The relative energy deficiency in sport (RED-S) syndrome was recently developed in recognition that male athletes display evidence of impaired physiological function that may be related to low energy availability (Mountjoy et al., 2014). Jockeys are unique among professional athletes in that they have to make weight daily, and to do so, they commonly undertake periods of food deprivation (Wilson et al., 2014). In this regard, we (Wilson et al., 2013a, 2015a, 2015b) and others (Dolan et al., 2012; Greene et al., 2013; Jackson et al., 2017; Leydon & Wall, 2002; Poon et al., 2017; Waldron-Lynch et al., 2010; Warrington et al., 2009) have consistently reported that male flat jockeys present with low bone mineral density (BMD), with Z-scores often lower than –1. Such low bone densities are often considered to be due to a combination of nutritional factors, including low energy availability and suboptimal micronutrient intake (Dolan et al., 2011; Greene et al., 2013; Martin et al., 2017; Wilson et al., 2013a, 2014), as well as a potential loss of calcium (Barry et al., 2011) due to the forced daily sweating that is often utilized as a technique to achieve daily riding weight (Warrington et al., 2009; Wilson et al., 2014). As such, low BMD is a continual cause of concern for jockey athlete welfare, considering the increased risk of fracture in the event of a fall (Dolan et al., 2012; Jackson et al., 2017; Wilson et al., 2012, 2015b).
Despite the well-documented reports of low BMD, it remains questionable if jockeys are athletes who truly exhibit symptoms of RED-S. Indeed, measured resting metabolic rate (RMR) does not differ from predicted RMR (Cunningham & Johns, 1980), either before (Wilson et al., 2015a, 2015b) or after dietary interventions (Wilson et al., 2015b). Furthermore, when considering the potential impact of low energy availability on endocrine function, it is noteworthy that male flat jockeys display testosterone, insulin-like growth factor 1, and sex hormone–binding globulin values all within normal ranges (Wilson et al., 2015b). Previous reports of low energy availability have also been largely ascertained from analysis of self-reported food diaries (Dolan et al., 2011; Leydon & Wall, 2002; Wilson et al., 2013a, 2013b), a method often critiqued for its unreliability (Braakhuis et al., 2003; Dhurandhar et al., 2015) and underreporting (Poslusna et al., 2009). Moreover, energy intakes of jockeys are significantly higher when food intake has been monitored via a wearable camera as opposed to the traditional food diary approach (O’Loughlin et al., 2013). Further evidence for likely underreporting of energy intake is also provided by our recent assessment of energy expenditure (via doubly labeled water) of male flat jockeys. Indeed, although self-reporting of energy intake was estimated at approximately 1,500 kcal/day, energy expenditure was calculated as 2,500 kcal/day, yet body mass did not significantly change during a 4-month data collection period (Wilson et al., 2018).
When taken together, it is therefore difficult to ascertain if the low BMD consistently observed in jockeys is in fact due to low energy availability and/or the lack of a consistent osteogenic stimulus arising from years of nonweight-bearing activity due to riding. Regardless of the precise contribution of each of the aforementioned factors, it could therefore be hypothesized that symptoms of RED-S should progressively worsen in accordance with the years of race riding. With this in mind, the aim of this study was to assess both measured and predicted RMR as well as hip and lumbar spine BMD in a cohort of apprentice and senior professional male flat jockeys.
Methods
Subjects
In total, 31 male professional flat jockeys currently race riding in Great Britain provided informed written consent to participate in this study. Apprentice jockeys (n = 17) were classified as those jockeys who were race riding at the time of the study with a “claim weight allowance” of 3, 5, or 7 lb. This “claim” is a reduction of weight from the allocated competition race weight for newly licensed professional riders who had not ridden a specified number of race winners, in order to incentivize racehorse trainers with a more favorable racing weight (lower), thus providing more chances for these riders. The senior jockey group (n = 14) consisted of those jockeys who had reached a specified total of winners, negating their “claim.” This group included a 21-year-old jockey who had been successful in a comparably short race riding career and had therefore reached the senior categorization in a relatively short time span. At the time of the study, none of the jockeys were taking any prescribed medication or nutritional supplements, though three jockeys (all senior jockeys) were smokers. The study received ethical approval from the National Research Ethics Service. A comparison of age, race riding experience, and anthropometrical characteristics is shown in Table 1.
