The Impact of Nutrition and Training Practices on Iron and Bone Status in Athletes

Athletes utilise various training and nutrition approaches to optimise training adaptation and competition performance. An important consideration is the potential impact of these practices on athlete health, and the subsequent effects on availability for training and competition and their longevity in the sport. Indeed, regular physical activity and mechanical loading is well-known to have significant benefits for bone health. However, excessive training load combined with insufficient recovery, including inadequate macro- and micronutrient support, places an athlete at increased risk for bone stress injuries. In particular, iron deficiency is also highly prevalent in athletes, especially in those with high training loads, and has been associated with an increased risk of low bone mineral density (BMD) in clinical populations. Considering the prevalence of and possible associations between bone stress injuries and iron deficiency, the studies comprising this thesis aimed to explore the impact of common training and nutrition practices on bone and iron metabolism in athletes.

Study 1 (described in Chapter 3): Bone mineral density and bone turnover marker values in a convenience sample of elite endurance athletes
Study 1 aimed to (1) examine BMD site discordance within and between sports, (2) investigate athlete bone turnover marker (BTM) ranges, and (3) explore possible links between bone and iron metabolism. A cross-sectional analysis of baseline data from athletes who participated in various studies within our research group over the period November 2015-January 2022 was conducted. Specifically, studies with baseline dual-energy x-ray absorptiometry (DXA) and where iron and bone markers were measured via venous blood sample collection prior to and following exercise were included. Significant discordance was shown between and within sports with lumbar spine BMD of rowers being higher than all other sports (p < 0.0001) and higher than the proximal femur site within rowers (p = 0.002). By contrast, racewalkers and runners had higher proximal femur than lumbar spine BMD (p  0.001). Age significantly contributed to the variance in both fasting carboxyterminal telopeptide (CTX) and procollagen-1 N-terminal peptide (P1NP) with higher concentrations observed in those younger than 25 years old compared with older athletes (p  0.001). In addition to age, assay method and resting interleukin-6 (IL-6) further explained fasting CTX concentrations (marginal R2 = 0.49). Post-exercise IL-6, however, was not significant in explaining post-exercise CTX at 1 h (p = 0.1). Finally, significant medium-strong correlations were present between fasting CTX and resting ferritin and hepcidin (p < 0.001).

Study 2 (described in Chapter 4): Short-term carbohydrate restriction impairs bone formation at rest and across prolonged exercise to a greater degree than low energy availability
Study 2 aimed to investigate the differential impact of energy and carbohydrate manipulation on BTMs at rest and across exercise, recognising the role of nutrition in bone adaptation to frequent mechanical loading. In a parallel group design, 28 elite racewalkers completed two 6-day phases. In the Baseline phase, all athletes adhered to a high carbohydrate/high energy availability diet (CON). During the Adaptation phase, athletes were allocated to one of three dietary groups: CON, low carbohydrate/high fat with high energy availability (LCHF), or low energy availability (LEA). At the end of each phase, a 25 km racewalk was completed, with venous blood taken fasted, pre-exercise, and 0, 1, 3 h post-exercise to measure CTX, P1NP, and osteocalcin (carboxylated, gla-OC; undercarboxylated, glu-OC). Following Adaptation, LCHF showed decreased fasted P1NP (~26%; p<.0001, d=3.6), gla-OC (~22%; p=.01, d=1.8), and glu-OC (~41%; p=.004, d=2.1), which were all significantly different to CON (p<.01), whereas LEA demonstrated significant, but smaller, reductions in fasted P1NP (~14%; p=.02, d=1.7) and glu-OC (~24%; p=.049, d=1.4). Both LCHF (p=.008, d=1.9) and LEA (p=.01, d=1.7) had significantly higher CTX pre- to 3 h post-exercise but only LCHF showed lower P1NP concentrations (p<.0001, d=3.2). All markers remained unchanged from Baseline in CON.

