In 1982 Jane Fonda proclaimed to America that “no pain, no gain” and “feel the burn” were the mantras they needed to follow to get into shape. What she couldn’t have known at the time was that a hormone secreted from muscles in response to exercise was aiding people in their weight-loss endeavors. Now, a group of scientists led by researchers at the University of Florida Health has learned more about how the hormone irisin helps convert calorie-storing white fat cells into brown fat cells that burn energy.

The investigators also discovered that irisin—which typically surges when the heart and other muscles are exerted—inhibits the formation of fatty tissue, making it an attractive target for fighting obesity and diabetes. Irisin appears to work by boosting the activity of genes and a protein that are crucial to turning white fat cells into brown cells. Additionally, the researchers found that irisin significantly increases the amount of energy used by those cells, indicating it has a role in burning fat.

In the new study, the research team collected fat cells donated by 28 patients who had breast reduction surgery. After exposing the samples to irisin, they found a nearly fivefold increase in cells that contain a protein known as UCP1, or uncoupling protein 1, that is crucial to lipid metabolism.

“Human primary adipocytes derived from 28 female donors’ fresh subcutaneous white adipose tissue (scWAT) were used to examine the effects of irisin on browning and mitochondrial respiration, and preadipocytes were used to examine the effects of irisin on adipogenesis and osteogenesis,” the authors wrote. “Cultured fragments of scWAT and perirenal brown fat were used for investigating signal transduction pathways that mediate irisin’s browning effect by Western blotting to detect phosphorylated forms of p38, ERK, and STAT3 as well as uncoupling protein 1 (UCP1). Individual responses to irisin in scWAT were correlated with basal expression levels of brown/beige genes. Irisin up-regulated the expression of browning-associated genes and UCP1 protein in both cultured primary mature adipocytes and fresh adipose tissues.”

The findings from this study were published recently in the American Journal of Physiology–Endocrinology and Metabolism in an article entitled “Irisin Exerts Dual Effects on Browning and Adipogenesis of Human White Adipocytes.”

“We used human fat tissue cultures to prove that irisin has a positive effect by turning white fat into brown fat and that it increases the body’s fat-burning ability,” explained senior study investigator Li-Jun Yang, M.D., professor of hematopathology at the University of Florida’s College of Medicine’s department of pathology, immunology, and laboratory medicine.

Dr. Yang and her colleagues also found that irisin suppressed fat-cell formation. Among the tested fat tissue samples, irisin reduced the number of mature fat cells by 20–60% compared with those of a control group. That suggests irisin reduces fat storage in the body by hindering the process that turns undifferentiated stem cells into fat cells while also promoting the stem cells’ differentiation into bone-forming cells.

Because more than two-thirds of U.S. adults are overweight or obese, according to the National Institutes of Health, knowing that the body produces small quantities of fat-fighting irisin highlights the importance of regular exercise.

“Instead of waiting for a miracle drug, you can help yourself by changing your lifestyle,” Dr. Yang stressed. “Exercise produces more irisin, which has many beneficial effects including fat reduction, stronger bones, and better cardiovascular health. Irisin can do a lot of things, and this is another piece of evidence about the mechanisms that prevent fat buildup and promote the development of healthy bones when you exercise.”