Comparison of Age, Race Riding Experience, Anthropometric Characteristics, and Hydration Status (UO) Between Apprentice and Senior Jockeys
| Group | |||
|---|---|---|---|
| Apprentices (n = 17) | Seniors (n = 14) | t test (p value) | |
| Age (years) | 19 ± 2 (17–24) | 32 ± 7 (21–49) | .01* |
| Race riding experience (years) | 3 ± 2 (1–8) | 16 ± 7 (5–33) | .01* |
| Height (cm) | 170 ± 5 (162–177) | 166 ± 5 (156–176) | .04* |
| Body mass (kg) | 56.2 ± 2 (46.6–60.4) | 56.4 ± 3 (51.2–62.9) | .83 |
| Body fat (%) | 13.7 ± 2.6 (7.6–18.5) | 12.5 ± 1.9 (9–15.5) | .14 |
| Fat mass (kg) | 7.5 ± 1.7 (3.9–10.4) | 6.8 ± 1.4 (4.9–10) | .22 |
| Fat-free mass (kg) | 46.4 ± 2 (44–51.3) | 45.7 ± 3.1 (40.8–51.8) | .48 |
| UO (mOsmol/L) | 816 ± 219 (200–1,080) | 704 ± 310 (200–1,090) | .31 |
Note. Data are presented as means ± SD (range). UO = urine osmolality.
*Significant difference, p < .05.
Design
In a cross-sectional design, both apprentice and senior jockeys (matched for body mass, fat mass, and fat-free mass) were assessed for both RMR and hip and lumbar spine BMD.
Experimental Procedures
After arriving in the laboratory in an overnight fasted state, jockeys were assessed for hydration status, BMD, and RMR. Hydration status was assessed from a midflow urine sample by measuring urine osmolality using a handheld refractometer (ATAGO U.S.A., Inc., Bellevue, WA). Jockeys were then measured for height and weight (Seca, Hamburg, Germany) wearing shorts, and they underwent a measure of whole-body composition, hip bone density, and lumbar spine bone density using a dual-energy X-ray absorptiometry scan (Hologic, Manchester, UK) for classification of Z-scores, matched for age, sex, and ethnicity. First, jockeys were asked to lie in a supine position and had their left foot affixed with Velcro (Middlewich, UK) to a Perspex (Leeds, UK) triangular platform to invert the head of the left femur for the measurement of hip bone density. Second, a box was placed under the popliteal crease of both knees of each jockey at ∼90° for the assessment of lumbar bone density. Finally, an assessment of full-body composition was undertaken in the supine position with inverted feet secured with Micropore surgical tape (Nexcare, Bracknell, UK) to allow for greater analysis of the neck of the femur. All measurements were performed within 12 min. Jockeys were then required to have RMR measured in a supine position using indirect calorimetry (Metalyser, Thatcham, UK). Jockeys were required to lie down for an initial 15 min before testing to allow for the dissipation of movement from the dual-energy X-ray absorptiometry analysis to the metabolic unit. Data were then collected for a further 30 min using the protocol described previously (Wilson et al., 2015a, 2015b).
Statistical Analysis
All data were analyzed using SPSS Statistics for Windows (version 22.0; IBM, Warwick, UK). Data were checked for normality, and independent t tests were used to compare data between apprentice and senior jockeys, as well as to compare measured RMR versus predicted RMR (Cunningham, 1980). Correlations between years of race riding and hip/lumbar spine BMD were made using Pearson’s correlation coefficient to ascertain the linearity between the two specific variables. All comparison data were reported as means (SD), and statistical significance was set at the level of p ≤ .05, with R2 values reported for correlation coefficient scores.
Results
Overview of Baseline Characteristics
A comparison of age, racing experience, and anthropometric characteristics between apprentice and senior jockeys is shown in Table 1. Apprentice jockeys were significantly younger and had fewer years of race riding experience than senior jockeys. Although apprentice jockeys were significantly taller than senior jockeys, there were no significant differences in body mass, fat mass (both absolute and percent), and fat-free mass between populations. Additionally, urine osmolality was not significantly different between apprentice and senior jockeys.
Resting Metabolic Rate
A comparison of RMR between apprentice and senior jockeys is shown in Figure 1. There was no significant difference (p = .48) in RMR between apprentice (1,459 ± 161 kcal/day) and senior jockeys (1,501 ± 165 kcal/day; see Figure 1a). In addition, measured RMR did not significantly differ from predicted RMR in either apprentice (1,459 ± 161 vs. 1,520 ± 44 kcal/day; p = .18) or senior jockeys (1,501 ± 165 vs. 1,505 ± 70 kcal/day; p = .92; see Figure 1b and 1c, respectively).
