The aims of the current study were, firstly to assess water and sodium intake habits of recreational ultra-runners during a semi self-sufficient MSUM conducted in a hot ambient environment; secondly to monitor serum sodium concentration, and hydration status using multiple hydration assessment techniques, along competition. In contrast to our hypothesis, water intake habits in the majority of UER were sufficient to maintain baseline euhydration levels whilst competing on consecutive days in hot ambient conditions. Despite sodium ingestion through foods and fluids under benchmark recommendations in the majority of UER, in accordance with our hypothesis, normonatraemia was observed in all UER along the MSUM. However, a novel finding was evidence of fluid over-consumption behaviours in a substantial number of UER, irrespective of running speed or gender, with asymptomatic hyponatraemia observed at certain points along the MSUM in n = 8 UER (corresponding to 42% population). The strength of the sample size (equating to 55% of all runners that participated in the MSUM), potentially gives a valid and reliable representation of current water and sodium intake habits and status of ultra-runners during semi self-sufficient MSUM conducted in hot ambient conditions, bearing in mind the diverse origins of the sampled population. Conversely, the extreme nature of the event and invasive techniques used to determine blood borne variables at frequent points along the MSUM created a potential barrier for obtaining full data sets from all participants. In light of this, only a sub-sample of the full population was used in determining blood borne variables.
Average total daily water ingestion through foods and fluids of UER was 7.7 L/day, and appears to be sufficient to maintain baseline euhydration levels throughout the current semi self-sufficient MSUM, in accordance with the maintained POsmol within normal clinical reference range both pre- and post-stage in the majority of UER. This ad libitum rate of water ingestion also provided a surplus of water required to supported the acute extracellular hypervolaemia observed in both SR and FR along competition (as indicated by 22.8% PV and 9.2% ECW increase in UER). This response is likely attributed to heat acclimatisation , since 72% of UER originated from countries that presented cold or thermoneutral environmental conditions (≤20°C), and arrived at the competition location <48 h prior the start of competition, without having previously heat acclimatised. These results also highlight practical relevance for self-sufficient MSUM, which normally provide water rations of ~12 L/day; suggesting that even with a water quota set by race organisers, euhydration can easily be maintained throughout competition. However, the quality of fluids ingested by UER could be adjusted, since the majority of fluid consumed was plain water, with nutrient rich fluids only accounting for ≤30% of overall fluid intake along the course of the MSUM. Educating UER to predominantly consume nutrient rich fluids throughout competition will contribute towards both euhydration and cater for meeting energy and nutritional needs on consecutive days of strenuous exercise .
Average total water intake through foods and fluids during running in UER was 4.5 L, with FR and male ultra-runners showing ability to tolerate higher rates of fluid consumption than SR and female ultra-runners, respectively. Thirst and appetite sensations, gastrointestinal distress during running, concomitant with training status and degree of exposure to competition beverages during training, are all are prime factors to fluid tolerance during exercise-heat stress [8, 11, 40, 41]. The majority of fluid consumed during running was plain water, with nutrient rich fluids only accounting for ≤26% of overall fluid intake. Such drinking behaviours during running are prime risk factor for hyponatraemia [17, 24, 26], with prevalence on hyponatraemia observed during a one stage 161 km running event ; and now during a 5 stage 225 km ultra-marathon in the heat, with 26% of UER sampled presenting exercise-associated hyponatraemia post-stage along the MSUM, irrespective of running speed, running distance, and/or gender; with incidence not accumulating over the MSUM. The interesting feature of the current study was that SNa indicative of hyponatraemia also occurred pre-stage (16% of UER sampled). Therefore, over-consumption of plain water during the recovery period and/or sub-optimal dietary sodium intakes may have also attributed to the incidences of asymptomatic hyponatraemia observed [6, 18, 19, 26]. Indeed, even though the majority of UER were actively adding external sodium supplementation to ingested fluids, sodium intake per ingested fluid volume during running and the recovery period were far below benchmark recommendation throughout the entire MSUM in UER [6, 18]. Conversely, despite the low sodium concentrations per fluid volume ingested, 58% of UER sampled remained normonatramic throughout the entire MSUM. These results are in accordance with previous studies suggesting that sodium supplementation may not be required during exercise in certain UER, since adaptations to increase sodium bioavailability and prevent losses (e.g. sweat, urine, and faeces) take place in response to periods of sodium deprivation or restriction [14, 23, 24].
