The impact of morning versus afternoon exercise on iron absorption in athletes

Purpose This study examined postexercise inflammatory, hepcidin, and iron absorption responses to endurance exercise performed in the morning versus the afternoon. Methods Sixteen endurance-trained runners (10 male, 6 female) with serum ferritin (sFer) < 50 μg·L−1 completed a 90-min running protocol (65% vV˙O2max) in the morning (am), or the afternoon (pm), in a crossover design. An iron-fortified fluid labeled with stable iron isotopes (57Fe or 58Fe) was administered with a standardized meal 30 min following the exercise and control conditions during each trial, serving as a breakfast and dinner meal. Venous blood samples were collected before, immediately after, and 3 h after the exercise and control conditions to measure sFer, serum interleukin-6 (IL-6), and serum hepcidin-25. A final venous blood sample was collected 14 d after each trial to determine the erythrocyte iron incorporation, which was used to calculate iron absorption. Linear mixed-modeling was used to analyze the data. Results Overall, exercise significantly increased the concentrations of IL-6 (4.938 pg·mL−1; P = 0.006), and hepcidin-25 concentrations significantly increased 3 h after exercise by 0.380 nM (P < 0.001). During the pm trial, hepcidin concentrations exhibited diurnal tendency, increasing 0.55 nM at rest (P = 0.007), before further increasing 0.68 nM (P < 0.001) from prerun to 3 h postrun. Fractional iron absorption was significantly greater at breakfast after the am run, compared with both the rested condition (0.778%; P = 0.020) and dinner in the am run trial (0.672%; P = 0.011). Conclusions Although exercise resulted in increased concentrations of IL-6 and hepcidin, iron was best absorbed in the morning after exercise, indicating there may be a transient mechanism during the acute postexercise window to promote iron absorption opposing the homeostatic regulation by serum hepcidin elevations.