It's an image familiar to every TV viewer - Anthony Hicks, 58, real-life front man of the Government's Quit Smoking campaign, propped in a hospital bed with his voicebox removed, eyes sunken, skin the colour of tobacco smoke, croaking intermittently about plans to spend Christmas with his daughter. Ten days later, the caption tells us, Anthony is dead - killed by cancer of the lung and larynx, a compliant victim of his own smoking habit.

There is no ambiguity in the NHS message - and those of us who don't smoke breathe a sigh of relief (because we can). We walk to the kitchen, take a ready-made lasagne from the fridge, place it in the microwave; if we're feeling generous to ourselves, we might fry a fillet steak in a little butter, eat it (guiltily) with a plateful of oven chips and treat ourselves to a can of lager and a tub of chocolate chip ice cream.

Unwittingly, we are placing ourselves in the same high-risk category as Anthony - victims not of cigarette smoke but of our Western diet. In its World Cancer Report, published last year, the World Health Organisation (WHO) quietly dropped the bombshell that 30 per cent of cancers in the West can be attributed to dietary factors - placing food second only to tobacco as a preventable cause of cancer. If we all improved our diet overnight (dream on), 2.1 million lives a year could be saved. Instead of 20 million people worldwide suffering from cancer, the figure could be 14 million (and a further half of those could be avoided by also banning smoking). Of the 10 million new cases every year, three million could be prevented by eating wisely.

What's more, as Western-style diets take hold in developing countries, they too are at increased risk of cancer. The evidence, says Sara Hiom, head of health information at Cancer Research UK, is in various migration studies conducted with Japanese women, who are prone to far lower rates of breast cancer than those in the West. 'Within one generation of migrating to America,' Hiom explains, 'their breast cancer risk was that of the indigenous population of America.'

This and other environmental factors have put us in the grip of a global cancer epidemic. By 2020, new cases are expected to rise from 10 million to 15 million a year. Within 20 years, the number of people living with cancer is projected to increase from 20 million to 30 million. All this could be prevented, the WHO implies, by giving up our drip-feed of fast, processed and convenience foods which are high in fat, sugar and salt.

Even if we consume none of these, we cannot feel smug. As we unpack our organic boxes and make our Saturday-morning forays to farmers' markets in search of free-range poultry, low-fat ostrich steaks and wholemeal bread, another danger lurks in the drinks cabinet. After obesity (cited by Cancer Research UK as an 'absolutely cast-iron' cause of the illness), excess alcohol consumption - defined by the WHO as 'more than two units a day' - is the biggest risk factor in cancers of the oral cavity, pharynx, larynx, oesophagus, liver and breast. Anthony Hicks, heavy smoker, RIP.

So there we were worrying about our children's heart disease, stroke and diabetes as a result of 'the obesity epidemic', when all along cancer was a cast-iron risk. But what can we feed our ill-fated progeny to prevent it?

In our unhealthy but strangely health-obsessed world, there is no shortage of suggestions. Broccoli is often cited by nutritionists and health writers as a kind of wonder vegetable because it contains vitamins A, C and E - a hat trick of antioxidants, compounds that 'scavenge' free radicals (oxygen particles that can damage body cells and possibly cause cancer). Butternut squash and sweet potato also score three out of three and, like most fruits and vegetables pigmented orange and yellow, are sources of beta-carotene - one of a group of antioxidants known as carotenoids. In fact, the body converts beta-carotene into vitamin A, which is found in green leafy vegetables among other things.

Another carotenoid with potential is lycopene, which is present in tomatoes. Some studies have suggested it may reduce the risk of prostate cancer. Tomatoes contain vitamin A and vitamin C as well, making them a doubly valuable addition to the arsenal. If your children can't stand fresh tomatoes, though, don't worry: several studies have shown that the processing of tomatoes makes lycopene more readily available for the body to use, so ketchup, tomato sauce and pizza topping (which make up three-quarters of the lycopene intake of Americans) are just as likely to prevent cancer.