BM loss ≥2% has previously been established as indicative of dehydration . In the current study the average exercise-induced BM loss in UER was 2.4%, which is relatively low compared with reports of >4% BM loss often observed after marathon and one-stage ultra-marathon competition . Moreover, FR and male ultra-runners showed a greater exercise-induced BM loss compared with SR and female ultra-runners, respectively. This is in accordance with previous reports indicating highly trained faster runners lose more BM during competition than lesser trained slower runners [16, 17]; but also present novel finding, showing that male ultra-runners lose more BM than female ultra-runner, possibly associated with running speed. Nevertheless, these results support the suggestion that SR, and now female ultra-runners, have the increased tendency for fluid over-consumption; with BM gains observed pre- to post-stage in 29% and 32% of SR and female ultra-runners respectively, compared with 21% and 22% of FR and male ultra-runners respectively. BM loss results from a combination of factors during the run phase of MSUM competition in the heat, including: water sweat, respiratory, and urine losses (taking into account potential gastrointestinal fluid reservoirs, awaiting intestinal transport, during the pre-stage period), weight loss associated with depletion of muscle glycogen stores, and/or faecal weight loss if evacuation is required during running [2, 6, 29]. Therefore, care is needed in using the degree of exercise-induced BM loss to programme fluid intake strategies during post-stage recovery, to correctly replace water sweat and obligatory urine losses. Water replacement rates after exercise of x1.5 BM loss have been recommended, and appear to be an effective and safe in replenishing exercise-heat stress induced body water losses . Using nutrient rich fluids during the post-stage recovery period (e.g. milk shake), will provide a plethora of essential ingredients required for an overall effective recovery [43–47].
The novel use of MBIA in the current field setting showed that euhydration was maintained throughout the entire course of the MSUM, with pre-stage hydration status actually improving along competition. The observed 9.2% increase in resting pre-stage ECW from pre-Stage 1 to pre-Stage 5 in UER, are likely associated with heat acclimatisation responses promoting acute extracellular hypervolaemia [48–50]. Exercise-heat stress raises circulatory osmotic pressure stimulating a vascular influx of plasma proteins (e.g. albumin) which draws fluid into intravascular compartments, and up-regulates hormones (e.g. aldosterone, vasopressin) responsible for renal water re-absorption [5, 51]. Even though PV expansion was not directly measured, using change in blood haemoglobin and haematocrit has previously been used as a valid method of estimating changes in PV during exercise-heat stress [16, 35–37]. Increases in PV were observed in the current MSUM in UER, but not in CON, which are in accordance with ultra-endurance specific heat acclimation protocols, reporting significant increases in PV (+7.9%) after two bouts of 2 hours running at 60% VO2max at 30°C . These results imply that simply starting MSUM competition in the heat will support a positive effect on hydration status if sufficient fluids are consumed to accompany heat adaptation responses.
Using urine measure of hydration to guide fluid intake and support euhydration is a common practice amongst ultra-endurance athletes during multi-stage competitions. Whereby, ultra-endurance athletes consume fluids based on their urine concentration (e.g. colour, osmolality). In the current study, pre-stage UOsmol and Ucolour rose substantially by Stage 2 and remained elevated throughout the MSUM in UER, with no change on CON; whilst the high UOsmol values post-stage are a common feature of exercise-heat stress . However, the substantial increase in UOsmol/POsmol ratio above clinical reference values in the majority of UER, irrespective of running speed and gender, at both pre- and post-stage, concomitant with progressive increases in PV and ECW observed as competition progressed, indicates UOsmol does not reflect body water content; suggesting urine measure of hydration are potentially inappropriate in monitoring hydration status and guiding fluid intake strategies in UER during MSUM in the heat. Moreover, all UER who presented asymptomatic hyponatraemia and BM gains post-stage had UOsmol/POsmol ratios above clinical reference values. Interestingly, male ultra-runners presented greater UOsmol/POsmol ratios compared with female ultra-runners Stage 3 onwards, likely due to the greater fluid over-consumption behaviours of female ultra-runners. However, differences in renal hormone responses between genders to exercise-heat stress should not be overlooked, and warrants further investigation. Even though a limitation in the current study is the absences of data on renal hormone responses during MSUM conducted in a hot ambient environment, previous ultra-endurance studies have reported a four-fold increase in vasopressin during a 109 km cycle race in the heat . Thus, from a practical standpoint, using results from urine measure of hydration techniques to guide hydration strategies during periods of exercise-heat stress could actually be dangerous for certain individuals, promoting fluid over-consumption (attempts to maintain clear urine) while renal water re-absorption is up-regulated . Using thirst as a mechanism to regulate fluid intake behaviour both during running and recovery appears to be a reliable and safer method in promoting euhydration maintenance along MSUM competition in the heat [13–15].