Athletes are required to spend a prolonged period of time at moderate altitude (2000-3000 m (Bartsch et al., 2008)) in order to accumulate a sufficient total hypoxic "dose" to stimulate erythropoiesis (Garvican et al., 2012). Indeed, current guidelines for simulated altitude exposure suggest that athletes should spend ~ 14 h-day-1 at 3000 m for 3 weeks (totalling ~ 300 hours of exposure) in order to observe a mean increase in haemoglobin mass ([Hb.sub.mass]) of 3-5% (Saunders et al., 2013). Likewise, a similar response may be achieved when 300 hours of exposure is accumulated at natural altitude (Garvican et al., 2012). Closer investigation of the time course of erythropoietic adaptation indicates that [Hb.sub.mass] increases approximately 1% per 100 hours of exposure at simulated or natural altitude of 2300 to 3000 m (Clark et al., 2009; Garvican et al., 2012). However, achieving such altitude exposures is often difficult for athletes due to limited availability of appropriate simulated or natural altitude, decreased living/sleeping and training quality (especially at altitudes above 3000 m), and interference with training or competition schedules (Neya et al., 2013).
To date, several models of hypoxic exposure have been established including; live high/train high (LHTH); live high/train low; live low/train high (i.e. intermittent hypoxic exposure during training) or intermittent hypoxic exposure at rest (McLean et al., 2014; Millet et al., 2013; Millet et al., 2010). The possible physiological and performance benefits of each of these modes of altitude exposure differ considerably with contributing factors including, normobaric vs hypobaric exposure (Saugy et al., 2014), the total elevation of exposure and the duration of exposure (Gore et al., 2013). Indeed, clear relationships exist between the dose of altitude exposure (measured as total duration (Bonetti and Hopkins, 2009; Gore et al., 2013), increases in [Hb.sub.mass] and/or associated improvements in aerobic capacity (Schmidt and Prommer, 2010).
Natural low altitude venues attract a considerable number of athletes each year with the common belief that even 'low' altitude (
It is plausible that improvements in performance following altitude exposure may be associated with altered training or the favourable belief that altitude training has been successful, rather than haematological alterations. Indeed, it has been suggested that placebo effects and/or a better training environment (i.e. high-quality training camps, increased focus on training and recovery, less distractions, change of venue, and people to train with on a consistent basis) may be responsible for some of the improvements in performance observed within altitude exposure research (Saunders et al., 2009; 2010; Siebenmann et al., 2012). Clearly, further research is warranted in order to examine the possible haematological alterations of low altitude exposure in order to assess the effectiveness of such exposure. As such, the aim of the present study was to examine the effects of 3 weeks of live high, train high exposure (LHTH) at low natural altitude (i.e. Perisher Valley, 1800 m) on [Hb.sub.mass], red blood cell count and iron profile in elite distance runners.
Sixteen elite or well-trained male and female distance runners were recruited from national and state sporting organisations and allocated into two groups (Table 1). These two groups comprised of participants that slept and trained at low altitudes (LHTH, n = 8) or remained near sea level for a period of 3 weeks (CONTROL, n = 8). All participants were provided with the procedures and risks associated with their participation in this study. Prior to data collection, written informed consent was obtained in accordance with the institution's Human Research Ethics Committee. All athletes were in a "pre-competition" phase meaning they were preparing to race in national and international track races following the winter build-up period. Therefore, all runners had significant prior aerobic conditioning, their training volume was high and included both threshold based interval sessions and some race specific interval sessions. Throughout the study, training intensity, duration and frequency were controlled by external coaches but athletes and coaches were asked not to substantially alter their training schedules during the study period.
The LHTH group, lived at a natural altitude of 1800 m (Perisher Valley, New South Wales, Australia) and trained at altitudes of 1700 to 2200 m (Snowy Mountains, Australia). Twice a week, these athletes also descended to 1000 m to perform high intensity training sessions (~2h) on a synthetic athletics track or running trails. The CONTROL group lived and trained near sea level for the duration of the study (Canberra, 600 m). All athletes were supplemented with oral iron (Ferro-Grad C, Abbott Laboratories, 105g elemental iron) on a daily basis to ensure any erythropoietic adaptations were not compromised by insufficient iron availability. At baseline, after two weeks and again at completion of the three-week intervention, haemoglobin mass ([Hb.sub.mass]) was assessed and a venous blood sample was taken from all participants.
[Hb.sub.mass] was measured using the 2-min carbon monoxide (CO) rebreathing method as described by Schmidt and Prommer (2005)...