Study 3 (described in Chapter 5): Factors influencing the hepcidin response to exercise: an individual participant data meta-analysis
Study 3 aimed to expand on the broader literature findings showing similar responses of bone and iron markers across exercise and in response to certain dietary interventions. Here, the aim was to amalgamate multiple small studies to investigate how both athlete and exercise session characteristics influence IL-6 and hepcidin concentrations through an individual participant data meta-analysis. Following a systematic review of the literature, a one-stage meta-analysis with mixed-effects linear regression, using a stepwise approach to select the best-fit model, was employed. Results demonstrated that exercise is associated with a 1.5- to 2.5-fold increase in hepcidin concentrations, with pre-exercise hepcidin concentration accounting for ~44% of the variance in 3 h post-exercise hepcidin concentration. Although collectively accounting for only a further ~3% of the variance, absolute 3 h post-exercise hepcidin concentrations appear higher in males with lower cardiorespiratory fitness and higher pre-exercise ferritin levels. On the other hand, a greater magnitude of change between the pre- and 3 h post-exercise hepcidin concentration was largely attributable to exercise duration (~44% variance) with a much smaller contribution from VO2max, pre-exercise ferritin, sex, and post-exercise IL-6 (~6% combined). Although females tended to have a lower absolute 3 h post-exercise hepcidin concentration (1.4 nmol.L-1, [95% CI -2.6, -0.3], p=0.02) and 30% less change (95% CI [-54.4, -5.1], p=0.02) than males, with different explanatory variables being significant between sexes, sample size discrepancies and individual study design biases preclude definitive conclusions.

Study 4 (described in Chapter 6): Sequential submaximal training in elite male rowers does not result in amplified increases in interleukin-6 or hepcidin
Study 4 addressed the lack of investigation of the effects of repeated training bouts completed in close succession on the IL-6 and hepcidin responses to exercise. Although this study design was set up to investigate the effect of calcium supplementation on CTX concentrations across the same period, it provided a unique opportunity to explore iron-calcium reciprocity and determine whether an intervention (i.e., calcium prior to exercise) designed to limit the impact of exercise on bone may result in an adverse effect on another body system. In a randomised, crossover design, 16 elite male rowers completed two trials, a week apart, with either high (1000 mg) or low (<50 mg) calcium pre-exercise meals. Each trial involved two, submaximal 90 min rowing ergometer sessions, 2.5 h apart, with venous blood sampled at baseline, pre-exercise, and 0, 1, 2 and 3 h after each session. Peak elevations in IL-6 (~7.5-fold, p<.0001) and hepcidin (~3-fold, p<.0001) concentrations relative to baseline were seen at 2 and 3 h after the first session (EX1) respectively. Following the second session (EX2), concentrations of both IL-6 and hepcidin remained elevated above baseline, exhibiting a plateau rather than an additive increase (2 h post-EX1 vs 2 h post-EX2, p=1.00). Pre-exercise calcium resulted in a slightly greater elevation in hepcidin across all timepoints compared to control (p=.0005), however no effect on IL-6 was evident (p=.27).

Summary and future directions
The research studies included in this thesis aimed to explore the impact of certain nutrition and training practices on bone and iron status in athletes and the potential interplay between the two systems. In summary, our findings demonstrate that: (1) Fasting CTX and P1NP concentrations decrease with age and, although limited by the lack of general population data, may be higher in athletes, (2) Resting IL-6 concentrations may contribute to fasting CTX concentrations, (3) Duration of exercise contributes significantly to post-exercise IL-6 concentrations and the magnitude of change in hepcidin concentrations from pre- to 3 h post-exercise, (4) Twice-daily training with a short recovery results in an elevated plateau in IL-6 and hepcidin concentrations, when supported by adequate energy and CHO consumption, (5) Short-term adherence to a ketogenic diet may be more detrimental to at-rest and across-exercise bone formation than the same period of LEA, yet bone turnover is maintained with a high EA/high CHO diet, (6) Significant BMD measurement site discrepancies exist between and within sports, raising the issue of the impact of mechanical loading on detecting at-risk athletes and the potential need for additional tools (e.g., BTMs) and protocols. Outcomes from this thesis support current recommendations that athletes should aim to achieve adequate energy and CHO availability to minimise potential detriments to bone turnover balance, at least in the short term. Furthermore, this work supports previous recommendations of morning, prior to or within 30 min following the first exercise session, iron consumption in order to maximise absorption. Invitations to expand the findings of this work extend to establishing a robust database of normative (and even sports-specific) values for athlete BTM and areal BMD ranges and employing longitudinal studies to assess the impact of BTM and iron status perturbations on BMD and bone architecture.

This work © 2024, Nikita Fensham. All Rights Reserved.