I am talking penises with Paul Merton. He's telling me about the evening he spent recently in the company of a donkey's giant genitals. No need. I've already seen the evidence. It was a huge brown thing as long as your forearm and presented to him roasted on a bed of cucumber and decorative orchids. Then it was whisked away, delicately spiced and sliced, and served up in a creamy, bubbling soup. Finally Merton put it in his mouth.

You'll be wanting to know - did he spit or swallow? The answer is that, after chewing for a few seconds, Merton ate the donkey penis, wincing only slightly. 'Mmm, that's good,' he said politely. Then, less politely: 'When I say it's good, I've no intention of ever eating it again in my life.'

Merton had been taken to a Beijing restaurant that specialises in penis cuisine. As you'll already know if you saw the first instalment of Paul Merton in China on Channel Five on Monday, he also sampled the testicles and forked penis of a snake and a sheep's penis that had 'the texture of knotted rope'. It was the latter that beat him.

'The donkey was all right,' he recalls. 'If you hadn't known what it was, and it was just slices on a plate, you could have mistaken it for lamb or beef. It was not a flavour I was familiar with but it was in that kind of area. But the sheep one was so intensely chewy, there was no taste in it and the texture was just horrible.' He spat it out. It was, in the words of Merton in the programme, a case of 'too many cocks spoil the broth'.

Food is a recurring theme in the four-part travelogue documentary. For the record, he turned his nose up at the idea of eating cicada, scorpion, sheep penis and dog but was happy to swallow donkey penis, snake penis and an inventive nomadic Tibetan dish that involves cooking lamb in its own stomach without the use of any stove or oven (the trick is to include red hot stones as part of the stuffing and to sew the stomach up). All rather more daring than the dim sum we're eating at Shanghai Blues, one of Merton's favourite Chinese restaurants, in London's Holborn.

If Merton, 50 this year, is now a relatively adventurous eater, this is a new departure. As we shall discover, he was not someone who grew up with a love of food. For six years, he lived on his own in depressing bedsits with almost no cooking facilities (he lived in one that only had a kettle). He was in his twenties before he learnt that people put their knife and fork side by side to signify that they have finished eating, and he had never been to a dinner party until he was married and in his thirties. Yet today he avoids too much gluten in his diet and believes in the power of 'superfood' vegetables to fight disease.

Meanwhile, back to China. The documentary - an entertaining attempt to make sense of a nation transforming itself from a third-world country to a superpower-in-waiting - contains a fascinating scene where Merton and his camera crew visit an area that specialises in restaurants serving dog meat. To film the sequence, Merton and his cameraman posed as holidaymakers. Diners, delighted and amazed to see European visitors to Dog Street, as the area is colloquially known, smiled and waved, merrily pointing to the food on their plates. 'They didn't know we were a Western film crew because we had a very small camera that looked like it might have been a home movie thing,' explains Merton.

If the restaurant-goers were pleased to see him, the Chinese government won't be. Merton's team had been refused permission to visit Dog Street and he went behind the back of the official minder who'd been assigned to accompany him for his six-week trek across the country.

'They didn't want us to go there because they didn't want us to put over that old-fashioned idea of China. But it's not an old-fashioned idea, because dog is a famine cuisine - there's a saying that the only thing with four legs that the Chinese won't eat is a table.' He is filmed discussing the issue with an obviously educated woman who speaks perfect English. 'And she did not know the phrase "pet dog". In a culture where you haven't got much food why would you have an animal that you have to keep alive with the food you could be eating?'

This time around, once seated at his table, Merton's bravery deserted him. He refused a mouthful of fried 'brown dog' - no one could tell him quite what breed he'd been served up - when the dish arrived. Now that he's had time to think through his decision, I ask him why someone who can swallow donkey penis suddenly came over squeamish.

A little earlier that day, he says, he'd filmed a sequence about how women from China's newly emerging middle class spend their new-found wealth by pampering their (ludicrously pompadoured) poodles. Eating what was on his plate for dinner 'might have been a little bit easier if I hadn't been walking dogs that day'. So he doesn't think eating such meat is inherently wrong, then? Not for a people in their circumstances, no: 'I'm not a vegetarian so it'd be hypocritical to go on as if dogs are in any way different to cows or sheep. A piece of meat is a piece of meat in many respects. I wasn't trying to impose my standards on them.'

So if a dog restaurant opened up tomorrow in Streatham? 'It wouldn't bother me that other people eat dog. It's up to them.' Then another reason for his on-screen refusal begins to emerge. 'There's also the practical side,' he says. 'If you start eating dog on TV you get very angry letters from people.'