Red peppers are excellent for vitamins A and C, while selenium (a mineral) is present in many plants, including broccoli, beets, cabbage and garlic as well as nuts, offal, fish, eggs and poultry. A review study in 2001 reported that selenium, an essential trace element, is necessary for the functioning of an enzyme that protects against oxidative damage to cell structures. In animals, it reduced the frequency of transplanted tumours and may provide protection in the later stages of human cancer.

Several other plant compounds (phytochemicals) have shown promise in the fight against cancer, albeit in limited trials in the laboratory and in animals. Glucosinolates (from cruciferous vegetables such as broccoli, Brussels sprouts, cabbage and pak choi) confer various health benefits while indole-3-carbinole, another constituent of these vegetables, has been shown to prevent colon cancer in mouse models. Flavonoids are another promising group. Quercetin (found in apples, onions, tea and red wine) blocks the hormone activity in human prostate cancer cells, while allicin (the main ingredient in crushed garlic) has been shown to inhibit the proliferation of human cancer cells in the colon, endometrium and breast.

In May, a study by Professor Ian Johnson for the Institute of Food Research found that a substance called allyl-isothiocyanate (AITC) stops colon cancer cells dividing relentlessly - the mechanism that causes tumours. AITC - a breakdown compound of sinigrin (found in brassicas such as broccoli, mustard, cabbage, horseradish, cauliflower, kale and wasabi) - is produced when particular vegetables are chopped, chewed, cooked or digested. Unlike chemotherapy drugs, it appears to target tumour cells without damaging healthy cells.

Despite this growing body of research, the World Health Organisation offers no advice about specific plant foods in its 2003 report. It simply lists fruits and vegetables - generally - as one dietary factor that may decrease the risk of cancer of the oral cavity, stomach and colorectum. Eat plenty of these (as traditional hunter-gatherers did, consuming more than 800 different plant-based foods on a regular basis) and you may be protected. It is what is lacking in our food - the vitamins, minerals, fibre, phytochemicals and micro-nutrients in a plant-based diet - that counts.

But surely working out what we need to eat to boost our chances can't be rocket science. If we know there are lower incidences of cancer in certain parts of the world, such as Japan and rural China, we can monitor what those populations are eating and identify what it is that we are doing wrong.

At the Oxford University laboratories of Cancer Research UK, principal scientist Professor Tim Key is doing precisely that. 'All our work here - and cancer epidemiology everywhere - is based on that premise,' he says. 'The reason we bother doing our work is that cancer rates vary around the world and have also varied with time in some countries - so it's clearly not genetics that determine it but something about lifestyle. Diet is one of the top candidates on the list.'

With such a bleak epidemic looming, the subject is being researched with a vengeance. In one project, Epic (the European Prospective Investigation into Cancer and Nutrition), scientists are analysing the food questionnaires of half a million people in 10 European countries to shed light on the links between diet and long-term health. The results are due in the next few years. Coordinated by the WHO and supported by the European Union, Epic is the largest such study ever undertaken - and Professor Tim Key is principal investigator of the Oxford cohort, a group of 65,000 human guinea pigs .

If anyone can help devise a cancer-busting diet, it is Professor Key - but his suggestions are conservative to say the least. The risk inherent in obesity is 'absolutely cast-iron and confirmed to be true', he says. 'For breast cancer in menopausal women in particular, obesity increases blood levels of oestrogen - and oestrogen is directly linked to breast cancer risk. The link with alcohol is also confirmed, so keep your weight down and don't drink too much.'

Eating lots of fruit and vegetables (as the WHO suggests) is 'only a probable', he says, 'and for no specific [plant] food is there convincing evidence that it's important'. Red meat is under 'serious investigation', he adds, because 'there is quite a lot of evidence that high intakes of meat - and it's stronger for processed and preserved meat than for fresh meat - may increase the risk of large bowel cancer. It's a hot topic of research.'

And what about boosting soya intake and cutting down on dairy products, like the rural Chinese who are less at risk from breast cancer? Of the first, he says the evidence 'has not been very strong' while the second is 'an interesting area'. I mention Professor Jane Plant's book Your Life in Your Hands, in which she puts forward a persuasive argument that giving up dairy produce helped her survive five diagnoses of breast cancer, but Professor Key is not convinced. 'In less developed countries where there isn't much food around,' he says, 'women start menstruating at a late age, they have several children and breastfeed them for a long time. These factors are all known to reduce breast cancer risk. Worldwide variation is not just to do with diet.'

