THIN is not only considered beautiful. It is also synonymous with being in good shape and that means being healthy. But the latest research into a wide range of diseases is forcing scientists to redefine what is meant by good shape. If you want to gauge how your weight is likely to affect your health, it makes more sense to reach for a tape measure than to stand on the scales. And this advice does not just apply to the frankly overweight - but to everyone. For it appears that the light-weight person, whose shape is straight up and down, may well be at greater risk of serious disease than an obviously fat person with a well-defined waist. Researchers now believe that the type of fat laid down round the middle is biochemically different from fat laid down in other parts of the body and appears to have harmful effects on hormones and blood fats. When big amounts of this fat are laid down in the abdominal cavity, the waist/hip ratio alters - providing a clear warning sign of future health risks. As a result, the difference between waist and hip measurements is no longer regarded as cosmetic, but as a crucial health indicator. Although fat distribution, which gives almost equal waist and hip measurements is most common among men - it has been found that women who take on this pattern acquire male cardiovascular disease risks. The most recent studies suggest that ''apple-shaped'' females face an increased risk of breast cancer - and now, it seems of infertility too. In February, the British Medical Journal published research showing that pear-shaped women are more likely to conceive than women who store fat round their middles. The finding was based on a study of 500 healthy females seeking artificial insemination because their partners were infertile. The smaller the difference between their waist and hip measurements, the greater the problems of conception. These findings held true even after other factors such as weight, age and menstrual cycle regularity were taken into account. This year, the Journal of the American Medical Association published a five-year study of deaths occurring among nearly 42,000 Iowa women aged 55 to 69 which showed mortality linked to waist/hip ratios. Scientists have devised a rather arbitrary ratio to help individuals to determine their risk. When women divide their waist measurement with their hip measurement they should come up with 0.75 or less. Men need a figure of about 0.9 for a lower risk. Some researchers give the male/female figures as 0.8 and one. However, they agree that the more unfavourable the ratio the greater the risks. ''We know that obesity is linked to ill health - and we hit upon weight as the way of measuring it,'' says Kayteen Khaw, professor of clinical gerontology at the University of Cambridge. Dieting to solve the problem may not only be unsuccessful but actually harmful. Weight cycling - yoyo dieting - has been linked with an increased risk of heart disease. So what should apple-shaped people do to minimise their risks? Exercise is particularly important, Professor Khaw says. She believes that differences in activity levels, rather than genetics, may explain why two people who eat the same amounts may lay down weight in different places. Research has shown that people who eat the most food are often at the lowest risk of heart disease. Smokers are more inclined to lay down fat centrally. ''It is ironic that many smokers do not want to give up because of fears of weight gain. We now think that even if they put on some weight, their fat distribution will be more favourable.'' In one respect, at least, apples are more fortunate than pears. As any British pear-shaped woman will confirm, dieting simply produces a smaller pear. However, a weight loss in apples is likely to lead to a much more favourable waist/hip ratio.