Is it possible that your blood group dictates your dietary type - that group As should avoid meat, for instance, while Os are natural carnivores? J
Do you know what your blood group is? Probably not, unless you are a blood donor. It is something that most of us bother about only on the way to casualty - unlike the Japanese, who all know their blood group because it is supposed to indicate their personality. They swap blood group details as we exchange astrological signs. But blood groups could tell us a lot more. The answer to many questions about health and diet may turn out to be written in blood.

In the last few months an American best-seller, Eat Right 4 Your Type, has had hard-boiled executives checking their blood groups, only half in jest. Phyllis Grann, head of the book's publishers, Putnam, was delighted to learn that her blood type, O, is the carnivorous one, while her two seconds-in-command suffered bruising to their macho egos when they discovered they were type As, who are vegetarians and more co-operative. Ms Grann commented, rubbing salt in the wound: "It makes sense that the people that I interact most closely with are As, instead of strong and ornery, like me."

The intriguing idea is that our blood group, far from being an arcane medical detail, is crucial to the sort of diet we ought to follow and the diseases to which we are vulnerable. Coming after years of advice to eat a low-fat, high-carbohydrate diet, it could put liver and steak back on the menu for some, and cut out dairy products for others.

According to the book's author, Peter D'Adamo, a naturopathic physician with a large practice in Greenwich, Connecticut, A types like him do well on vegetables, but for other people, too many, or the wrong ones, can be a disaster. What Os need, for instance, is the sort of dish that has not been seen on the tables of chic restaurants for years: fatty steaks and protein-rich liver.

It all began with D'Adamo's father (also a naturopathic physician) who had worked in various European spas and noticed that some of his patients did well on vegetarian and low-fat diets, while others in fact got worse. He began blood-typing his patients and discovered that type O patients thrived on meat and protein-heavy diets and aerobic workouts, while type As did better if they ate soy proteins, combined with more gentle physical activities such as yoga and t'ai chi.

D'Adamo confesses to having been sceptical about this until he came across the well-established medical fact that type Os tend to get more ulcers. "They have more acidic stomachs, for digesting all that meat they need," he explains. "Type As, on the other hand, have a lower ulcer rate but a higher rate of stomach cancer, linked to lower levels of stomach acid production."

According to the scenario set out in the book, which is due to be published here later this year, O groups were the first to evolve about 50,000 years ago, when humans were hunter-gatherers and were eating a fairly carnivorous diet. Nowadays Os are found everywhere, but they are especially common among Africans and American Indians.

Next to appear were the As, between 25,000BC and 15,000BC, whose diet was more vegetarian. Today many Japanese and western Europeans are As.

Then came Bs - 15,000BC to 10,000BC - now most common among eastern Europeans, east Indians, northern Chinese and Koreans, who began as cattle herders and are now highly tolerant of dairy products. However, they are at greater risk of bladder and kidney infections because they tend to lack an antibody that gives protection against certain bacterial diseases. The most recent arrivals are the ABs - the "modern diet blood group", according to D'Adamo - who can tolerate a wide range of food types. They appeared in Europe around 900AD.

"It's a very interesting idea," says David Weatherall, a professor of medicine at Oxford University. "We've known for some time that there is a link between blood groups and disease - O and ulcers, for instance - and it does seem likely that a particular group developed because it gave the people who had it protection against various infectious diseases such as malaria. I suppose there could be a link to food; it would be interesting to test it."

Blood groups were first discovered at the end of the last century as a result of research into why some blood transfusions failed. As we now know, you cannot put type A into someone with type B because markers on the surface of the blood cells identify the newcomer as foreign. D'Adamo's book takes this idea further and suggests that certain foods may be rejected in the same way.

The way this works is via markers, known as lectins, that are found on the surface of both body cells and those of plants. Sometimes they are incompatible, and when this happens they clump together and everything works far less efficiently. Bananas, for instance, are highly beneficial for Bs and ABs, neutral for type Os, and to be avoided by As. Long term, the clash between the lectins can lead to the degenerative diseases that are so common in the West.

So far the evidence for the theory comes largely from clinical work with patients, but large-scale studies should not be hard to do. D'Adamo's chronology is certainly open to question: there were plenty of early human vegetarians before 50,000BC, so what happened to them? But putting dates to something that leaves as little trace as changes in diet is hard. The meat-eating American Indians are something of a problem, too, since humans didn't arrive in the Americas until comparatively late - 15,000BC to 12,000BC, long after carnivorous Os developed - and they came from what is now northern China, from people who are now Bs and happy with dairy products.

But, at a practical level, the approach obviously works for some.

"Different foods have different effects depending on your blood type," says D'Adamo. "For instance meat, which is fine for Os, will tend to slow down digestion and metabolism for As, while carbohydrates have the opposite effect." It certainly gives a new twist to losing weightn

Know your blood type

Blood type is O: Eat meat (high protein, low carbohydrate). Cut out wheat and most other grains. Engage in vigorous aerobic exercise. Your risk factors for ulcers and inflammatory diseases such as arthritis increase if you eat incorrectly for your type.

Blood type is A: You should be largely vegetarian (high carbohydrate, low fat). Engage in gentle exercise such as yoga or golf. Meditate to deal with stress. Your risk factors for cancer and heart disease increase if you eat incorrectly.

Blood type B: You should have the most varied diet of all the blood types, one including meat; yours is the only blood type that does well with dairy products. Engage in exercise such as moderate swimming or walking. Your risk for slow-growing viruses that attack the nervous system increases if you eat incorrectly for your type.

Blood type AB: You have most of the benefits and intolerances of types A and B. Engage in calming exercises and relaxation techniques. You have the friendliest immune system.