It isn't the Holy Grail I am looking for, and I begin to wonder about all those cancer-busting carotenoids, flavonoids and phytochemicals cited by health writers and nutritionists. Isn't there a shred of truth in it? 'Everyone wants to know which fruits and vegetables they should be eating,' Professor Key acknowledges. 'Is it brassicas, is it garlic? None of these things are sorted out. Reports in the media and in scientific journals are usually based on a single study which may identify an interesting effect in a laboratory - but that doesn't mean it will work in people. The only scientifically based advice we can give is: 'Eat a variety of fruits and vegetables, and base your choice on the ones you like and can afford.'

It's good advice, but no good at all to those of us who are indecisive in the supermarket aisle (or who never look at price labels). But in California, naturally, the selection of nutritionally perfect fruit and veg has been perfected as a science. David Heber, MD, a scientist at the UCLA Center for Human Nutrition in Los Angeles, has devised a formula to help consumers with the weekly shop. In his review article 'Vegetables, fruits and phytoestrogens in the prevention of disease' (published in the Journal of Postgraduate Medicine ), he divides foods into seven colour categories and suggests we eat one type from each group every day - red (tomatoes and related products), red/purple (berries, grapes, red wine), orange (carrots, mangoes, pumpkin), orange/yellow (cantaloupe, peaches, oranges, papaya), yellow/green (spinach, avocado, honeydew), green (broccoli, cabbage, cauliflower) and white/green (leeks, onion, garlic, chives).

Apart from the red wine (presumably less than two glasses a day), it is not so different from the superfoods programme outlined by Dr Steven Pratt, a Californian plastic surgeon, in his book, Superfoods: Fourteen Foods That Will Change Your Life. It recommends a diet of 14 colourful foods (such as broccoli, tomatoes, pumpkin, soy, spinach, wild salmon, oranges and black tea) containing the range of nutrients we need. Though mainly for weight loss, the diet 'can change the course of your biochemistry', Dr Pratt claims. 'These foods can help stop the cellular damage that can develop into disease,' he says - an optimism shared by Katherine Tallmadge of the British Dietetic Association. 'The reasoning sounds good,' she says. 'These are the kinds of healthy wholefoods we should be eating.'

The truth is, few of us will heed this advice, somehow believing that cancer will strike elsewhere - and in the distant future. We have organic muesli for breakfast, we eat up our greens (once or twice a week). Why should it happen to us? For one group of people, however, the link between cancer and diet is real. Already diagnosed, often with weeks or months to live, they will seize dietary advice with both hands because it may prevent secondary tumours occurring and brace their immune systems for the toxic onslaught of orthodox chemotherapy.

Michele Eve, 40, a mother of three living in Bristol, was diagnosed with breast cancer four years ago this month and immediately went on a cancer-busting diet. She was not prepared to wait for the randomised, double-blind placebo trials of over-cautious scientists to tell her what she should be eating.

'I was in a bookshop,' she says, 'and the first book I picked up was about the Bristol Cancer Help Centre. I just feel so lucky that it's in my home town.'

While undergoing chemotherapy ('I was very ill indeed,' she says) and eventually surgery, Michele adopted the lifestyle changes recommended by the holistic charity. In addition to counselling and complementary therapies (including shiatsu, meditation and spiritual healing), these involved switching to a plant-based, wholefood diet with lots of fruit and vegetables, pulses and whole grains but very little meat and dairy produce. 'I gave up fish as well and went completely vegan,' she says, 'though that was my choice.'

Four years later (and just one year short of being medically 'clear'), Michele is completely well - something she attributes both to orthodox treatment and the Bristol approach. 'When they took the lump out, the specialist couldn't believe it,' she says. 'All the cancer cells had gone - which is very unusual. He seemed amazed, but I wasn't. I think the diet helped me fight the cancer and it was the one thing I could do something about immediately when everything else was up in the air. When given a 50:50 chance, you ask yourself "Which half am I going to be in, then?" You can make a choice.'

