Circulating met-enkephalin in trained athletes during rest, exhaustive treadmill exercise and marathon running.

Twelve trained male athletes were subjected to a progressive intensity treadmill exercise test to total exhaustion and at a later date took part in the Comrades Marathon. Blood was taken in order to determine whether plasma met-enkephalin levels increase in an exercise-intensity dependent manner. The mean concentrations in f mol/ml were as follows: post-exercise levels (266) were significantly higher than basal levels (172), and the post-marathon levels (378) rose significantly above the post-exercise levels. The magnitude of the responses is thus correlated with the amount of work performed and would suggest that this opioid peptide is involved in physiological actions that perhaps extend beyond the pharmacological actions of opiate drugs.