ABSTRACT Objective To assess the effect of a program of vigorous physical exercises on the serum concentration of free and total L-carnitine, in male inmates at a prison in Boyacá, Colombia.Methods Pre-post intervention population-based study. 44 male prisoners with overweight and/or obesity, from a jail in Boyacá, Colombia were randomly assigned into two groups: an intervention group and a control group. The intervention consisted in participating in a vigorous exercise program over twelve weeks. Anthropometric measures and levels of free and total L-carnitine were every four weeks.Results There were significant increases in serum levels of free and total L-carnitine in the intervention group compared to the control group. Concurrently, in this group there was a reduction in body mass index (BMI), while in the control group there were no changes.Conclusion In overweight and/or obese patients, the routine practice of vigorous exercise plus caloric restriction offers significant benefits in reducing body fat volumes through the mechanisms of energetic consumption of long chain fatty acids.