Family doctors should offer immediate health checks to those carrying a lot of weight around the stomach, a leading obesity expert says.
Someone who has a pot belly, but might not otherwise be obviously overweight, could be at risk of heart disease or diabetes.
Measuring their waist is a quick and effective way of telling how healthy they are, according to Jean-Pierre Despres, chairman of the Department of Nutritional Sciences at Laval University in Quebec, Canada.
'You can take two individuals the same age and the same weight, but the one carrying more fat in the abdominal cavity, what we call visceral adipose tissue, is more at risk,' said Despres, who studies the way lipids - or blood fats - work.
Up to 70 per cent of our body shape is determined by our genes, which control the hormones that decide where fat is stored. Many women put on weight around the thighs and bottom, for example, because higher levels of oestrogen cause fat to accumulate in the lower half of the body.
People with an 'apple-shaped' figure, with weight mainly around their midriff, are known to be at higher risk of heart disease and diabetes.
But scientists increasingly think abdominal fat is the most dangerous, because it is more likely to be absorbed in the bloodstream and raise cholesterol levels.
Abdominal fat cells also have a much greater influence on the hormone insulin, and are more likely to cause insulin resistance, where normal levels fail to control the blood sugar levels.
Despres uses CT (computerised tomography) scans, showing a cross-section of the body, to reveal how much fat has been laid down around the organs. But simple blood tests can reveal whether people are developing a resistance to insulin which could turn into diabetes. A key indicator, Despres believes, is the number of small LDL (low-density lipoproteins) in the blood.
'It is the concentration of these particles that can predict risks quite accurately,' he said. Despres would like doctors to be more aware of patients' body shape as a way of alerting them to potential problems.
'The waist circumference is the best indication of the amount of abdominal fat,' he said. 'It is so simple and it's a much better indicator than weight or body mass index. There is no reason why doctors cannot spot someone walking into their surgery, probably with a different medical problem, and book them in for tests that will look at the cholesterol levels.'
He said that, in women who are going through the menopause, it is common to see the waist size rising without their weight really increasing much. 'The weighing scales can be misleading,' he added.
Diet and exercise can play an enormous role in modifying genetic influences, said Despres. 'We live in a very toxic environment in terms of fat,' he said. 'People take the elevator into shopping malls and they sit in front of the TV eating pizza.
'The genes have always been there, but people can do a lot to keep the weight down once they know the risks.'