On a damp, doleful September day, I make my way to Bristol and find the reception area of the Bristol Cancer Help Centre suffused with natural light. There is something about the tall windows, high ceilings and minimalist decor of the former convent that makes it seem sunnier indoors than out - and when I peer into the relaxation room, where participants in a five-day residential course are sharing their experiences of cancer, I notice they are sitting not on polypropylene chairs (standard issue in institutions) but reclining sun loungers arranged like radiating spokes around a nutrition therapist.

'Would you like a drink?' asks Clare Ben jamin, who has invited me here to find out about Michele Eve's diet.

'A coffee would be nice,' I say, but the request is met with an apologetic smile and a tray of herbal teas.

At once I am reminded of the lack of consensus when it comes to treating cancer with diet. In the best-known and most controversial approach, Gerson therapy (invented by German-born doctor Max Gerson in the Thirties), coffee is the very thing that is believed to work - albeit administered by enema rather than bone-china cup. When combined with a strict nutritional regime - principally 13 glasses of fruit juice a day and injections of vitamin B12 and liver extract - coffee is said to dilate the bile ducts in the liver and help it excrete cancer-causing toxins.

In June, Prince Charles provoked a venomous attack by the medical establishment for suggesting Gerson therapy had worked for a cancer sufferer he knew and merited further research. 'I have no time at all for "alternative" therapy that places itself above the laws of evidence,' blustered Professor Michael Baum of University College London. 'The power of my authority comes with a knowledge built on 40 years of study and 25 years of active involvement in cancer. Your power and authority rest on an accident of birth.'

Professor Karol Sikora, Britain's most eminent cancer specialist, was equally damning of the diet, saying there was no rationale for it. 'Why would a coffee enema work?' he asked. 'It's popular at the higher end of the middle classes, partly because it's expensive and there's an element of religious mania to it. But the idea that huge amounts of vitamin C can cure you of cancer is wrong.'

Yet here at the Bristol Cancer Help Centre (where sufferers are urged to take supplements of 500-2,000mg of vitamin C a day, in addition to seven or more portions of fruit and vegetables), Professor Sikora is an ally. Visit the centre's website and there he is in his lilac shirt, clashing tie and serious spectacles, saying: 'Bristol represents the gold standard for complementary care in cancer.' Climbing the stairs, I pause on the landing beneath a portrait of HRH, the centre's patron. Somehow, the Bristol Approach satisfies both sides in a polarised and very angry debate. When I ask even the most conservative cancer specialists about the centre's dietary advice, nobody is prepared to knock it.

'Well, we're not exactly making people eat live lizards,' says Jane Sen, the professional chef and cookery writer who helped devise the Bristol guidelines. Looking at our lunch menu, chalked on a blackboard in the award-winning restaurant (the centre was named Caterer of the Year in 2000 by BBC Radio 4's Food programme), I can see what she means. I'm not a great fan of soybean curd, but Malay-spiced tofu with aubergine, mustard-seed rice, sesame stir-fried broccoli, red pepper, spinach and sprout salad doesn't sound bad, even when lightly drizzled with peanut vinaigrette. I ask Jane Sen what is so cancer-busting about it.

'The colour is important,' she insists, 'because a colourful plateful [green broccoli, bright red peppers] means you are eating a variety of ingredients and getting all the plant nutrients you need. Make sure you have something raw in every meal [like the shredded spinach in this one] because raw food still has all its vital energy.' Sprouting seeds should be consumed three times a week, she says, because they are 'the freshest things you can eat', with a raised nutritional value. 'Think of all the energy needed for a tiny thing like that to grow to such a height.'

Somehow, it sounds right - but it's hardly Professor Baum's kind of science. What I am after is a forensic breakdown of the meal's active ingredients. How does each one fight or prevent cancer - and are any of the alleged benefits scientifically plausible? In its factsheet 'The scientific basis for the Bristol approach to cancer care', the centre offers a few clues. Soya products (such as the Malay-spiced tofu I am eating) are listed along with six other foods and micro-nutrients that are 'known to be actively beneficial in cancer prevention and reduced recurrence': they are cruciferous vegetables (such as broc coli), tomatoes, onions, garlic, omega-3 fish oils and the mineral selenium.

