It is all Martine McCutcheon's fault. This time last year, the "blood group diet" was emphatically a fringe practice - popular enough on the wilder shores of American food-faddery, perhaps, but strictly non-mainstream here. There wasn't, after all, much medical backing for the central claim of the diet's originator - a bow-tied doctor from Stamford, Connecticut called Peter D'Adamo - that your blood type holds the key to your optimum diet: that eating only those foods suited to your blood would, as D'Adamo hyperbolically put it, "lead you back to the essential truths that live in every cell of your body and link you to your historical, evolutionary ancestry."
But that was before the EastEnders star went public with her enthusiasm for the diet which, she announced, had made her "look and feel totally different". Elizabeth Hurley and Hugh Grant were soon reported to have followed suit, and D'Adamo's book - published in the UK as The Eat Right Diet - has enjoyed sustained success in the bestseller charts since.
In some parts of the world - especially Japan - one's membership of blood group O, A, B or AB has long held a roughly similar status to one's star sign, a piece of information treated as essential by dating agencies and even by some recruiters. But D'Adamo (a naturopathic doctor, as opposed to a medical one) broke new ground in proposing a scientific mechanism to explain why blood type - previously of medical interest mainly for matching the donors and recipients of blood transfusions - could hold the key to nutritional health.
The theory is bewitchingly simple. Each of the four blood types - characterised by the presence or absence of certain molecules, called antigens, on the surface of red blood cells - emerged at different stages of human evolution and so, D'Adamo argues, members of each group should stick to the foods and exercise styles prevalent at that time. Type O emerged 50,000 years ago, when hunting and gathering were dominant, so today's type Os, "the hunters", are best suited to a meat-heavy diet, low in dairy products, and to intense exercise. Type As, "farmers", whose blood group dates from the dawn of agriculture, should be largely vegetarian, while type B, the "nomad", dating from times of mass migration, is suited to most foods except wheat, shellfish, chicken and some pulses. (D'Adamo never really gets to the bottom of AB, the rarest blood group, which he labels "the enigma" and on which he places the fewest dietary restrictions.) In Britain, most of us are hunters: 45% of the population is type 0, 43% type A, 9% type B and just 3% type AB.
Too much of the wrong foods for your blood type, D'Adamo argues, and lectins - a type of protein - in the food will cause clotting reactions in the blood, leading to obesity and numerous illnesses including irritable bowel syndrome and cirrhosis of the liver. But with enough of the right foods, weight loss is pretty much inevitable: "I'd dare to say," D'Adamo writes of the first type Os, "that there wasn't an overweight Cro-Magnon on the planet!"
The popularity of the theory - like any that postulates a genetic, rather than environmental, cause for obesity - is partly due to the fact that it explains why certain diets work for some people and not others, without accusing the failures of being cheats guzzling chocolate when nobody's looking. Unsurprisingly, the theory is proving a hit with food manufacturers too - already, a product line of teas matched to blood groups, entitled Sip Right 4 Your Type, has been launched by the California-based company The Republic of Tea.
But D'Adamo's theories have drawn aggressive fire, not just from rival dieticians - who often have vested interests or who advocate vegetarianism and take exception to his anti-vegetarian recommendations for type Os - but from mainstream medicine as well. Most nutritionists simply refuse to accept the basic mechanism at the heart of the theory.
"It's cobblers," says Professor Tom Sanders, head of the department of nutrition and dietetics at King's College, London. "There's no phsyiological reason to believe that energy balance should be altered by the type of antigens your blood has on it - the idea is laughable. But because most people don't understand science, you can get away with it: it appeals because it's a pseudo-scientific explanation for why some people seem able to maintain their weight and others need only to look at a cream bun to put on weight."
One of the theory's key flaws is that it seems unable to account for the massive rise in obesity in recent decades to today's levels (in Britain, one in five women and one in six men is obese, while a further 45% of men and 33% of women are overweight). Needless to say, what people were eating in the years before this increase began did not resemble D'Adamo's prescriptions.
In any case, critics argue, blood type simply does not have the significance that D'Adamo ascribes to it. First discovered in the early 1900s by the Austrian physician Karl Landsteiner, the ABO system for distinguishing blood cells has since been complemented by a huge number of alternative classifications. While there is evidence that a small number of conditions are correlated to blood type - Type As seem to be at a significantly increased risk of stomach cancer - such examples are far too rare to lend support to D'Adamo's theory.
Tim Gorski MD, a US doctor and contributor to publications advocating a sceptical stance towards alternative therapies, calls D'Adamo's theory "awesome nonsense... people who would scoff at proposal that a car's paint colour determines what sort of gasoline or oil should be used for the vehicle, or that a home's exterior brickwork determines how the furniture inside should be arranged are nevertheless apparently willing to consider this particular delusion." (D'Adamo was unavailable for comment this week, his personal assistant explained, the popularity of the blood type diet meaning his diary was completely full for some time.)
Advocates of the diet do not deny that other factors, both genetic and environmental, have an impact on obesity. The blood type diet is, D'Adamo insists, "not a panacea": it might work for you and it might not, and if it doesn't that doesn't mean the theory is wrong. Whatever the latest weight-loss theory, it seems, having your cake and eating it is still the preferred option for dieticians.
• The easiest way to discover your blood type is to donate blood. Contact the National Blood Service (0345 711711). The Eat Right Diet is published by Century Books, price £6.99.
