An assessment of fractures, bone mineral density and habitual bone metabolism in female distance runners
thesisposted on 2022-09-07, 13:35 authored by Jennifer A. Higgins
Background Exercise is generally considered osteogenic, promoting the accrual of bone during growth and preventing its loss during ageing. However female distance runners (FDR) present a paradox to this consensus. Despite regular engagement in weight bearing activity, reports of low bone mineral density (BMD) are evident amongst up to 45% of FDR. Up to 60% of FDR report at least one previous stress fracture; an injury that occurs in response to repetitive sub-maximal loading. There is variation in the prevalence of low BMD and/ or fracture history within the extant literature. Understanding whether poor bone health poses a significant problem amongst FDR, through evaluation of its magnitude was an aim of this thesis. BMD is the recommended measure of bone health. But due to its static nature, it does not enable understanding of the mechanisms of exercise induced adaptation or maladaptation. BMD represents the net effect of formation and resorption processes over a period of time. Changes in BMD take time. These changes represent the net effect of bone metabolism, constituted of the formation and resorption processes, over that time period. Bone metabolism itself has been proposed as an independent risk factor for fracture. Elevated bone metabolic activity is associated with elevated fracture risk. Assessment of bone metabolism, using bone turnover markers (BTMs) may therefore provide a more appropriate index of current fracture risk. Bone metabolism, notably resorption, has been shown to increase in response to endurance exercise. This response is maintained for a number of hours. The effect of habitual exercise is less clear. In this thesis altered bone metabolism was proposed as a measure of current fracture risk amongst FDR. The central aim of the thesis was to examine whether FDR display altered bone metabolism relative to a non-athletic reference control (CON) population. Approach: Two study designs were used to address the central research questions. Females 18-35 y were recruited in both. The first assessed fracture history amongst 119 FDR using an online survey. Stress fracture and fragility fractures that occurred at any time up to the time of data collection were included. Fractures were deemed confirmed if they had been diagnosed by x-ray or magnetic resonance imaging (MRI). Of the 119 respondents 39 FDR also had BMD evaluated at the spine and hip using dual energy x-ray absorptiometry (DXA). A 7 day study was used to assess habitual bone metabolism in a cohort of FDR (n=7) who were compared to CON (n=11). Homeostatic bone metabolism was assessed using fasted, early morning blood samples collected at a standard within participant time. CTX and P1NP were measured. Total daily resorption was assessed from 24 h urine samples. uNTX, fDPD and fPYD were measured. Concurrently training was captured using training logs. Energy expenditure was assessed by Sensewear Pro3™ (SWP3). Menstrual status was assessed by self-report questionnaire. Body composition and BMD were assessed by DXA. A sub-group of FDR (n=5) also had energy expenditure assessed by double labelled water (DLW). In order to understand whether any observed effect was attributable to exercise, or was compounded or due to nutrient intake, dietary intake was also evaluated. The energy requirements of FDR were compared to those of CON. The validity of SWP3 was assessed through comparison of energy expenditure measured by DLW, assessed in 24 h epochs. The energy and nutrient adequacy of FDR was then compared based on dietary intake recorded using estimated food intake records (eFIR). Findings: Amongst 119 FDR 35% had a history of fracture. The sub-group (n=39) who had BMD assessed had a similar fracture history (36%). Prevalence of low BMD was 31%. However sensitivity of BMD to identify those with fracture history was only 14% suggesting that fracture risk is not adequately explained by BMD alone. Resorption (assessed by CTX) was 31% lower and formation (assessed by P1NP) 15% lower in FDR relative to CON. 24 h bone resorption (assessed by uNTX) was 37% greater in FDR. Menstrual dysfunction was evident amongst 86% (6/7) FDR and 0% of CON, indicative of low EA (LEA). However measured EA amongst FDR (45 ± 9 kcal.kgFFM-1), indicated that the FDR in this study addressed their 10% elevated energy requirements, incurred through a threefold greater exercise energy expenditure. FDR also consumed adequate (1474 ± 287 mg) calcium but only consumed 57% of the vitamin D recommendation. Conclusion: The greater total resorption displayed by FDR is indicative of an exercise induced response. However the lower homeostatic resorption and formation suggests the response is transient. Whether transient increases in resorption are necessary for osteogenesis or are harmful to bone is currently unclear. The lower homeostatic bone metabolism may be indicative of an adaptive response. But in the absence of an FDR cohort without menstrual dysfunction this finding must be interpreted with caution for now.