Recently, Nanci Guest, a registered sports dietitian and PhD candidate at the University of Toronto, recruited 100 athletes for a test. They all rode a 10-km time trial. Before each test ride, an athlete was given a caffeine supplement: either 4 mg/kg of body weight, 2 mg/kg of body weight or a placebo. Guest wanted to see who exactly got a benefit from the caffeine and who didn’t. She figured the results were connected to an athlete’s genes.