Because iron is essential for oxygen transport to working muscles, it plays an important role in energy production during exercise [1
]. Iron deficiency leads to a decreased exercise capacity during endurance events [2
]. Among endurance athletes, iron deficiency is prevalent as a result of their prolonged training and repetitive ground impact [4
]. Several physiological mechanisms have been proposed to explain the impaired iron status, including gastrointestinal bleeding [5
], hemolysis [6
], lack of iron in the daily diet [7
], and loss via sweat [8
]. However, the detailed mechanisms underlying exercise-induced iron deficiency among athletes remain unclear.
Hepcidin, a 25-amino-acid peptide hormone, is a key mediator of iron homeostasis [9
], and it may represent another mechanism of iron deficiency in response to exercise training [10
]. Hepcidin regulates iron balance by binding to the iron export protein ferroportin, which inhibits iron efflux from enterocytes, hepatocytes, and macrophages [13
]. Hepcidin expression is upregulated by increased iron stores [14
] and inflammation [15
]. Because pro-inflammatory cytokines, such as interleukin-6 (IL-6), also stimulate hepcidin production, sustained inflammation may increase hepcidin, resulting in iron deficiency and anemia [16
Strenuous exercise causes inflammation, and a marked increase in IL-6 levels is observed after prolonged exercise [18
], which stimulates hepcidin production. Serum hepcidin levels are elevated around 3–6 h after a single bout of exercise [11
]. However, information about the cumulative effect of daily training on hepcidin levels in female athletes is still limited. Although an increased prevalence of iron depletion was found previously during a competitive season in endurance athletes [22
], whether an increased training volume augments hepcidin levels in female athletes has not been fully examined.
Thus, the purpose of the present study was to compare serum hepcidin levels and iron metabolism between two periods involving different training volumes in female long-distance runners. We hypothesized that increased training would elevate serum hepcidin levels.
The major finding in this study was that serum hepcidin levels were elevated significantly with an increase in the monthly running distance (INT) in female long-distance runners.
Several physiological mechanisms have been suggested to explain exercise-induced impairment of iron status. In addition to the typically reported factors, excessive physical activity may aggravate the iron status via increased hepcidin levels [12
]. In the present study, we observed that elevated serum hepcidin levels were associated with increased training intensity. This is consistent with results indicating that serum hepcidin levels were elevated in young tennis players during the tournament season [28
]. Dzedzej et al. reported that hepcidin and IL-6 levels were significantly higher in professional basketball players than in non-athletes at the end of the season [29
]. Moreover, seven days of running training increased basal urinary hepcidin levels significantly [30
]. Thus, increased training seems likely to augment hepcidin secretion. In the present study, well-trained female long-distance runners (top level of university runners in Japan) were recruited, and they were regularly involved in daily training twice a day, approximately 18 h per week. Since most elite athletes perform more than one training session within a day, it is considered that the present findings can be widely applicable for athletes with iron deficiency in different types of endurance events.
In previous studies involving cell culture [15
] and animals [32
], IL-6 stimulated hepcidin biosynthesis. Pedersen and Hoffman-Goetz also demonstrated that strenuous exercise induces a marked increase in IL-6 levels [4
]. The detailed mechanism of the exercise-induced hepcidin response is not fully understood, but previous studies revealed that acute exercise initially increases IL-6 and subsequently hepcidin levels 3 h after the exercise [11
]. Unfortunately, we were not able to determine the influence of exercise-induced IL-6 elevation on hepcidin responses. Based on prevalent reports that demonstrated that exercise acutely increased IL-6 [18
], we evaluated plasma IL-6 levels at baseline only, not after exercise. However, exercise-induced acute elevation of IL-6 during daily training may cause sustained elevation of hepcidin with a concomitant increase in the risk of iron deficiency.
In the present study, 31% (LOW) and 37% (INT) of subjects were classified as iron-deficient (serum ferritin level < 20 ng/mL). Ferritin levels were reduced in female endurance athletes [22
] and professional soccer players [33
] during the competitive seasons. Somewhat surprisingly, a significant positive correlation was observed between serum hepcidin and ferritin levels during both LOW and INT. However, caution should be taken with the interpretation of this correlation, because acute inflammation increases ferritin levels [34
], and increased ferritin levels due to sustained inflammation might augment hepcidin levels to maintain iron homeostasis [35
]. Recently, Dzedze et al. suggested that baseline hepcidin levels were correlated significantly with serum ferritin levels in basketball players during the competitive season [29
]; however, the relationship was not evident at the end of the season. Serum ferritin is commonly considered an indicator of iron status; elevated ferritin levels during an intensified training period may reflect sustained inflammation.
A strength of the present study was the dietary assessment conducted, whereas most previous studies failed to present detailed information regarding daily dietary intake. Nuviala et al. reported that the energy intake in female athletes with iron deficiency (serum ferritin level <20 ng/mL) was 2176 ± 530 kcal/day. In the present study, the daily energy intake was 2140 ± 130 kcal/day (LOW) and 2318 ± 343 kcal/day (INT) [36
]. Unfortunately, we were not able to evaluate energy availability due to the lack of data on energy expenditure during training. In the present study, serum hepcidin levels tended to be higher in subjects with iron supplementation. Although the efficacy of iron supplementation on performance improvement is not fully evident, iron overload has been reported in competitive athletes [37
]. A high dose of iron supplementation may stimulate hepcidin production to maintain iron homeostasis [38
]. In the present subjects, the average iron intake was 14.4 ± 0.4 mg/day (LOW) and 17.3 ± 1.3 mg/day (INT), respectively. These values were above or close to previously reported values (e.g., 14 mg/day) [39
]. Considering that a recommendation for iron intake in female endurance runners is 18 mg/day [41
], the amount of iron intake in the present subjects was moderate.
Several limitations should be considered in interpreting these results. First, we did not have a control group (group not in training), because all subjects were competitive athletes. Second, energy expenditure during training was not assessed. Pasiakos et al. reported that changes in hepcidin levels after a four-day military training regimen were related to the total daily energy expenditure [42
]. Thus, 24 h energy expenditure data will be of value in determining whether lower energy availability is a predominant factor in elevated hepcidin levels during the intensified training period.
From a practical viewpoint, the present findings may suggest a cumulative effect of daily endurance training on hepcidin elevation in well-trained female endurance athletes. Since hepcidin levels were associated with increased training volume (running distance), athletes and coaches should pay attention to iron status during intensified training periods. Future studies are required to confirm whether nutrition intervention (e.g., increased carbohydrate intake, appropriate energy availability) will attenuate hepcidin elevation and the risk of iron deficiency in female endurance athletes.