—(a) Measured RMR of apprentice and senior jockeys, (b) measured versus predicted RMR in apprentice jockeys, and (c) measured versus predicted RMR in senior jockeys. RMR = resting metabolic rate.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 28, 4; 10.1123/ijsnem.2017-0371
—(a) Measured RMR of apprentice and senior jockeys, (b) measured versus predicted RMR in apprentice jockeys, and (c) measured versus predicted RMR in senior jockeys. RMR = resting metabolic rate.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 28, 4; 10.1123/ijsnem.2017-0371
—(a) Measured RMR of apprentice and senior jockeys, (b) measured versus predicted RMR in apprentice jockeys, and (c) measured versus predicted RMR in senior jockeys. RMR = resting metabolic rate.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 28, 4; 10.1123/ijsnem.2017-0371
Hip and Lumbar Spine BMD
There was no significant difference in either hip Z-score (−0.9 ± 1.1 vs. −0.8 ± 0.7; p = .84) or lumbar spine Z-score (−1.3 ± 1.4 vs. −1.5 ±1.0; p = .70) between apprentice and senior jockeys, respectively (see Figure 2a and 2b). Years of race riding did not display any significant correlation with either hip (R2 = .01; p = .72) or lumbar spine Z-score (R2 = .04; p = .29; see Figure 2c and 2d).
—(a) Hip Z-score and (b) lumbar spine Z-score of apprentice and senior jockeys. (c) Hip Z-score and (d) lumbar spine Z-score plotted against years of race riding experience.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 28, 4; 10.1123/ijsnem.2017-0371
—(a) Hip Z-score and (b) lumbar spine Z-score of apprentice and senior jockeys. (c) Hip Z-score and (d) lumbar spine Z-score plotted against years of race riding experience.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 28, 4; 10.1123/ijsnem.2017-0371
—(a) Hip Z-score and (b) lumbar spine Z-score of apprentice and senior jockeys. (c) Hip Z-score and (d) lumbar spine Z-score plotted against years of race riding experience.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 28, 4; 10.1123/ijsnem.2017-0371
Discussion
Despite consistent reports of low BMD in male jockeys, it is not yet known if male jockeys exhibit true symptoms of the RED-S. Given the rationale that low energy availability is a contributing cause to low BMD, the aim of this study was to test the hypothesis that both hip and lumbar spine BMD should progressively worsen in accordance with the years of riding. Importantly, we demonstrate no differences in hip or lumbar spine Z-score between apprentice and senior jockeys and also observed no associations between years of race riding and BMD. In addition, RMR was not different between jockey cohorts, whereas measured RMR was also not different from predicted RMR in either apprentice or senior jockeys.
A well-documented negative consequence associated with RED-S is low BMD (Mountjoy et al., 2014). Given that jockeys have to make weight daily, it has therefore been suggested that jockeys are an athletic population especially sensitive to exhibit symptoms of RED-S, including impaired BMD (Wilson et al., 2014). Confirming previous data from our group and others (Dolan et al., 2012; Greene et al., 2013; Leydon & Wall, 2002; Poon et al., 2017; Waldron-Lynch et al., 2010; Warrington et al., 2009; Wilson et al., 2013a, 2015a, 2015b), we also report that the BMD of the jockeys studied here was significantly lower than clinical norms. Indeed, we report that 20 of the 31 jockeys demonstrated low bone mass (Z-score < −1; Barrack et al., 2017) in the lumbar region (10 apprentice and 10 senior), with 13 jockeys also presenting with low bone mass at the hip (six apprentice and seven senior).
