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% vVO_2max) 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.