A hormone that regulates the amount of fat stored in the body was found to be responsible for the increase in blood pressure that occurs with weight gain.

Researchers from Monash University and the University of Cambridge found that the hormone leptin is the reason why being obese or overweight is a major risk factor for the development of high blood pressure and cardiovascular disease. Whilst a number of factors may be involved, the precise explanation for the link between these two conditions has been unclear.

For the study, researchers studied mice and humans who 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 -- hence it is sometimes referred to as the "satiety hormone."

Researchers Sadaf Farooqi showed that some obese people who were lacking the hormone leptin because of a genetic disorder had low blood pressure despite being very heavy. This was also the case for people lacking the gene for the leptin receptor in the brain, meaning that the brain was unable to respond to the hormone.

Based on the findings, researchers determined that leptin signaling is necessary for obesity-induced increased blood pressure. The clinical studies in severely obese humans showed that these observations are relevant to humans.

 "We now know that leptin regulates both our weight and our blood pressure through its action on the brain," Farooqi said in a statement. "Targeting this action could offer a useful way of helping people fight obesity and associated problems such as high blood pressure and heart disease."

Modeling the human condition, researcher Michael Cowley's team in Australia showed that mice with normal leptin signaling developed an increase in blood pressure when they became obese on a high fat diet. These effects 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.

The researchers are now investigating the precise pathways in the brain by which leptin acts to regulate blood pressure.

The findings are detailed in the journal Cell.