Day one

It is towards the end of my first 24 hours as a vegan, at the point when I discover I have already failed, that I begin to despair. How was I supposed to know there would be dairy products in a bag of nuts? Who the hell decided that putting dried milk powder in with the lemon and coriander flavoured cashews and macadamias was a good idea? Sainsbury's, as it happens. Or bloody, sodding Sainsbury's as I now like to call them.

It is such a disappointment. I liked those nuts. I reckoned that I could quite easily get through my vegan week - not just meat and fish-free, but dairy, egg and honey-free - if it involved a snack as tasty and satisfying as this one. Nuts could be my friend. Nuts could be my salvation. High in protein. Rich in luscious fats. All a big boy with a taste for the animal could need. At lunchtime on that first full day I had stood in the kitchen scoffing the macadamias, as my wife Pat pleaded to be allowed to try them.

'You can't have these,' I said, with a solemn shake of the head. 'They're only for vegans.'

'Oh go on.'

'Are you a vegan?'

'Well no, but...'

'Then you can't have them can you.' And I popped another in my mouth.

'Sod you,' Pat said. 'I'm going to make myself a ham and cheese bagel.'

I told her to go right ahead; that all those animal and dairy fats wouldn't make her feel good, either about the planet or herself. If you've led a life like mine, rich in shame and filth, there are few opportunities for being sanctimonious and you have to seize those that offer themselves quickly. Which was what I was doing when I examined the ingredients of this bag of nuts and noticed that, in among the listings for sunflower oil and salt and lemon and coriander leaf was a reference to lactose from cows' milk. Apparently it works as a thickener for flavouring powders.

What's most irritating about this is that I had been warned it might happen. It was one of the things I was told by my vegan neighbour Nikki when I went round to her house and fell to my knees shouting, 'Help me! I have to be vegan for a week! What is it you people do?'

Nikki and her partner Ian have been vegans for a few years now and, whisper it, are not at all pale and wan and listless. They are vibrant, dynamic, lovely people. They know that, at our house, we delight in the slaughtering of many animals for our table, but we have never let a little light farmyard genocide get in the way of our friendship.

That said, Ian clearly thought it was hilarious. 'Couldn't have happened to a nicer bloke,' he said, when he heard. This was odd. It made it sound like something he regarded as a lifestyle choice was actually a form of punishment, one which I too was about to suffer. Over the next few days, when he saw me in the street, he would point and laugh.

Nikki was more nurturing. She allowed me to look at the contents of her fridge. There was something white and wobbly in a plastic container. 'Silken tofu,' Nikki said. 'You blitz it if you want to add creaminess to a dish.' I asked her what it tasted of. She told me it tasted of nothing and looked at me as if I had slightly missed the point. There was Alpro soya milk and soya yoghurt. There was instant miso soup in sachets and sweet chilli sauce. There were puy lentils and long-grain rice which takes longer than your natural life to cook and split peas, which are quicker, but really, why would you?

There was also a bottle of maple syrup, 'which you can use as a replacement for honey'. Until that moment it hadn't occurred to me that honey was forbidden in the vegan diet. After all, honey doesn't come pouring out of the tiddly bee's honey duct, does it? It is manufactured outside their delightful stripy bodies. But it belongs to the bee so it can't be used. After all we like the bee. We do not steal from the bee. I make a note to use maple syrup from now on. Like a quarter of a million other Britons, like Oprah Winfrey recently, I am going to be a vegan. I feel fellowship with the bee.

That was when Nikki mentioned the milk powder thing. 'Turns up in lots of unexpected places. Cereals are full of it. That's the one you have to be careful of.'

I was, frankly, intimidated. I wasn't sure if I was up to it. Still, I was determined to give it a go. The truth is I have been very, very rude about vegans in the past. The American food writer and chef Anthony Bourdain once called them the 'paramilitary wing of the vegetarian movement', a sentiment I share. I regard vegans, even the nice ones like Nikki and Ian, as people who are preoccupied by the negative: it is all about what they don't do. It seems to me joyless and unfulfilling. If life is about happiness in the here and now rather than in the future - and I think it is - veganism seems ultimately self-defeating.

