Regular physical activity and exercise interventions are suspected to have anti-inflammatory effects depending on exercise modality, thereby potentially reducing the risk and progress of several chronic diseases. Alterations in the kynurenine pathway may represent a link between inflammatory responses following acute exercise and chronic anti-inflammatory properties, such as increased levels of regulatory T-cells (T_reg). Here, we hypothesize that acute exercise activates the kynurenine pathway and physical fitness is associated with proportions of circulating anti-inflammatory T_reg in older healthy women. Nineteen older healthy female participants (55 years (SD: ± 5.6)) completed a cardiopulmonary incremental exercise test (CPET) with spirometry on a bicycle ergometer until exhaustion with maximum oxygen uptake (VO₂max) as outcome. Blood samples were taken before (T0) and one minute after (T1) the CPET. Levels of tryptophan, serotonin and kynurenine were determined by enzyme-linked immunosorbent assays. Flow cytometry was used to identify proportions of T-cell subsets. Both, kynurenine (p = 0.003, d = 0.40) and the kynurenine/tryptophan ratio (p = 0.034, d = 0.48) increased significantly after acute exercise. Moreover, participants` VO₂max was strongly correlated with T_reg levels (p < 0.001, r = 0.689). This is the first study indicating a kynurenine pathway activation following acute exercise in older healthy women. The observed correlation between T_reg levels and VO₂max emphasizes a potential link between short-term upregulated kynurenine levels and longer-term anti-inflammatory properties of exercise. Future research is needed to clarify to what extend acute exercise-induced activations of the kynurenine pathway contribute to T_reg differentiation.

• The Kynurenine Pathway can be activated by acute exercise in older healthy women

• Higher VO₂max values are associated with increased levels of regulatory T-cells

• Both findings within one sample suggest a potential interaction between acute exercise-induced Kynurenine Pathway activation and chronically elevated anti-inflammatory capacity