The finding could help doctors determine who is at risk of heart disease.

Heart disease is a major health problem. Deaths from the condition are falling in the UK, but not as rapidly as in many other countries.

Some risk factors for heart disease, such as a family history, cannot be altered.

But habits such as smoking, stress, diet and exercise can be changed.

Repair

US researchers looked at endothelial progenitor cells.

It's likely that the changes seen reflect non-apparent early signs of vascular disease rather than being a predictor Professor Andrew Hattersley, Exeter University

These are "parent" cells to endothelial cells which line the blood vessels and allow communication between vessels and circulating blood cells.

In diseases like atherosclerosis, where arteries are blocked by fatty deposits, the endothelial layer becomes damaged and the vessels do not function properly.

The body calls on endothelial progenitor cells to fight against the condition.

It is thought new cells are generated in the bone marrow, circulated in the bloodstream and used to form new blood vessels or repair damaged ones.

Risk factor

The US researchers studied 45 men, aged around 50, who all had various levels of cardiovascular risk, but no history of actual disease.

They measured clumps of endothelial progenitor cells in their blood and how well blood vessels functioned.

They also calculated the patients' risk of heart disease, looking at factors such as cholesterol level, diet and smoking.

It was found that those with the lowest levels of endothelial progenitor cells had the highest risk factor.

There was also a strong correlation between circulation problems and low levels of the cells.

The researchers then studied cells from people with high or low risk factors for heart disease in a laboratory.

They discovered after seven days that more cells from people at the highest risk showed signs of ageing.

Damage

Professor Arshed Quyyumi, who led the research, said: "Cardiovascular health is dependent on the ability of the blood vessels to continually repair themselves.

"Evidence has shown that cardiovascular risk factors ultimately lead to damage to the endothelial layer of blood vessels.

"We can now speculate that continuing exposure to cardiovascular risk factors not only damages the endothelial layer, but may also lead to the depletion of circulating endothelial progenitor cells.

"Thus, the net damage to blood vessels and hence the risk of developing atherosclerosis depends not only on the exposure to risk factors, but also on the ability of the bone marrow-derived stem cells of endothelial origin to repair the damage."

He said further research was needed to determine whether there was a definite cause and effect relationship between a decrease in endothelial progenitor cells and cardiovascular disease.

Professor Quyyumi added: "We are hopeful that further research will show that endothelial progenitor cells are a useful marker for cardiovascular disease risk."

Andrew Hattersley, professor in the department of diabetes and vascular health at the Peninsular Medical School, Exeter University said the study added to knowledge about the biology of blood vessels.

He added: "It's likely that the changes seen reflect non-apparent early signs of vascular disease rather than being a predictor."

The research was published in the New England Journal of Medicine.