I also have a special hatred for the kind of cookery such an eating regime tends to produce. I don't hate meat and dairy-free cookery per se. I love the southern Indian vegetarian culinary tradition which is mostly vegan - all those crisp puri filled with nuts and sweet chutneys; all those spiky curries and rotis and dhals. I also like much of the Japanese vegetarian repertoire which just happens to be dairy and egg-free. What I don't like are pretend meat dishes: the veggie burgers and sausages, the pretend lasagnes and moussaka, where ingredients go to die. I simply demand that a dish be good because it is meat-free, not in spite of the fact.

That said, I recognise there are strong moral arguments for such a diet. Our planet's population is growing. Feeding us all at an affordable price is becoming harder and harder. It takes eight tonnes of grain to raise one tonne of beef. In terms of pure calories, the farming of meat is not efficient. Cattle also fart a lot, contributing significantly to global warming. I know all this. The question that has to be asked is whether a vegan diet is the answer. My view is no. Mother nature intends us to be omnivores and I don't think rewiring the planet's population is practical. I also don't believe you can be genuinely happy and vegan. I think the two are mutually exclusive. The thing is, Morris dancing and incest aside, it's hard to criticise something unless you've tried it. My week as a vegan will add substance to my argument. These are the sacrifices I am willing to make for the moral low ground.

And so, one lunchtime, which I intend to be my last as an omnivore for seven days, I go with friends to the Albemarle restaurant at London's Brown's Hotel. I order razor clams baked in luscious dairy fat-rich butter with breadcrumbs. I have a huge, rare bloody rib-eye steak the size of a golden Labrador puppy, and finish with a chocolate mousse thick with cream. I feel my arteries harden to the touch and I am profoundly happy.

In preparation for my adventure I had visited a local vegetarian health-food store. It is vegan central. I had bought rice noodles and smoked tofu and sweet chilli sauce and spring onions and that evening I use them to make a reasonable stir fry. I even decide that smoked tofu, with its dense, almost meaty texture, isn't half bad. The next morning I decide that soya milk and soya yoghurt are just about OK on my All-Bran, even allowing for the slight aftertaste of sawdust. At lunchtime I have more noodles and smoked tofu in miso soup. I eat fresh fruit. I feel virtuous and proud.

And then I make the milk-powder cashew-nut discovery and I am downhearted. Later it gets worse. I make a rather good Thai green curry full of roasted mushrooms and baby corn cobs and caramelised onions. It is fabulous; rich and dense and spicy and fulfilling and entirely vegan. Until, that is, I study the ingredients on the jar of Thai green curry paste: it's got crushed shrimp in it. Bloody hell. This isn't fair. I bought it at the uber veggie, all-the-wheatgrass-you-can-eat health-food shop. And if I can't rely on that place to see me right, whom can I rely on? I regard myself as an oppressed minority. I feel bitter and self-righteous. Though that may just be indigestion brought on by a surfeit of smoked tofu.

No matter. I decide to forget about today. The nut thing had already made it a write-off. What's a little shrimp paste between friends? I am determined that tomorrow I will be a better vegan. Which is to say one who doesn't actually eat any dairy products or shrimps.

Day two

I want to be a glamorous vegan. To do this I must go to where glamorous vegans go. There really is only one choice: the Gate, a vegetarian restaurant in Hammersmith. Gwyneth Paltrow goes to the Gate. And Madonna. And Stella McCartney. And Woody Harrelson, which is slightly worrying but we'll let it pass. It's where cool veggies go. Except not today, because none of them is here in this light, bright whiteout of a dining room, which apparently used to be an artist's studio. I don't care. I'm here for evolved vegan food. I want to see what it's possible to do with vegetables and pulses and a bit of wit.

To be honest what I'm really here for is someone else to take the strain. I'm less than two days into my vegan experiment and I'm already exhausted by the fiddly business of working out what to eat. Omnivores can open the fridge and go: sandwich, fry-up, steak. Piece of protein. Butter. Heat. Job done. Omnivores get to paint from a full palette. As far as I can see vegans are working in sepia. It's bloody difficult.

I know a lot of vegans regard this as a virtue. Ah, they say, it forces you to think. It's a challenge. I have a very sophisticated word for this: bollocks. If I hear someone has conquered Everest or traipsed single-handed to the North Pole, I don't think what a lightweight I am for living such a simple easy life. I think, if that's what makes you happy so be it, but it's not for me. That's now how I feel about vegan cookery. The fact that it's difficult does not make it better. What's more, in my experience, most vegans are simply not up to the challenge.

