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The Impact of Morning versus Afternoon Exercise on Iron Absorption in Athletes.

PURPOSE
This study examined post-exercise 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 completed a 90 min running protocol (65% vV[Combining Dot Above]O2max) in the morning (AM), or the afternoon (PM), in a crossover design. An iron-fortified fluid labelled with stable iron isotopes (Fe or Fe) 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 pre-, immediately post-, and 3 h post- 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-modelling was used to analyse the data.


RESULTS
Overall, exercise significantly increased the concentrations of IL-6 (4.938 pg⋅mL; p=0.006), and Hepcidin-25 concentrations significantly increased 3 h following 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 pre- to 3 h post-run. Fractional iron absorption was significantly greater at breakfast following 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).While exercise resulted in increased concentrations of IL-6 and hepcidin, iron was best absorbed in the morning following exercise, indicating there may be a transient mechanism during the acute post-exercise window to promote iron absorption opposing the homeostatic regulation by serum hepcidin elevations.