Body Mass Changes Across a Variety of Running Race Distances in the Tropics

The mean decrease in BM across all races in this study, with a greater magnitude change as the race length increases, is not unexpected. Mass loss from oxidation of metabolic substrates for energy production and fluid losses from sweat inevitably leads to a decrease in BM, and this loss is amplified by greater energy requirements from increasing race durations and distances [7]. However, sub-analysis in the breakdown of ranges of % ?BM revealed a wide range of ?8.0 to 4.1 %, and this is comparable with those previously reported in endurance running events [6, 8, 16]. The presence of a positive change in BM highlights runners who gained BM during the race. The concept of excessive fluid consumption leading to overhydration with resultant body mass gain has been well established [8]. In this study, 6 % (106/1887) of runners appeared to drink beyond fluid excretion rates with a slightly higher incidence of body mass gain noted in the longer distance categories (42-km and above). Excessive concern over dehydration and hyperthermia developing during exercise in tropical climates may have led to overzealous fluid consumption in our runners, similar to other ultraendurance races [5, 10, 17]. In addition, overdrinking behavior could have been reinforced by companies selling sports drinks [18, 19] despite the fact that all sports drinks are hypotonic with regard to sodium and potassium content [3].

However, even with excessive consumption of fluid, mass gain should not normally occur during exercise [7]. The excretory capacity of kidneys is between 750 and 1440 mL/h and the rate of sweating estimated to be 500 mL/h [20]. Thus, even in slow runners, fluids may be safely ingested in excess of 1.5 L/h without the development of water retention. The mechanism of mass gain during exercise has been attributed to the failure of antidiuretic hormone (ADH) suppression, which leads to a reduction in the excretion of free water by the kidneys [21]. There is increasing evidence suggesting several non-osmotic stimuli that can lead to a submaximal suppression of ADH during exercise, including intense exercise, nausea, and/or vomiting, plasma volume contraction, hypoglycemia, pain, and other yet-to-be determined factors [3, 22–24].

Relationships between race time and hydration levels have previously demonstrated a positive association between finishing time and % ?BM (as a surrogate for hydration status) [25]. Our study weakly supported these findings. This is in contrast with the current literature which demonstrates that runners participating in longer distances (42-km and beyond) are at a greater risk of overhydration [8, 10, 14, 17, 16, 26–28]. This discrepancy may result from other variables not studied, such as the experience levels of the runners taking part in these shorter races. Slow pace running might have also contributed to lower sweat rate and longer duration for fluid consumption, increasing the risk of overhydration [3, 11, 24]. In our study, we also noted a comparatively lower proportion of runners who were dehydrated (loss of BM post-race) in the shorter race distances (0 % for 10-km and 11 % for 21-km). This contrasts to a higher incidence of dehydration in longer race distances that ranged from 21 to 67 %. It is postulated that increasing race distance and thus race duration contributes to a higher degree of dehydration, which could account for why the shorter distance runners did not face a significant issue of dehydration. Therefore, while post-race BM loss increases in longer race distances due to increased substrate oxidation, it is noteworthy that increased BM could instead occur due to reasons aforementioned. While the current study is unable to critically evaluate the relationship between race time and changes in body mass, it does establish the foundation for future biochemically based investigative work across a wider variety of running distances.

In this study, we reported eight cases of EAH diagnosed based on clinical symptoms and biochemical verification of EAH in runners who had presented to the medical tent. However, the actual incidence may be higher, as runners who had not presented to the medical tent would not have been captured in this group. While the prevalence of symptomatic EAH may appear low, it should be emphasized that EAH accounted for 17 % of all cases that presented to the medical tent. The majority of the current literature on EAH focused on longer distance races, such as ultra-marathons. Recent evidence, however, has documented the occurrence of symptomatic EAH in much shorter duration and distance events such as half-marathons and sprint races [12, 13]. As such, we augment the current literature by providing the largest study to date with the inclusion of a wider range of shorter race distances (10- to 25-km) to better reflect the running population at large. However, the runners diagnosed with EAH in this study were those participating in longer (42-, 84-, and 100-km) races, supporting previous data that reported longer distances and longer exercise duration as established risk factors for developing EAH [16, 29]. Despite a similar percentage of runners who were overhydrated in the shorter race distances (10- to 25-km), the lack of documented, symptomatic, EAH could possibly result from either an insufficient time for EAH symptoms to manifest or the runners’ preference to “endure it through” rather than report to the medical tent immediately following race.