Anyway, today it's someone else's problem. I have some falafel made from broad beans which are really just the victory of the deep-fat fryer. I've said it before: anything can be made palatable by a dip in boiling fat, even Ann Widdecombe. Still, I quite enjoy them. I follow that with aubergine teriyaki which is nothing of the sort. Teriyaki suggests a sweet, sticky, dark soy-based sauce. This is a roll of chargrilled aubergine stuffed with ground-down mushrooms and something called pesto, which isn't. On the side are some stir-fried noodles with vegetables. Pretty much the same thing I made for myself a few nights before. I reach a conclusion: ethnic is the default position for the vegan.

At the end I have a vegan rhubarb crumble, with a topping like cement and vegan custard. I ask the manager how they make a vegan version of custard, when the real thing is a combination of eggs and milk. He spits out the words: cornflour, water, flavourings. So he's not a vegan then? 'I'm not even vegetarian. Just in the restaurant business. Though vegetarians are some of the nicest customers.' I suggest it's because they're grateful for somewhere to go. 'Could be.'

I ask him what his favourite restaurant is. He says: 'St John.' Ah, St John, where they serve roast bone marrow. Where they chargrill chitterlings. Where I once helped eat a whole suckling pig. Why did he have to mention St John? Bastard. Bastard, bastard, bastard.

Day three

I am becoming one of those people I hate. You know the ones. The people who turn the simplest of questions - fancy a cup of coffee? - into a long, drawn-out lecture on lifestyle choices and the importance of a balanced diet that doesn't scar the planet. Right now, I'd be quite happy to scar the planet. I'd happily burn holes in it. Anything to stop 'would you like a cup of coffee' becoming such a minefield. I simply cannot bring myself to ask whether my hosts have soya milk, because then the big speech has to begin, the one about how I'm experimenting with veganism and all the dilemmas it poses, and I can barely bring myself to hear it let alone give it.

I am invited to a swish hotel for the launch of the St Alban's Food and Drink Festival, where I am to take part in an event. The chef has done fabulous things with fillet steak on toast and treacle-cured bacon. There are plates of charcuterie and cream-burdened pastries. I cannot eat any of it. And when I explain to the poor chef, who has worked so hard, why this should be he looks at me with a mixture of pity and disgust. Too right mate. I feel the same way. I accept a glass of orange juice and try to look serene.

There's another problem that's bugging me: finding something to eat when I am outside the house. In the world of the cheese sandwich, the vegan options are few and far between. I realise it is lunchtime and suddenly become anxious. What am I to do? Run into a restaurant and become one of those picky eaters they hate? Demand a vegan option? I'd rather nail my own tongue to the table than do that. The solution? Go ethnic. But of course. I pull up a stool at a branch of Yo! Sushi and give thanks for the Japanese. A few Japanese pickle maki rolls and vegetable dumplings later the job is done. I am fed.

But not entirely. My normal diet is carbohydrate light. I eat a lot of meat, fish and dairy. I eat very little bread, rice or noodles. The situation is now reversed, and all those carbs are making me feel almost fed but not at all satisfied. My body is complaining. Or at the very least, it is making serious adjustments. A work colleague, who is trying to set up a meeting, asks me about my movements over the next few days. 'Decidedly loose,' I say, quickly. 'I've just turned vegan.'

Day four

Damn it all. I just ate some cow. I am such a bad vegan. Mind you, I think it's excusable. At very short notice I have started filming an edition of Channel 4's Dispatches, about the rising cost of food. (It will be screened tomorrow night.) We are shooting a sequence in a Chinese restaurant, about the way the Chinese are becoming more affluent and eating more beef. Consumption has risen 64 per cent in the past 10 years.

I have ordered a bunch of vegan dishes - some lovely crunchy cauliflower, which is deep-fried with salt and chilli, some braised aubergine, a plate of green beans with mustard seed - but there is also a beef dish. I do my thing for the camera, explaining about China's growing beef habit and, to illustrate the point, pick up a cube of meat with my chopsticks. It dangles before my lips. The crew knows I am being vegan. They know there is something forbidden just inches from my lips. There is a hush as I talk. But we all know the sequence doesn't work unless, at the end, I pop the meat into my mouth. And I do. It's delicious. It's the best piece of beef I have ever tasted. Immediately I feel guilty. Something weird has happened to me. I appear to give a damn about being vegan. Furiously I attack all the vegan dishes as if, by eating those to excess, I will atone for my filthy sins of the flesh.

