It's common knowledge that shivering in the cold is part of the body's attempt to stay warm. According to new research into the mechanisms involved, shivering releases a hormone that stimulates fat tissue to produce heat so that the body can maintain its core temperature. This hormone, irisin, is also produced by muscle during exercise. The findings, which are published in the February 4 issue of the Cell Press journal Cell Metabolism, demonstrates that the act of shivering produces calorie-burning brown fat and improves metabolism.

Through experiments conducted in healthy volunteers, Dr. Francesco S. Celi of the National Institute of Diabetes and Digestive and Kidney Diseases and his colleagues found that the irisin, produced when the body shivers, is released in proportion to shivering intensity. Furthermore, the amount of irisin secreted as a result of shivering is of similar magnitude to that of exercise-stimulated secretion. The team also found that when human fat cells in the laboratory were treated with FNDC5, a precursor of irisin, the cells burned more energy and released more heat.

The results suggest that exercise-induced production of irisin could have evolved from a similar mechanism that occurs following shivering-related muscle contractions in order to burn calories and generate heat. "This research may reveal why exercise increases secretion of a hormone that makes the body maintain its internal temperature," said Dr. Celi, who is currently at Virginia Commonwealth University. This may help explain why exercise increases secretion of a hormone that, paradoxically, makes the body feel hotter.

The findings also suggest that exploiting the muscle-fat crosstalk that is mediated by irisin may represent a new strategy to treat or prevent obesity. The results may even help people embrace feeling cold. "Perhaps lowering the thermostat during the winter months could help both the budget and metabolism," said Dr. Celi.