An international team of researchers said Thursday they have uncovered for the first time how obesity increases the risk of high blood pressure.

In a study published in the U.S. journal Cell, a group led by Professor Michael Cowley of the Monash University in Australia revealed that leptin, the hormone associated with fullness or satiety, is the key to the increase in blood pressure that occurs with weight gain.

Being obese or overweight is a major risk factor for the development of high blood pressure and cardiovascular disease. While a number of factors may be involved, the precise explanation for the link between these two conditions has been unclear.

Cowley, in collaboration with researchers from Britain and the U.S., studied mice and humans that have problems producing or processing the hormone leptin and compared them with "healthy" individuals to see whether this hormone could provide the link.

Leptin is made by fat and circulates in the bloodstream to reach the brain, where it acts as a signal for energy reserves, adjusting both energy expenditure and the sensation of hunger.

The group showed that some obese people who were lacking leptin because of a genetic disorder had low blood pressure despite being very heavy.

Their research also found that mice with normal leptin signaling developed an increase in blood pressure when they became obese on a high fat diet.

These effects, however, were not seen in mice that lacked leptin or where leptin was unable to work because of a defect or block on the leptin receptor.

These experiments demonstrated that leptin signaling is necessary for obesity-induced increased blood pressure, the researchers said.

Clinical studies in severely obese humans also showed that these observations are relevant to humans.

"High blood pressure is a well-known consequence of obesity," Cowley said in a statement.

"Our study explains the mechanism behind this link, showing that leptin, a hormone secreted by fat, increases blood pressure."

The researchers said targeting this hormone could offer a useful way of helping people fight obesity and associated problems such as high blood pressure and heart disease.