Search the term "peptides" and a long list of online retailer options will pop up. You'll be offered vials of molecules with funky names that sound like exoplanets: GHK-CU, CJC12-95, Thymosin Beta-4.

Studies on the neurons of mice suggest our own human endurance may have more to do with the brain than our physique. Regular exercise strengthens muscles and improves endurance—but new research ...