The present invention provides methods for assessing the effect of exercise on an individual by measuring changes in the concentration of a compound in the individual's blood. Specifically, the invention measures changes in the concentration of an anti-inflammatory signal molecule or endogenous morphine. Anti-inflammatory signal molecules include, for example, adrenocorticotropin (ACTH), cortisol, and interleukin-10 (IL-10). A first embodiment of the invention provides a method for assessing the effect of exercise on an individual comprising the steps of measuring a concentration of at least one compound in the individual's blood before exercise, measuring a concentration of the at least one compound in the individual's blood after exercise, and comparing the concentrations of the at least one compound.