Stored fat won’t let you lose weight?

The fatter we are, the more our body appears to produce a protein that inhibits our ability to burn fat, suggests new research published in the journal Nature Communication. The findings may have implications for the treatment of obesity and other metabolic diseases.

Most of the fat cells in the body act to store excess energy and release it when needed, but some types of fat cells, known as brown adipocytes, function primarily for a process known as thermogenesis, which generates heat to keep us warm. However, an international team of researchers from the Wellcome Trust-Medical Research Council Institute of Metabolic Sciences at the University of Cambridge, UK, and Toho University, Japan, have shown that a protein found in the body, known as sLR11, acts to suppress this process.

Researchers investigated why mice that lacked the gene for the production of this protein were far more resistant to weight gain. All mice — and, in fact, humans — increase their metabolic rate slightly when switched from a lower calorie diet to a higher calorie diet, but mice lacking the gene responded with a much greater increase, meaning that they were able to burn calories faster.

Further examinations revealed that in these mice, genes normally associated with brown adipose tissue were more active in white adipose tissue (which normally stores fat for energy release). In line with this observation, the mice themselves were indeed more thermogenic and had increased energy expenditure, particularly following high fat diet feeding.

The researchers were able to show that sLR11 binds to specific receptors on fat cells — in the same way that a key fits into a lock — to inhibit their ability to activate thermogenesis. In effect, sLR11 acts as a signal to increase the efficiency of fat to store energy and prevents excessive energy loss through unrestricted thermogenesis.

When the researchers examined levels of sLR11 in humans, they found that levels of the protein circulating in the blood correlated with total fat mass. In other words, the greater the levels of the protein, the higher the total fat mass. In addition, when obese patients underwent bariatric surgery, their degree of post operative weight loss was directly proportional to the reduction in their sLR11 levels, suggesting that sLR11 is produced by fat cells.

In their paper, the authors suggest that sLR11 helps fat cells resist burning too much fat during ‘spikes’ in other metabolic signals following large meals or short term drops in temperature. This in turn makes adipose tissue more effective at storing energy over long periods of time.