'GRANDPA, how high is your cholesterol level? And have you had problems with your blood pressure?' Parents, grandparents - and great aunts and uncles for that matter - be warned. These are the type of questions you will be fielding when the latest American health trend arrives in Britain.

Although it is a truism that the best way to ensure good health is to choose healthy parents, genetic inheritance is normally ignored when it comes to advice on healthy eating. For years we have been told to lower our fat intake, up our fibre intake and count calories; but now it seems that all that should be taken with a pinch of - once forbidden - salt.

The diet debate frequently focuses on families with health problems, but in fact most families are not at risk; about 50 per cent of heart disease in America is accounted for by just 5 per cent of families. A massive school project in Texas compiled health charts for 25,000 families and found that only 3,000 families (12 per cent) were seriously at risk of heart disease, strokes or cancer.

Members of the families at risk were then able to start thinking about diet and exercise, while members of the families considered not to be at risk were able to be more relaxed about their lifestyles.

In Genetic Nutrition, to be published next month, Dr Artemis Simopoulos, president of the Center for Genetics Nutrition and Health in Washington and former chair of the nutrition co-ordinating committee office of the director of the National Institute of Health, has pulled together a mountain of research on the genetics of disease, and for the first time related genetics to the latest findings in nutrition.

'If you come from a family with no major diseases, you should feel comfortable eating anything you fancy,' she says. 'There is no reason to avoid eggs, butter or red meat for instance, provided you don't become overweight.'

But what has all this to do with asking grandpa probing questions? The idea behind Dr Simopoulos's book is that once you know what sort of diseases run in your family and so have an idea what health problems you could be vulnerable to, you can vary your diet accordingly. But most people have only a sketchy idea of their parents' medical histories, let alone those of their aunts or uncles, so the first step in tailoring your diet to your genes is to compile a medical family tree.

There are 30 questions in the book, on topics including heart attacks, breast cancer, diabetes, smoking and congenital defects, to be answered by parents, aunts, uncles and grandparents. Depending on the size of your family, there could be dozens of questionnaires to fill in. The results are presented on a chart 2ft by 3ft so that your doctor can see at a glance what you should watch out for.

Dr Simopoulos claims the results can be life-saving. 'Take Charles. He knew there was a history of heart attacks in his family so he took up exercise. What he didn't know, because he hadn't done a chart, was that there was also hypertension (high blood pressure). He was dead before he was 40. 'There are very definite dietary changes that you can make for hypertension - if he had known about that he might have lived.'

There are other anecdotes in the book, including one about the sisters who compiled their charts and discovered they had a high risk of cancer. They had scans that identified their own cancers very early.

Dr Simopoulos does not believe that margarine is healthier than butter. She claims, controversially, that a component of some polyunsaturated fats - omega-6 - is a dietary villain. Polyunsaturated margarines contain omega-6. 'We should reduce omega- 6 as much as possible and cut out margarines altogether,' Dr Simopoulos says. 'Margarines contain trans fatty acids that occur only rarely in nature. They increase the risk of heart disease by raising the 'bad' cholesterol and lowering the 'good'. They also make you put on more weight than other oils.'

Another example in the book of the link between genes, diet and health is that of the Finns and the apo-E gene, which affects the body's ability to handle cholesterol. The gene comes in three forms - 2, 3 and 4. Apo-E4, common among Finns, causes the body to absorb large amounts of cholesterol from food, while apo-E2, common in the Japanese, means that however much fat is eaten, cholesterol levels will not rise much unless the person is overweight. The effects of apo-E3 are in between.

During famine, the body's ability to extract maximum levels of fat from a meagre food supply is an advantage. But now it means the Finns have one of the highest levels of coronary heart disease (CHD) in the world.

Treatment for someone with the apo-E4 gene is a good example of how genetic nutrition can make dietary advice much more specific. For a man with apo-E4, exercising and keeping the weight down is not enough - a low-fat, low-cholesterol diet is necessary.

Dr Simopoulos believes that women who have the gene should also avoid omega-6 polyunsaturates, because they lower the levels of the good sort of cholesterol, which protects against heart disease. Instead, women should go for omega-9, which is found in fats such as olive oil and peanut oil.

About 63 per cent of CHD cases are considered to be heritable; half are determined by single genes that affect fats carried around the bloodstream. Dietary advice varies, depending on which gene is involved. Someone with dangerous cholesterol levels because of apo-E4 might well not be overweight, but someone who was at risk because of high levels of fatty acids in the bloodstream almost certainly would be, and so would need a low- calorie diet.

A different approach might be taken if a risk of heart-disease came from excessive clotting. One of the proteins that affects clotting is homocystine, and high levels of this are genetically determined. Research suggests that folic acid may reduce high levels of homocystine and so reduce the risk of clotting.

High blood pressure is another area in which, depending on the genetic defect present, different dietary regimes are required. For one person, hypertension could involve high levels of fats in the bloodstream and be improved by exercising, losing weight and cutting down on salt. For someone else, reducing salt intake would not have an effect but a ban on alcohol and upping potassium intake - with yoghurt and plantains, for instance - would bring the pressure down.

If someone's health chart shows hypertension in the family, strenuous efforts should be made not to become overweight, Dr Simopoulos advises.

Sometimes interpreting the family tree can involve detective work, such as with a condition known as haemochromatosis in which the blood is too rich in iron. Although 10 per cent of Americans have the gene that produces this, it is often undetected. The result can be cirrhosis of the liver, CHD or diabetes. When these three killers appear regularly in a family, haemochromatosis may be the cause. Once spotted, however, it can be combated if the sufferer exercises, gives blood and does not eat liver. Sufferers should also steer clear of iron supplements and extra vitamin C, which increases iron absorption.

Although doctors inquire about the family histories of people who seem at risk from heart disease, nothing as detailed as a personal genetic chart has been suggested before. The response to it has been favourable.

Timothy Cox, professor of medicine at Addenbrooke's Hospital in Cambridge says: 'It sounds as though it might be as powerful a tool for spotting heart disease early as testing the general population for cholesterol levels.'

Parents and grandparents should resign themselves to impertinent questions and sharp-eyed youngsters scanning them for such tell-tale signs as the vertical ear lobe crease that indicates a tendency to heart disease, or the yellowish, fatty bumps below the eyelids that are associated with high levels of cholesterol.