The researchers suggest their discovery may help explain why overweight people find it very hard to lose weight.
The researchers, from the University of Cambridge in the UK and Toho University in Japan, report their findings in the journal Nature Communications and suggest they may change the way we treat obesity and other metabolic diseases.
In contrast to the more abundant white adipocytes, or white fat cells, that store energy, our bodies contain much smaller amounts of brown adipocytes, or brown fat cells, that burn fat to keep us warm - a process known as thermogenesis.
The new research reveals that a protein known as sLR11 appears to suppress thermogenesis in fat tissue.
The team found that mice unable to produce the protein were more resistant to weight gain when put on a higher-calorie diet. Compared with mice that did not lack the protein, their metabolic rates increased so they burned calories faster.
The researchers also found that in the mice lacking sLR11, genes that are normally active in brown fat tissue were more active in white fat tissue.
The higher the BMI, the harder it is to burn off fat
On further investigation, the team discovered that the protein binds to specific receptors on fat cells so as to block their ability to trigger thermogenesis and convert fat to heat.
Moreover, it appears that sLR11 increases the efficiency of storing energy in fat and stopping any excess being lost to heat generation.
In a final part of the study, the team turned to humans. There, they found higher blood levels of the protein were linked to higher total body fat.
Also, when they looked at obese patients undergoing bariatric surgery, they found the amount of weight loss following surgery was in line with falls in sLR11, suggesting the protein is released by fat cells.
The researchers suggest sLR11 plays an energy conserving role to prevent energy wastage in fat tissue, and this role is exaggerated in overweight and obese people, with the result that the higher a person's BMI, the harder their body fights to conserve energy.
Joint first author Dr. Andrew Whittle, of the Wellcome Trust-MRC Institute of Metabolic Science at Cambridge, concludes:
"Our discovery may help explain why overweight individuals find it incredibly hard to lose weight. Their stored fat is actively fighting against their efforts to burn it off at the molecular level."
Thermogenesis is attracting a lot of interest as a target for treating obesity, diabetes and other metabolic disorders, because it appears to offer a way to safely dispose of excess fat.
The study highlights a potential way to help people who need help in the other direction - with treatments that target thermogenesis to increase efficiency of fat storage - such as patients with anorexia nervosa.
This work follows other research Medical News Today learned about recently that suggests new treatments for diabetes and obesity may arise from studying fat cell metabolism. In that study, researchers found that as fat cells develop, they vary the types of nutrients they consume to grow and make fat and energy.