The project is separated on two studies. Study A: young volunteers (n=40) are supplemented either with carnitine with leucine or carnitine alone for 24 weeks. Before the start, in the mid-point, and after finishing the supplementation the following primary outputs variables are performed: maximal isokinetic knee extensor peak torque (Biodex System 4 Pro), body composition analysis (InBody720), skeletal muscle carnitine concentration, oral glucose tolerance test. Study B: volunteers over 65 years old (n=40) are supplemented either with carnitine or placebo for 24 weeks. Before the start, in the mid-point, and after finishing the supplementation the following primary outputs variables are performed: body composition analysis, blood cytokines and carnitine concentration, blood lipid profile and global physical activity questionnaire.