Who would have thought that being an A or an O - positive or negative - could have a say in what one must eat or not eat, and the kind of exercise one should focus on? But the fact is that today, following a diet by one's blood group or type is catching on and showing impressive results too.

Kanika Kalia, a 27-year-old documentary filmmaker based in Delhi, is A positive. She's been following a diet prescribed for this blood type for five months now, and is happier being on it. "I started this diet on the advice of my mother's friend in Singapore. I am now about 9 kgs lighter, my energy levels are on an all-time high and I have become a very disciplined eater. I intend to follow this style of eating throughout my life," says Kalia.

Each of us has a certain blood type - identified by particular antigens on the surface of our red blood cells. An individual can be of blood type A, B, AB or O. In addition, there is another antigen Rh, the absence of presence of which adds the positive or negative status to an individual's blood type.

Says Dr Shikha Sharma, a nutrition expert based in New Delhi, "The idea is that by following a specific diet for a particular blood type you will not just lose weight but also improve your overall health."

Peter J D'Adamo, MD, a naturopathic physician in the US, gave the world this new way to eat through his book, 'Eat Right 4 Your Type', in 1996. Based on visitors to his website and his private practice, D'Adamo estimates that about two million people worldwide are following the blood type diet today.

"In India too, this diet is catching on fast. Although it is more oriented towards Europeans, the South Asian community needs to modify it to according to the kind of food they eat," says Sharma.

D'Adamo's theory is based on four decades of work that he and his father did on the connections between blood type, food, and disease. He propagates that your blood group type is the key to how you burn your calories, which foods you should eat, and how you benefit from certain types of exercise. D'Adamo's nutrition plan is based on the premise that our blood type reflects our internal chemistry and determines the way nutrients are absorbed in the body. He also believes that levels of stomach acidity and digestive enzymes are linked with blood type.

Instead of counting calories and fat grams, this diet provides a detailed list of food to be avoided or included. There is also advice on supplements, stress and exercise for each blood type. Here's a low-down from 'Eat Right 4 Your Type':

Blood Type O should basically stick to a high protein diet (including red meat), low carbs, fruits and vegetables. Cut out wheat and most other grains, corn, and avoid dairy products and most nuts. Must engage in vigorous physical exercise.

Blood Type A must ideally be vegetarian - high carbohydrate, low fat. Basically, stick to fruits and vegetables. This blood type is supposed to have relatively thicker blood, and a sensitive immune system. Must not consume dairy products, animal fat and meat. Should engage in gentle exercise such as yoga or golf, and meditate to deal with stress.

Blood Type B can have the most varied diet of all the blood types. Should consume a balanced diet of fruits and vegetables, grains, fish, dairy products. Chicken is best avoided. Should engage in moderate exercise such as swimming or walking.

Blood Type AB has most of the benefits and limitations of types A and B. Should consume a mostly vegetarian diet, and only on rare occasions some fish, meat (no chicken), and dairy products. Should engage in calming exercises and relaxation techniques. Considered the friendliest immune system of all the blood types.

Dr D'Adamo believes that this diet brings about weight loss, restores the body's natural genetic rhythm to achieve health and vitality, and helps in resisting allergies and infections, combating stress, and in delaying ageing.

Although Dr Sharma thinks that more research is required on the blood type diet, at the practical level she has found that it works. "Also, it is very similar to our age-old and time-tested Ayurveda diet principle - 'everybody responds to different kinds of food'," she says.

Dr Anjana Kalia, an Ayurvedic practitioner in New Delhi agrees that there are similarities. "In the blood type diet, dairy products are not recommended for the O blood group. Research and our hands-on experience is showing that Ayurveda too advises low dairy product consumption for particular constitution types, many of whose blood type is O."

The flip side is that following a blood type diet can be practically difficult - especially for family members with different blood types. Even supporters of the diet say it is difficult to stay within its confines.

For instance, D'Adamo's diet advocates that individuals with type B forgo shrimp, pork, chicken, ice cream, American cheese, peanuts, black beans, whole wheat bread and tomatoes. Social eating in this case tends to be a real challenge. But D'Adamo has a counter - 75-80 per cent compliance works well for most people, he says. So you can have "forbidden" food now and then.

But not everyone agrees with him. And Sharma pitches for the middle ground. Different diets suit different people, she says, so trying out this diet is a good idea. And one need follow it only if one feels comfortable.