Most diet pills promise you'll lose weight without breaking a sweat, which always makes me nervous. But here is a new concept: a pill that makes you want to exercise. It is not a pill yet, but scientists found when they injected high amounts of a hormone called erythropoietin, or EPO, mice became more active.

You may know EPO as a performance-enhancing drug banned by sports governing bodies including the Olympic Games and the Tour de France. Athletes are attracted to EPO because the hormone can increase red blood cells in the bloodstream, giving the body more oxygen, which equates to better performance.

EPO is naturally produced by cells in the kidney that can sense when oxygen levels start to dip. EPO binds with receptors in the bone marrow, where it stimulates red blood cell production. Even though our bodies make EPO, more of it is risky.

In the past fifteen years, eighteen cyclists have died suddenly in their sleep from using EPO. Injected in repeated small doses, too many red blood cells can end up in the bloodstream, thickening it and clogging capillaries. The risks are stroke and heart attack. Athletes are more vulnerable since they tend to become dehydrated, further thickening the blood.

However, researchers found that instead of repeated small doses, when given in acute high doses, EPO crosses the blood-brain barrier and helped mice run better, even though it did not boost red blood cell production. This meant EPO acted as a brain hormone that motivated the mice to exercise more, without changing its physiology.

If proven safe, EPO can help people who are obese and quicken recovery for people rebuilding muscle mass after a long illness.