Journal scientifique

ates many of the benefits of exercise, such as resistance to metabolic diseases. To find out more, a team from the Dana Farber Cancer Institute in Boston, analyzed factors secreted by muscle and regulated by PGC1-alpha. One of these factors turned out to be a new hormone which had gone unnoticed because it is hidden within a complex molecule. Further study showed that levels of that hormone jumped in mice and humans after bouts of exercise. When the hormone was added to mouse subcutaneous white fat cells at an early stage of development, it made the cells more likely to become "beige" fat cells. These, like brown fat, are equipped to burn body fuel to generate heat.

"The hormone carries a message from muscle to fat tissue," says lead author Pontus Bostrom. Bruce Spiegelman, director of the lab, named the hormone irisin after the Greek messenger goddess Iris.

Speculating about applications for a hormone that mimics the effect of exercise is easy, but explaining its existence is "a little trickier", says Bostrom. Why would exercise put in motion events that ultimately produce fat cells that expend energy more easily than they conserve it? Jan Nedergaard, who studies brown fat at the Wenner-Gren Institute at Stockholm University, Sweden, says that irisin "could be of interest for all obesity-related issues" but was also perplexed by its origins. Bostrom and colleagues speculate that it may be down to shivering. If the muscle contractions of shivering cause the body to produce irisin, which then influences the development of brown fat that generates heat, the hormone may have evolved to help stave off hypothermia.

Science JokesA neutron walked into a bar and asked, "How much for a drink?" The bartender replied, "For you, no charge."How many theoretical physicists specializing in general relativity does it take to change a light bulb?Two. One to hold the bulb and one to rotate the universe.