Listen up, kids: any problem you'll ever have, ranging all from being fat to (now) being lazy, can be effortlessly be solved by taking drugs. Don't want to exercise? Just inject yourself with some of this new hormone, and you'll be on your way to the gym before you can say "recombinant human erythropoietin!"
Erythropoietin (or Epo, as your doctor/dealer will call it) is a hormone that's found naturally in your kidneys and liver. Its job is to regulate production of red blood cells, and as such, injecting yourself with more of it will increase your athletic performance by making your body more efficient at delivering oxygen to your muscles. Epo has been known to work this way for a long time, and professional athletes have been covertly (and illegally) using it for over a decade.
The new discovery, made by researchers at the University of Zurich, is that Epo can also motivate you (and by you we mean mice in a study) to exercise in the first place, without stimulating blood cell production at all. The study used three groups of mice: mice that had been genetically engineered to produce the human version of Epo, mice that had the human version of Epo injected into their brains, and Epo-free mice. In testing, the mice with the Epo showed significantly higher running performance without any corresponding increases in red blood cells.
What seems likely here is that rather than motivating the mice to exercise specifically, Epo works as a targeted mood enhancer that (for mice, at least) stimulates activity. The researchers say that this effect "might be used in patients suffering from depression and related diseases," but the editor of The FASEB Journal (in which the research is published) was a bit more optimistic: "If you can't put exercise in a pill, then maybe you can put the motivation to exercise in a pill instead."