-up as I ate it. I'd expected to want to throw up at the first bite, but instead I felt satiated and bubbling with energy. Two days later, I had a small, organic lamb chop for lunch and the following day, a leg of corn-fed chicken.

By the end of the week, I felt overwhelmingly full of life. My tongue was pink again and when I went to the gym I was able to run for 40 minutes without stopping, instead of flaking out after 20.

Since re-establishing myself as a carnivore, I've also lost 5lb in one week, perhaps because after eating a meal that contains meat I don't feel hungry again until the next meal-time.

My vegetarian friends are, of course, horrified. I agree that it's a drastic leap, but I love making snap decisions and I love testing my willpower.

When I decided to give up smoking four years ago, I simply opened my sitting-room window, tossed a full pack of Marlboro Lights out on to the street, closed the window and never smoked again. Of course, everyone agrees that smoking is bad for you.

But the best health decision I've ever made was giving up vegetarianism.

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