Few studies on EAH have been performed in the Asian context [14], and this study adds to the existing literature by providing figures for the prevalence of symptomatic EAH in a tropical climate where temperature and humidity are generally higher [10, 14] compared to the Western context. According to the study by Hoffman et al. [10], races with higher ambient temperatures were associated with the highest incidences of EAH. This suggests that countries that experience higher ambient temperatures all year round could be at greater risk of EAH, underscoring the importance of knowledge regarding EAH in these climates. Hoffman et al. [10] also reported that EAH with dehydration was more predominant than EAH with overhydration in races with higher ambient temperatures. The prevalence of overhydration, based on percent changes in body mass, of 6 % in our study was much lower compared to the figure of 34.9 % quoted in the study by Hoffman et al. [10] This disparity is however more likely to be a result of a combination of factors including race distance, interval of fluid stations, racing speed, and other confounding factors in addition to the ambient temperatures alone.

Female gender has also been found to be a risk factor for EAH, with more dire symptomatology and outcomes with equivalent levels of hyponatremia [29, 30]. The underlying etiology remains unclear, but some studies have suggested that estrogen increases ADH secretion, lowers the thirst threshold, and blunts the drive to consume sodium [31, 32]. Other than estrogen, another factor contributing to female gender being a risk factor for overhydration may possibly be a differential behavior towards hydration. This is a factor that has yet to be explored more critically. However, Almond et al. [11] demonstrated that even though the incidence of EAH is higher in women, there were no statistical significance differences after adjusting for race time and BMI. These findings were similarly reflected in our study, where the incidence of overhydration (7 % in women versus 6 % in men) and EAH (0.3 % in women versus 0.5 % in men) was similar. We recognize, however, that the lower percentage of women (19 %) in our study sample could have led to the underreporting of results. We thereby propose to conduct longitudinal studies in the future with the inclusion of more women for more balanced gender analyses.

Various strategies have been proposed to reduce the occurrence of EAH, but the primary preventive strategy remains avoidance of overdrinking during a race. However, the threshold for overdrinking is often difficult to define in practice. Current recommendations suggest that fluid intake be based on the sensation of thirst because it reduces both the risk of dehydration and overhydration [3]. Reducing the availability of fluid stations at more than 3 km apart was also shown to be effective in reducing the incidence of EAH [3, 33]. More importantly, information on fluid balance should be disseminated to the runners to educate them on the current recommendations and increase the awareness of EAH. Outdated advice such as “drink beyond thirst” should be corrected. This strategy has been shown to be similarly effective in various studies [33, 25]. Other strategies proposed include placement of on-site weighing scales and sodium supplements. While usage of weighing scales would allow runners to screen for post-race body mass gain and seek early medical help, the absence of it does not exclude EAH as body mass loss has been reported in some cases of runners with EAH [10]. The usage of sodium supplements remains controversial. While sodium ingestion during a race may attenuate the fall in blood sodium concentrations, when body mass losses are fully replaced, it does not prevent the development of EAH if overdrinking continues [34, 35]. Current literature also recommends the avoidance of excessive sodium intake [3]. Therefore, the cornerstone in the prevention of EAH remains the prevention of overdrinking.

There are a few limitations worth noting in this study. First, the amount of fluid consumed during the race was not determined, as these data were too logistically challenging to collect. Second, initial hydration status was not assessed before the race and a small proportion of runners may have started either dehydrated or overhydration. Third, only symptomatic runners were admitted to the on-site medical tent and had their blood sodium levels measured; thus, our numbers do not represent the exact prevalence of EAH. Their body mass changes could not be evaluated, as they were incapable of standing on the weighing scale. And finally, the relationship between changes in body mass may not accurately reflect changes in total body water in field settings although runners should never gain body mass (indicative of overhydration) during exercise.