So that was part of the calculation - that there may be a campaign against you if you're seen eating one of Lassie's friends? 'Well, maybe,' he says, sounding unsure. He smiles. 'By talking about this I don't want to be targeted as the man who nearly ate dog. Nearly is the key thing.' Big laugh. 'I know how people feel about it - some British people are completely potty about animals, they place them above human beings, so I really don't want to start tangling with people who ...' he hunts for the right words, 'may not have my best health as their number-one priority. I think you know what I am saying.' I do. It's time to move on.

Merton, the son of a Tube train driver and an NHS nurse, grew up as Paul Martin (the name change came later, when he wanted to join Equity) in Robert Owen House, a low-rise block of council flats in Fulham. It was a church-going Catholic household: every Sunday there was a wafer at communion and a traditional roast afterwards. But much of the food served up by Merton's mother in the Fifties and Sixties was drab - and often from the freezer compartment of the fridge. There is no sense of Merton, now a multimillionaire of course, looking down his nose when he describes what his mother gave the family, but equally no attempt to romanticise a diet that appears pretty unappealing from this distance - and didn't do a great deal to excite him back then.

'The food wasn't adventurous,' he says. This was food as fuel, not a source of pleasure. 'First of all, lunch was called dinner and dinner was called tea,' he says. 'I remember fish fingers, rissoles - stuff that Bird's Eye started to sell in the late Fifties, all that instant food. Something out of a packet. I remember when the Vesta curry came out - my first awareness of it was sometime in the mid-Sixties. That seemed extremely exotic.'

Schoolfriends never came round for food - nor did any of his parents' pals. 'Nobody ever came to our place to eat and we never went to anybody else's house to eat. You wouldn't think of doing it. People would have thought: "Why would we come to you to eat? Are you saying we don't have good food at our house?"'. Such thinking may also be 'tied in with the fact that the food wasn't particularly good, so why would we invite people round to have potatoes, peas and meat pie when that's probably what they're having anyway?'

The sentiment has, to some extent, stuck with him all these years later. Does he ever throw dinner parties now? 'No, I don't. This goes back to a class thing.' Instead, he'll invite friends round to his house for a game of snooker or to watch a film. The first dinner party he went to was 'probably when I hosted them, when I was married to Caroline [Quentin]. She is solidly middle class. I must have been 34 or 35.'

People will look at the quick-witted urbane performer on television and be amazed that he was such a late (and reluctant) arrival at the dinner-party scene. t middle class - though I'm not sure I am - but equally the working-class stuff hasn't just disappeared. It's there in your background, how you are and how you relax. 'The class thing is interesting. I'm not sure I'm no I went to a BBC dinner party a few years ago which was thrown by John Birt when he was BBC director general and he did this thing where, after each course, you get up and move along a place.'

There's something similar that sometimes happens at Oxbridge high table, I say.

'Oh, like passing the port to the left? All these rules that you don't know. Even simple things like, in the early days of going into restaurants, not knowing that the knife and fork together is a signal that says "take away my plate".'

You didn't know that?

'No.'

You didn't do that at home?

'No. You just left them there. You saw the plate was empty. I'd never heard of the notion that putting the two together to signify that you have finished.'

When Merton found out, he 'wasn't embarrassed about the fact that I didn't know but I thought, "That sounds like a good idea".' So at what stage did he discover the knife and fork trick? 'When I first started going into restaurants. I don't want to make myself sound like a complete ignoramus but I was probably in my twenties. Seriously. I know it probably seems really retarded but I never went into that world.'

Again, this gap in his knowledge is likely to surprise people, I suggest.

'I suppose so. It's one of those quirks. It's one of those things that working-class people don't necessarily know. It doesn't come into the area of manners, does it? We understand manners, we know that you don't sit with your elbows on the table, you don't spit, that's obvious. But you can be from a working-class background and not know the etiquette of putting your knife and fork together because nobody ever talks about it.'

The gap in knowledge is also likely to be another product of the fact that it is only since he has had money that Merton has allowed food to become a source of pleasure. He says: 'I can remember thinking when I was about 19 or 20 that food was a bit of a boring thing that you just had to do to keep going. And if you could've taken a little tablet - as, you know, the astronauts will have one day - to fill you up, I would have gone for that method.'

By that time, Merton had a steady but uninspiring job as a civil servant. He'd wanted to be a comedian since he was a child, and realised that his career, such as it was, at the Tooting employment office had left him wishing his life away.

