You become what you believeA week later, the participants were given a result, based not on their actual data, but rather on one of two groups into which they had been randomly placed. Some were told they had the form of a gene called CREB1 that makes a person tire easily; others were told they had the high-endurance version. Then they ran on the treadmill again. This time, those who had been told they had the low-endurance version of CREB1 did worse on the test, even if they had the other variant. Compared with their results on the first test, on average their bodies removed toxic carbon dioxide less efficiently, their lung capacity dropped, and they stopped running 22 seconds sooner, the team reports today in Nature Human Behavior. And those who thought they had the high-endurance form of the CREB1 gene ran slightly longer on average before feeling hot and tired, regardless of what gene variant they had. “Simply giving people this information changed their physiology,” Turnwald says. The team also tested a second group of 107 people for its version of FTO, a gene that influences how full we feel after eating. Some versions can also predispose people to obesity. Participants ate a small meal and rated their fullness. After being told, at random, that they had a version of FTO that made them hungrier than average or one that made them easily sated, participants ate the same meal. Those told they had the “hungry” version of the gene didn’t feel any different. But those who were told they had the other version felt less hungry on average after eating; they also had higher blood levels of a hormone that indicates a feeling of fullness.