Exercise brings down body weight because it draws on fat reserves that muscle can burn as fuel. But the heart also plays a similar role.
Sheila Collins and colleagues at the Sanford-Burnham Medical Research Institute detail how hormones released by the heart stimulate fat cell metabolism.
These hormones turn on a molecular mechanism similar to what's activated when the body is exposed to cold and burns fat to generate heat, the Journal of Clinical Investigation reports.
"Exercise is always going to raise your blood pressure somewhat, so there's the potential that these heart hormones -- called cardiac natriuretic peptides (CNP) -- are being released and contributing to the breakdown of fats," said senior study author Collins, according to a Sanford statement.
"Over a period of time, natriuretic peptides could also be leading to an increase in the numbers of brown fat cells, which we know are very important for protection against diet-induced obesity, at least in lab experiments," added Collins, professor at the Diabetes and Obesity Research Centre at Sanford-Burnham.
Brown fat cells, unlike white fat cells typically linked to body fat, not only store fat but also readily convert calories into energy -- a process that malfunctions in obesity, according to a Sanford statement.
Collins and her team found that the metabolic effects caused by CAPs depend largely on the ratio of two different kinds of receptors -- message-receiving proteins -- on the surface of fat cells.
One, called NPRA, is a "signalling" receptor and its presence helps boost brown fat cells and burn white fat. The other, called NPRC, is a "clearance" receptor and seems to prevent natriuretic peptides from activating NPRA, resulting in a greater accumulation of white fat cells.
When exposed to cold in this study, mice had elevated amounts of natriuretic peptides. As a result, fatty acids were mobilized and the calorie-burning brown fat machinery was turned on in these mice.