'I know what it's like getting up on a Monday morning, walking in through the office door at five to nine, looking up at the clock and thinking: "The next time I look at you with a sense of satisfaction it'll be five o'clock on Friday."'

So he chucked his job in. He gave himself a five-year 'apprenticeship' to make it in showbusiness. But the lack of money meant he had to move into a single rented room. It was 1982 - and Merton was to remain in bedsitland until just two years before he joined Have I Got News For You and achieved fame.

'I was in bedsits from early 1982 through to 1988, I think. A long time. At first, I was earning virtually no money. I was signing on, doing occasional gigs for maybe £20 each. I did 15 or 20 gigs in that first year.

'In the first bedsit, I had no facilities at all. The only thing I had to cook with was an electric kettle. So I would make porridge by pouring hot water onto a bowl of oats. If you haven't got much money, porridge is one of the cheapest and best things you can eat because it keeps you going, gives you energy.' If he was feeling a little less virtuous, Pot Noodles - still relatively new - were another food source. Or, 'I'd go to the supermarket and buy a loaf of Mother's Pride bread, which I think was something like 25p at the time, and a pot of fish paste and eat that. Or a bit of Bovril, something with a bit of flavour to it.'

If you think this makes him sound miserable, you've got it wrong, he says - and not merely because he managed to escape (and in some style: he has homes in Islington and Sussex). 'This isn't a hard-luck story and I don't want it to sound like one, because I was freeing myself up to write stuff and pursue a career in comedy.'

As a child, he'd often retreat to his bedroom to listen to his records, 'so as to be in one room on my own wasn't a massive culture shock for me'. The loneliness was 'kind of what I was used to - being sent to Coventry at school, not really having lots of friends. I know I won't have to, but I could go back to that environment again.'

Reading these words on paper, this sounds a little morose. But that is not at all the sense in which it was delivered. Though Merton does not, thank God, feel the need to pep up each turn of the conversation with a wisecrack, lunch with him is a jolly and enjoyable affair. It involves a lot of laughter - there's a little of Sid James in Merton's cackle. And he seems proud to have been a 'bedsit boy', a phrase he uses several times about himself. 'I think success and happiness in life is about contrast in many ways,' he explains.

His life has certainly dished up those contrasts. Four years ago, he had to deal with a very cruel blow. His second wife, Sarah, died in his arms. She'd been suffering from breast cancer.

They approached her illness together and with a belief in the healing effects of diet (albeit working alongside conventional drugs). A few months before Sarah died, he told an interviewer: 'We juice two kilograms of carrots every day - they are exceptionally good for the immune system and very versatile: carrot and apple is beautiful, carrot and pineapple is wonderful ... There are things you can do: diet, mental attitude, spiritual attitude, all these things are very important.'

Today, despite the loss of his wife, he sticks by that conviction. 'You get that terrible diagnosis, the prognosis. So what can you do? Well, a large percentage of cancers are caused by poor diet - it's not just smoking and asbestos and stress - so you look to do things that can boost the immune system as much as possible. Standard doctors don't know anything about nutrition, so some of them tend to pooh-pooh this sort of thing as faddy diets. But the truth of the matter is that if you're juicing a lot, OK it's not going to cure the cancer, but you feel great. And you want to help your body as much as possible.

'I'm completely different to how I was when I was 19. Now I seek out purple sprouting broccoli' - he utters the words as if they are obscure Latin labels in a medical textbook - 'it's fantastically good stuff. And it's tasty as well. You look at what the superfoods are - cauliflower, broccoli - these things have been shown to fight cancer, or at least to help your immune system build up an immunity to it. And we know that if we eat really good food we feel great. If we eat chips, we feel sluggish.'

Yet that new respect for food and his enjoyment of it cannot entirely rub out the effects of all those years that taught him that spending time in the kitchen was anything but a pleasure.

He reflects on where he is today. 'I'm not living in a bedsit, and I've got more than 21p to spend on bread and fish-paste sandwiches, but I still don't cook at home. I've got very little cooking equipment. But,' he adds, 'there are hundreds of restaurants nearby. If you've got £50 in your pocket, you can go into a restaurant and have lunch, so I do that.'

· Paul Merton in China continues tomorrow on C5. Shanghai Blues, High Holborn, London WC1, 020 7404 1668, www.shanghaiblues.co.uk

· This article was amended on June 4 2007. Vespa makes scooters, not curries. We meant Vesta. This has been amended.