Nonetheless, despite the consistent reports of low BMD in jockeys, it is not yet certain whether such data are true symptoms associated with RED-S. Indeed, we observed no differences in hip or lumbar spine Z-scores between apprentice and senior jockeys, as well as reporting no positive association between years of race riding and BMD (see Figure 2). The latter point is especially important considering that, in some cases, senior jockeys presented with 20–30 years of race riding experience. For example, when comparing jockeys who had ridden for the longest periods (i.e., >20 years) with the least experienced jockeys (i.e., <1 year), it is clear that such individuals display similarly low BMD at both the hip and lumbar spine. In consideration of other symptoms of RED-S, we also observed no differences in RMR between apprentices or senior jockeys, as well as no differences in measured versus predicted RMR in either cohort (see Figure 1). When such findings are considered with previous data highlighting marked evidence of underreporting of energy intake (O’Loughlin et al., 2013) as well as direct assessments of energy expenditure (Wilson et al., 2018), it remains questionable if male jockeys truly exhibit low energy availability. Indeed, despite the potential impact of low energy availability on endocrine function, we also previously reported that male flat jockeys display testosterone, insulin-like growth factor 1, and sex hormone–binding globulin values all within a clinically normal range (Wilson et al., 2015b). Given the cross-sectional nature of the study and the lack of a control group (given that there is no appropriate control group for jockeys), we cannot exclude the possibility, however, that the jockeys experienced an initial reduction in BMD during their adolescent years, and this state has persisted without further significant reductions. Future studies may now wish to assess BMD in adolescent jockeys prior to them commencing significant amounts of horse riding.
When taken together, it is therefore possible that the low BMD reported in jockeys is in fact predominantly due to the lack of an osteogenic stimulus associated with years of nonweight-bearing activity due to riding activities (Olmedillas et al., 2012), as opposed to low energy availability per se. While we acknowledge that the apprentice jockeys presented with considerably less race riding experience than their senior counterparts, it is noteworthy that apprentice jockeys are likely to be from horse-racing families and, hence, may have spent much of their adolescence engaged in riding activities (Greene et al., 2013). Additionally, apprentice jockeys potentially had inadequate intake of key micronutrients important in bone development, such as vitamin D and calcium (Wilson et al., 2013a). Such loading patterns are particularly important given that peak bone mass occurs at the end of the second decade of life (Baxter-Jones et al., 2011). Further studies are now required to accurately quantify the physical loading patterns, energy availability, and progression of bone mass of prospective senior jockeys throughout their childhood and adolescence and to assess if any of these variables correlate with poor bone health.
Support for a lack of an osteogenic stimulus is also provided by the observation that one of the apprentice jockeys studied here presented with a hip and lumbar Z-score of 2.2 and 1.8, respectively (see Figure 2). Indeed, this athlete was a former amateur boxer of international status and, hence, had a training history of high load-bearing activity, such as daily running and circuit- and resistance-based training. Interestingly, despite potential low energy availability in boxers (Morton et al., 2010), it is noteworthy that amateur boxers exhibit greater BMD in hip and lumbar regions (in a hierarchical manner) when compared with age-matched recreationally active individuals and a cohort of professional jockeys, respectively (Dolan et al., 2012). It is therefore possible that the negative effects of transient periods of weight cycling (i.e., multiple training camps per year) on markers of bone turnover in combat athletes (Prouteau et al., 2006) may be offset by the high osteogenic stimulus of habitual training activities (e.g., both amateur and professional boxers may run 5–10 km 5–6 days/week), as well as the return to normal body mass within 7–10 days postcontest. Furthermore, in a review of studies looking at the influences of participation in ball sports on bone health development in young athletes, Tenforde et al. (2015) concluded that activities within these sports, primarily jumping and multidirectional movements, may serve as a prehabilitation strategy for future stress fractures, including for running and swimming sports, which generally are devoid of such activities. From a clinical application perspective, it may therefore be suggested that practitioners who advise aspiring jockeys on injury prevention should also include such activities within their training modalities.
By contrast, an alternative explanation for the anomalies identified in the bone health of jockeys is that jockeys are an “atypical” population, given that they are significantly smaller in size and stature than the average western European male (Kidy et al., 2017). Clearly, further studies are now warranted utilizing much larger cohorts of age- and weight-matched athletic and nonathletic control subjects. In addition, histochemical analysis of bone fragments (as collected following any break or fracture) would also allow for definitive classification of osteoporosis and osteomalacia.
In summary, we report that purported symptoms of RED-S (e.g., hip/lumbar spine BMD and RMR) display no differences between apprentice and senior male flat jockeys and that such parameters do not progressively worsen with years of race riding. This, therefore, suggests that there is no clear association between long-term participation as a jockey and impaired skeletal health. When considered with previously published data examining underreporting of energy intake and direct assessments of energy expenditure, we suggest that poor bone health in jockeys is not due to low energy availability per se but rather the lack of an osteogenic stimulus associated with riding. Further studies are now required to directly test this hypothesis using a large cohort of age- and weight-matched athletic and nonathletic control subjects. Additionally, future studies should also attempt to longitudinally track the physical loading patterns, energy availability, and progression of bone mass of prospective senior jockeys throughout their childhood and adolescence.
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