Almost 35% of overweight or obese individuals are free of any metabolic disorder. This may be explained by a favourable fat distribution. However, those individuals also have a higher level of physical fitness. Therefore, deleterious cardiometabolic effects of excessive fat mass ("fatness") might be counterbalanced by regular physical activity leading to high cardiorespiratory fitness ("fitness"). The present article first analyzes the various pathophysiological mechanisms explaining why muscular exercise has beneficial effects and second, describes the relationship between "fitness" and "fatness" and their respective cardiometabolic consequences at various ages: adolescents, adults and elderly people.