["The Journal of Physiology, EarlyView. ", "\nAbstract figure legend Acute aerobic exercise induces LGALS1, FSTL1, OGN and C1QTNF3 in both skeletal muscle and adipose tissue, with LGALS1 and FSTL1 upregulation corresponding to increased circulating galectin‐1 and follistain‐like 1. Red and yellow brick bars represent the contribution of skeletal muscle and adipose tissue, respectively, integrating tissue size and gene induction. Fstl1 shows tissue‐mass‐dependent coupling in both depots (red bars: skeletal muscle, yellow bars: adipose tissue), while galectin‐1 reflects a muscle‐dominant, additive pattern. \n\n\n\n\n\n\n\n\n\nAbstract\nExercise‐induced secreted factors (exerkines) are proposed to coordinate systemic health benefits, yet their human tissue sources and the physiological rules governing transcript‐to‐serum coupling remain incompletely defined. We integrated multi‐tissue transcriptomics with matched serial serum profiling to identify exercise‐regulated exerkines and determine whether tissue mass moderates their systemic appearance. Sixteen healthy, sedentary young men performed a single treadmill bout calibrated to expend 300 kcal at 70–75% of maximum heart rate. RNA sequencing was performed on periumbilical subcutaneous adipose tissue, vastus lateralis skeletal muscle and whole blood collected before and immediately after exercise, with serial serum quantification up to 120 min post‐exercise. Acute exercise altered adipose and muscle transcriptomes, with a minimal whole‐blood response, and upregulated LGALS1 (galectin‐1), FSTL1 (follistatin‐like 1), OGN (osteoglycin) and C1QTNF3 (C1q and tumour necrosis factor‐related protein 3) in both tissues. Serum follistatin‐like 1 and galectin‐1 concentrations increased significantly immediately after exercise and during recovery. Despite robust transcript induction, OGN and C1QTNF3 did not translate into early circulating increases, indicating candidate‐specific kinetics and constraints beyond transcript abundance. Moderation models revealed divergent transcript‐to‐serum coupling: follistatin‐like 1 coupling was moderated by tissue mass in both muscle and adipose depots, whereas galectin‐1 followed a muscle‐dominant additive profile independent of mass scaling. These findings propose galectin‐1 and follistatin‐like 1 as human exerkines and indicate that body composition regulates systemic exerkine signatures after exercise, providing a mechanistic framework for understanding physiological variability in exercise‐induced adaptations.\n\n\n\n\n\n\n\n\n\nKey points\n\nExercise releases circulating factors that may contribute to health benefits, but the human tissues responsible for their production remain unclear.\nRNA sequencing was performed in human skeletal muscle, subcutaneous adipose tissue and whole blood, with targeted serum protein quantification over 2 h after a single treadmill bout.\nExercise increased LGALS1 and FSTL1 transcripts in skeletal muscle and adipose tissue, with concurrent increases in their circulating protein products immediately post‐exercise and during recovery, whereas OGN and C1QTNF3 transcripts increased robustly without an early post‐exercise elevation in their circulating protein products.\nBody composition moderated transcript‐to‐serum coupling: FSTL1 (follistatin‐like 1) showed mass‐sensitive coupling, whereas LGALS1 (galectin‐1) displayed a muscle‐dominant additive profile.\nGalectin‐1 and follistatin‐like 1 emerge as exercise‐induced exerkines whose circulating responses vary with body composition, supporting their prioritization as candidate mediators of exercise‐related health benefits and a framework for interpreting physiological heterogeneity in adaptation.\n\n\n"]