This new-found conviction is a good thing because tonight I have to cook an entirely vegan dinner party. Arguably it is the one time Nikki and Ian will be able to eat round at my house, and I am determined to do them proud. But I also have a rule: I cannot use any ingredients pretending to be from the non-vegan world. No fake cheese, soya milk or bogus meat. The food I serve must just happen to be vegan.

The starter is easy, if labour intensive. It is asparagus season. I marinate armfuls in a mixture of soy, lemon juice and maple syrup, then chargrill them ahead of time on the skillet. There will be bowls of olives and marinated artichokes and a few Kettle Chips because, well, what's not to like about deep-fried potato and salt?

The main course is more of a challenge because I don't want to succumb to the ethnic clichés, but nothing else seems to work. In the end I make a powerful Thai red curry, using a paste that contains no shrimps, and fill it with roast butternut squash and taut little Thai aubergines and coriander. We finish with a huge platter of fresh fruit and a plate of my wife's chocolate-chip biscotti, made to an entirely vegan recipe.

Afterwards, as we sip our vegan wine - many wines are filtered using animal products - Nikki and Ian talk about the strains of their diets: how they have to plan holidays around what they eat, how they sometimes take ingredients with them when they go away. They tell me not feel too bad about the milk-powder thing, because it's happened to them, too. 'I try to be realistic,' Ian says. 'I just try to do the best I can.' I am trying to do the same; I don't mention the beef episode.

Certainly the true vegan lifestyle sounds exhausting. Actually scratch that. I am no longer merely a spectator. I know for a fact that veganism is exhausting. This dinner party took more work than any other I have ever cooked. I am knackered.

Day five

It is Sunday and, coincidentally, the restaurant I have reviewed for this newspaper is a smartypants vegan joint called Saf in London's Shoreditch, which makes a weird version of cheese out of nuts tortured to within an inch of their lives, and where a plate of mushed-up, stacked vegetables is called a lasagne. It is nothing of the sort. In the review I say that I am pleased vegans have somewhere funky to go. 'Then again,' I write, 'I'm also glad fully grown men who like to wear nappies and pretend to be babies have clubs they can go to indulge their desires, too. It doesn't mean I want to be a member.'

Predictably my email inbox is filling up with outrage and bile from furious vegans, who do not know I am one of them. Frankly, I am amazed they have the energy to type. This morning I barely have the energy to read. My diet is making me feel awful. I phone Heston Blumenthal of the Fat Duck for some insight. I tell him what I am doing and there is a long hiss of breath from the end of the phone. He has known me quite a while, and understands my attachment to animals, particularly dead cooked ones.

I want to know why the food I have been eating has been so much less satisfying. He tells me it's because it's low in fat. 'Flavour molecules dissolve in fat,' he says, 'so you're not getting the flavour hit you're used to.' I shouldn't feel bad about this, he says. 'We are genetically hard-wired to crave fat,' he says, 'because it's a source of energy.' Going vegan, doing it properly, is therefore not something you can embark upon casually. Very quickly you will move on from simple matters of taste - what you like to eat and what you are willing to deny yourself - to something much more elemental: what your body thinks it needs. I realise now that going vegan takes serious bloody willpower. It takes more than a magazine commission. It takes commitment.

I do not have it. I know this for sure when, at the gym, I weigh myself and discover that for the first time in six months of weight loss, my weight has gone up. All that carbohydrate has shoved me up a kilo. It is the last straw. I am tired. I am grouchy. I am out of ideas. And now, to add insult to injury, I am getting fat. I was meant to be vegan for a week. Then again, isn't five days a working week? Won't that do? I decide it will have to. My wife asks me what I'm planning for my vegan dinner that evening. I say, 'Spare ribs and chorizo from the Portuguese grill house up the road.'

'Isn't that on the slightly liberal side of the vegan regime?'

'It was just a phase I was going through,' I say, trying to sound nonchalant. Pat says she's relieved. And, by God, so am I.