The genetic influence on the placebo effect specific to exercise performance
Placebo treatments can be used to elicit many different physiological responses; however, the underlying mechanisms responsible remain unclear. Recent research has shown the possibility of a genetic influence on the placebo response in patients with mood disorders. In this study, we attempted to establish a similar relationship in healthy college-aged students. Force production was measured by isometric knee extension of the quadriceps muscles using maximum voluntary contractions (MVC). Subjects were given placebo treatments disguised as an undisclosed sports supplement with the information that the supplement was previously shown to provide immediate strength improvements following ingestion. Subject DNA was genotyped for two genetic polymorphisms, tryptophan hydroxylase-2 (TPH2) and monoamine oxidase A (MAO-A). These particular polymorphisms were chosen for study based on previous research and their possible relationships to athletic performance. Results showed a 4.4% improvement in peak force with the ingestion of the placebo for both men and women (p < 0.05). We also found that the average placebo effect was similar for both genders (3.37% improvement in males, 7.47% in females). Neither polymorphism displayed a significant effect on the presence of the placebo response. We concluded that while a placebo response was evident with MVC isometric force production, TPH2 and MAO-A were not likely to be responsible for the effect.