However, when I run this list past Liz Butler, the nutritional therapist at the Bristol Cancer Help Centre, she is more circumspect than the promotional literature. 'Tomatoes are a source of lycopene,' she says, 'but lycopene is not a magic bullet like a drug. There has to be a balance of things in the diet; if you look at one antioxidant in isolation, you're just messing about.' It's the range and variety of micro-nutrients that counts, she explains, and the way these compounds work 'synergistically'. In one study conducted for the Institute of Food Research, due to be published shortly, anti-cancer agents were found to be up to 13 times more effective when several foods in a wholefood diet were combined.

The best approach, say Bristol nutritionists, is to eat lots of plant foods with known benefits for general health (not just those that may protect against cancer), with an emphasis on 'adding foods to the diet' rather than subtracting them - the opposite principle to most diets.

One controversial addition is supplements in capsule form, since most experts say a balanced diet delivers all the vitamins and minerals we need, and the approach has even been linked to an increased risk of cancer. Beta-carotene supplements (which are recommended on the Bristol programme) slightly increased the risks of recurrence in a trial involving people with lung cancer, and another study showed that high vitamin C intake interfered with apoptosis - the natural cell death that stops tumours occurring. 'We recommend supplements because people who are ill, fatigued or busy may not get an ideal diet every day,' Liz Butler explains. 'There is also evidence that minerals in crops are declining due to intensive farming.'

Cancer sufferers are also advised to cut down on foods 'associated with an increased risk of cancer'. These include red meat, dairy products, smoked and salt-cured foods, refined sugars, processed foods, caffeine, alcohol and table salt. Game and poultry should be organic; fish should be non-farmed and from deep unpolluted waters, and oily fish should be small varieties such as sardines and pilchards which carry fewer contaminants. Despite such rigours, hundreds of people passing through the Bristol centre say they have benefited from the guidelines while on courses - and 92 per cent continue to follow them at home.

So, does the Bristol approach - and Jane Sen's diet - stand up to scientific scrutiny? The only study that has ever looked at the outcome of Bristol patients compared to others (published in the Lancet in September 1990) concluded that women with breast cancer who had attended the centre fared worse - though campaigners and scientists later argued that the study was flawed. When I ask if the centre keeps its own records of recovery and mortality rates, the answer is no. However, yet another supportive clarion call comes from Professor Karol Sikura. Patients on the Bristol programme show fewer side effects during chemotherapy, he says - an idea that comes as no surprise to nutritional therapist Liz Butler. 'The rationale behind everything we do is restoring people's biochemical balance,' she says. 'Then, their immune system is running well and they benefit more from their conventional medical treatment.'

In the fight against cancer, then, the message is clear: legitimate science is leading us inexorably towards a vegan diet. Last year, the WHO made its statement about fruit and vegetables. In March, the Food Standards Agency published the results of research it had funded into the link between colorectal cancer and red meat. 'The results support other published studies showing that higher levels of red meat consumption increase the risk,' it concluded.

With dairy products, too, there is mounting evidence that avoiding them may reduce the risk of breast cancer. Though cautious about Professor Jane Plant's reasoning in Your Life in Your Hands (in which she attributes her own recovery to abstinence), Professor Tim Key at the Epic project confirms that part of her argument is correct. 'It relates to a hormone called insulin-like growth factor 1 (or IGF-1),' he says, 'and there is some evidence that if you eat more dairy products, blood levels of this hormone go up. There is also some evidence that women with high blood levels of this hormone are more likely to get breast cancer. So that is quite a tantalising story.'

Three months ago, the Lancet concluded that there probably was an association between animal products and some forms of cancer and further studies will be published soon, Professor Key reports. 'In fact, we've been working on that here and have shown that vegans have low levels of this hormone - and they don't eat any dairy products.' Yet, in his ruthlessly reductionist way, the principal investigator for Epic-Oxford says the link between dairy products and cancer is 'not an established fact'. In terms of advising the public, he concludes, 'the truth is that we don't know. Some studies have shown the reverse. However, if people want to take a gamble and say, "I'll give up dairy products because it might turn out to be true", fair enough.'

It doesn't sound like a gamble to me.

· Bristol Cancer Help Centre helpline: 0845 123 2310; www.bristolcancerhelp.org