The impact stress of heel strike during running has been identified as the largest contributor to the destruction of red blood cells (hemolysis), as a result of a force dependent relationship between heelstrike1,4. Although it is possible that the hemolysis induced by the impact stress of running may have adverse affects on the iron status of an athlete, it is questionable as to whether a single hemolytic episode will cause clinically significant iron loss3. It has been suggested that daily, or twice-daily hemolytic episodes during a block of hard training may have a cumulative effect on exercise-induced hemolysis and iron status, which could potentially put athletes at a significant risk of developing an iron deficiency3. To date however, no research has looked at the effect of consecutive running sessions on the degree of hemolysis incurred by endurance trained runners. Methods Ten well trained male endurance runners completed two experimental trials in a randomised, counterbalanced order. The two trials included (a) a one running session trial (T1) including 10 x 1 km interval (INT) repeats at 90% peak VO2 velocity, and (b) a two running session trial (T2), comprising a continuous 10 km run at 70% peak VO2 velocity (LSD), and a 10 x 1 km interval run at 90% peak VO2 velocity (INT) completed 12 h later. Venous blood samples were collected and analysed for free hemoglobin (Hb) and serum haptoglobin (Hp) as markers of hemolysis. Additionally, Interleukin-6 (IL-6) and urinary hepcidin levels were also assessed. Results The running velocity, average 1 km split times, heart rates and blood lactates were all significantly faster and higher in the T2INT when compared to the T2LSD (p<0.05). There were no differences between the T2INT and the T1INT sessions (p>0.05). Free Hb levels showed a significant time (p=0.0001) but no trial effect (p=0.507) between T1 and T2. The time effect showed that the free Hb levels were significantly increased at the completion of the T2INT and the T1INT trials (p=0.013 and p=0.002, respectively), and also at the conclusion of the T2LSD run (p=0.003), when compared to pre-exercise levels. The increased post-T2LSD free Hb levels had decreased significantly by the commencement of T2INT (p=0.042), approximately 12 h later, but were still significantly higher than the pre-T2LSD values. Finally, the post-T2LSD free Hb levels were significantly lower than those recorded post-T2INT (p=0.013). Serum Hp showed a significant time (p=0.003) and trial effect (p=0.001) between T1 and T2. The time effect showed that the post-T1INT serum Hp levels were significantly lower than pre-T1INT (p=0.037). Within the T2 trial, the serum Hp levels fell significantly at the conclusion of the T2LSD (p=0.006), and remained at the same levels 12 h later (pre-T2INT), still significantly lower than pre-T2LSD (p=0.011). The post-T2INT serum Hp levels then fell significantly further from pre-T2INT levels (p=0.002), being also significantly lower than those recorded post-T2LSD (p=0.001). Additionally, both the pre-T2INT and post-T2INT serum Hp levels were significantly lower than those recorded at the pre-T1INT and post-T1INT time points (p=0.034 and p=0.027, respectively). Discussion/Conclusion The T2INT session resulted in significantly greater increases to free Hb and decreases to serum Hp than those seen after the T2LSD run. This supports the notion that exercise intensity may have an effect on the hemolytic response to exercise2. Furthermore, the T2INT decreases in serum Hp was also significantly lower than that seen after the T1INT trial. It has been suggested that twice-daily hemolytic episodes could have a cumulative effect in athletes, and that this may eventually begin to influence iron stores4. This theory is supported by the serum Hp data of the current investigation. Furthermore, the lack of difference between the free Hb levels recorded post-INT between the T1 and T2 conditions may also be explained by the greater decreases in serum Hp after T2INT, since it is likely that this corresponds to more Hp-Hb complexes being created to deal with the cumulative stress of multiple running sessions. These cumulative increases in hemolysis must be considered by coaches working with athletes that struggle to maintain a healthy iron balance, with the possibility of non-weight-bearing, cross-training alternatives in place of a second running session on the same day, to avoid further taxing already compromised iron stores.
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