The notion that the more weight people carry, the more difficulty they'll have slimming down isn't a surprising one, but an element of mystery still surrounds the exact mechanism that makes it increasingly difficult to shed that extra baggage. Scientists are now claiming to have zeroed in on a key factor, identifying a protein that stops fat cells from burning energy, a molecule they say could become a key target in treating obesity and other metabolic conditions.

The human body contains two types of fat. White fat stores excess energy, releasing it if it is needed or resulting in flabby love handles if it is not. The other type, brown fat, mostly performs the role of burning fat to generate heat and keep the body warm, a process known as thermogenesis. As our understanding of this has improved over recent years, research has uncovered proteins that may promote thermogenesis when targeted with diabetes drugs and chemical compounds, potentially creating more "good fat". But little is known about the proteins that inhibit it.

Now an international team of scientists has found a protein called sLR11 that suppresses thermogenesis. It was found that mice lacking the gene for the production of sLR11 were less capable of gaining weight, had higher amounts of energy and burned through calories faster, especially after feasting on high fat foods. A closer look revealed that sLR11 sticks to certain receptors on fat cells, like a key sliding into a keyhole, preventing thermogenesis being triggered.

Measuring sLR11 levels in humans showed that the protein's levels in the blood were linked with total fat mass. Observations of obese patients following bariatric surgery showed that the amount of resulting weight loss was proportional to reduced sLR11 levels, suggesting to the researchers that fat cells are responsible for generating sLR11.

So the more overweight a person is the more likely it is that they'll be carrying higher levels of sLR11, which in turn hinders the crucial thermogenesis process. The researchers say that the identification of the protein could lead to drugs designed to block its activity. Conversely, its effect on the body could be mimicked to encourage weight gain or save energy for treating conditions like anorexia nervosa.

The research was published in the journal Nature Communications.