The aging process is associated with an increased risk of chronic diseases and a decline in physical and cognitive functions. Physical activity, particularly endurance exercise training, can counteract these changes, but the underlying biological mechanisms are largely unclear. The "HEAL" project investigates whether endurance exercise training mediates anti-aging effects via the metabolism of the amino acid tryptophan in middle-aged adults. The results could provide new insights into the effects of physical activity on healthy aging and contribute to the development of personalized prevention strategies.