By Richard Black BBC science correspondent

Scientists in the United States have discovered three things which help predict how long someone is going to live.

In a study published in the journal Science, they report that the length of a person's life is related to their body temperature, and to levels of two chemicals, insulin and DHEAS, circulating in the blood.

If we can learn what these individuals are doing then perhaps we could help the rest of us to live longer, too

Dr George Roth

On average, people who have a lower body temperature live longer, as do those with lower levels of insulin, and those with higher levels of DHEAS.

The researchers are not entirely sure of the reasons behind this finding but experiments with animals suggest it may be to do with a lower metabolic rate.

They say that unravelling the mechanisms behind the finding may lead to new ways of helping people to live longer.

Of mice and men

The suggested link between lifespan and metabolic rate comes from experiments in which animals are maintained on strict calorie-controlled diets.

These animals tend to live longer than normal - up to 40% longer. They also have lower than average body temperature, low levels of insulin, and high levels of dehydroepiandrosterone sulphate (DHEAS).

George Roth from the National Institute of Ageing in Baltimore in the United States has been working on a long-term study of factors affecting human lifespan.

Called the Baltimore Longitudinal Study of Ageing, it has been running since 1958 and has enrolled around 1,500 people.

Longevity clues

Following the lead given by the animal diet experiments, Dr Roth's team decided to analyse body temperature and levels of insulin and DHEAS in men enrolled in the Baltimore study.

"Men with lower temperature and insulin and those maintaining higher DHEAS levels have greater survival than respective counterparts," he writes in the Science journal. At present, there are not enough data to say precisely how big these effects are.

The average human body temperature is around 37 degrees Celsius but individuals differ by around a degree.

The men in the Baltimore study are not on diets. So something else must be keeping body temperature and insulin down, and DHEAS up, in the men who are living longer.

"It could be genetic, it could be something else in their lifestyle apart from diet," Dr Roth told the BBC. "If we can learn what these individuals are doing then perhaps we could help the rest of us to live longer, too."