As with most of the neuroscience research, there exists a counter evidence to this - Boecker, H., Sprenger, T., Spilker, M.E., Henriksen, G., Koppenhoefer, M.

As with most of the neuroscience research, there exists a counter evidence to this - Boecker, H., Sprenger, T., Spilker, M.E., Henriksen, G., Koppenhoefer, M., Wagner, K.J., Valet, M., Berthele, A., Tolle, T.R. (2008). The Runner's High: Opioidergic Mechanisms in the Human Brain. Cerebral Cortex DOI: 10.1093/cercor/bhn013[http://cercor.oxfordjournals.org/content/18/11/2523.full http://cercor.oxfordjournals.org/content/18/11/2523.full]