Previous data showed that the administration of high doses of melatonin improved the
circadian system in athletes. Here, we investigated in the same experimental paradigm
whether the antioxidant properties of melatonin has also beneficial effects against
exercise-induced oxidative stress and muscle damage in athletes. Twenty-four athletes
were treated with 100 mg.day-1 of melatonin or placebo 30 min before bedtime during
four weeks in a randomized double-blind scheme. Exercise intensity was higher during
the study that before starting it. Blood samples were collected before and after
treatment, and plasma was used for oxygen radical absorption capacity (ORAC), lipid
peroxidation (LPO), nitrite plus nitrate (NOx), and advanced oxidation protein
products (AOPP) determinations. Glutathione (GSH), glutathione disulphide (GSSG)
levels, and glutathione peroxidase (GPx) and reductase (GRd) activities, were
measured in erythrocytes. Melatonin intake increased ORAC, reduced LPO and NOx
levels, and prevented the increase of AOPP, compared to placebo group. Melatonin
was also more efficient than placebo in reducing GSSG.GSH-1 and GPx.GRd-1 ratios.
Melatonin, but not placebo, reduced creatine kinase, lactate dehydrogenase, creatinine,
and total cholesterol levels. Overall, the data reflect a beneficial effect of melatonin
treatment in resistance-training athletes, preventing extra- and intracellular oxidative
stress induced by exercise, and yielding further skeletal muscle protection against
exercise-induced oxidative damage.