Dehydration is reported as a common finding in endurance athletes. Generally, dehydration is defined as a loss of more than 2% in body mass during endurance performance (10,13,45,52–54). It is supposed that dehydration leads to a rise in body core temperature, and when fluid losses are fully replaced, athletes can lower their core temperature (14) and thus reduce considerably their risk of dehydration (3,13,18,19).
Several studies showed that dehydration impaired endurance performance (11,45,49,54,61,65). Most of these studies were preferably performed as laboratory experiments under controlled laboratory conditions. Athletes were investigated while cycling on a stationary ergometer (4,6,12,42,48,67), rowing on a stationary ergometer (8,59), or running on a treadmill (20). The temperatures were kept constant during the laboratory trials and varied between 21 and 32°C (4,42,59,67). Generally, the number of investigated subjects in these laboratory settings was rather low between 6 and 8 participants (4,6,12,20,42,48,67). The intensities were rather high between 70% V[Combining Dot Above]O2max (42) and 80% V[Combining Dot Above]O2max (6) or even at maximal intensity to exhaustion (48,67). In some instances, the subjects completed a time trial on a cycling (12,42) or on a rowing ergometer (59). Other subjects were running at 70% V[Combining Dot Above]O2max on a treadmill (20) or rowing on an ergometer at maximum intensity (8).
The performance duration of these laboratory settings was generally no longer than 6 hours; therefore, only little is known about the association between dehydration and ultraendurance performance, defined as an endurance performance lasting for 6 hours or longer (72). The change in body mass has been investigated in ultraendurance athletes; however, the effect of dehydration on ultraendurance performance has almost not been explored yet. Sharwood et al. (55) described no evidence that a loss in body mass was related to an impaired performance in 297 ironman triathletes. Laursen et al. (39) concluded that a body mass loss of up to 3% was tolerated by 10 male ironman triathletes, and the changes in body mass were not related to Ironman finishing times. Recent studies investigating ultramarathoners reported that body mass losses were significantly and positively associated with race times in 23 ultramarathoners in a 24-hour run (24). In the “Marathon des Sables,” more than 230 km within 7 days in the desert, the athlete with the greatest body mass loss was the fastest one (74).
During the last years, studies were performed to investigate changes in body mass and hydration status in ultraendurance athletes (24,27,29,30,32–38,74). In some instances, no dehydration was found in ultraendurance athletes such as in mountain bike ultramarathoners (33) and triple iron ultratriathletes (32). In a recent study on ultrarunners in a 24-hour ultrarun, it was even questioned whether ultrarunners really dehydrate (38). In general, body mass decreased during an ultraendurance performance as has been demonstrated for ultrarunners (24,35,36,38,74), ultracyclists (33,37), and long-distance triathletes (27,32,39,55). For male ultraswimmers, however, no change in body mass was described (30). It must be assumed that the decrease in body mass was not only because of dehydration (33,38) but also because of a decrease in solid masses such as skeletal muscle mass (27,32–36) and fat mass (32,33,36–38,74).
These results on ultraendurance athletes showed that (a) a decrease in body mass such as skeletal muscle mass and fat mass is a common finding in ultraendurance athletes and (b) a loss in body mass seems not to be associated with a decrease in ultraendurance performance. For ultramarathoners, there are a few data about the relationship between body mass decreases and ultraendurance performances (24,74). Although Sharwood et al. (55) presented a rather large sample of 297 Ironman triathletes, only a small series of ultrarunners have been investigated. Regarding the data of Kao et al. (24) and Zouhal et al. (74) with rather small samples of 23 and 16 ultrarunners, respectively, we intended to investigate the association between body mass changes and ultraendurance performance in a larger sample of ultramarathoners. Apart from body mass changes, we included also parameters of hydration status to better investigate fluid metabolism and dehydration.
The aims of this observational field study on a sample of 50 recreational, male, 100-km, ultramarathoners were therefore to investigate (a) whether ultramarathoners in a 100-km ultramarathon undergo a decrease in body mass, (b) whether an eventual decrease in body mass was because of dehydration, and (c) whether performance was impaired in case of a decrease in body mass. Based on present literature, we hypothesized that (a) body mass would decrease and (b) a decrease in body mass would be related to a decreased running speed.
Experimental Approach to the Problem
The organizer of the “100 km Lauf Biel,” in Switzerland, contacted all the participants of the 2010 race before the start via a separate newsletter and informed them about the planned investigation. The race took place on June 11, 2010. The ultramarathoners started the 100-km run at 10:00 PM They had to climb a total altitude of 645 m. During these 100 km, the organizer provided a total of 17 aid stations offering an abundant variety of food and beverages. The athletes were allowed to be supported by a cyclist to have additional food and clothing, if necessary. The temperature at the start was at 21.7°C, dropped to 15.6°C during the night, and rose to 18.1°C the next day. Humidity was at 52% at the start, rose to 62% during the night, and to 69% the following day. Barometric pressure was at 1,007.8 hPa at the start, rose to 1011 hPa in the night, and was constant at 1,011.9 hPa the next day. During the night, the sky was covered but no rain was falling. Body mass, body composition, and parameters of hydration status were determined pre race and post race to investigate potential associations between changes in both body mass and body composition with either running speed or parameters of hydration status.
Fifty-six male ultrarunners volunteered to participate in the study; 50 athletes finished the ultramarathon successfully within the time limit of 19 hours. The characteristics of anthropometry and training are represented in Table 1. The study was approved by the Institutional Review Board for the Use of Human Subjects of the Canton of Zurich, Switzerland, and appropriate informed consent has been gained.
On inscription to the investigation, the participants were instructed to keep a training diary until the start of the race. The investigator provided an Excel file where athletes could record their training. All training units in running until the start of the race were recorded, showing distance in kilometres and duration in minutes.
Before the start of the race between 05:00 PM and 10:00 PM, body mass, body height, the circumferences of the limbs, and the thicknesses of 4 skinfolds (mid upper arm, abdominal, midthigh, and midcalf) were measured on the right side of the body. With these data, fat mass and skeletal muscle mass, using an anthropometric method, were estimated. Body mass was measured using a commercial scale (Beurer BF 15; Beurer GmbH, Ulm, Germany) to the nearest 0.1 kg after voiding of the urine bladder. Body height was determined using a stadiometer to the nearest 1.0 cm. The circumferences and the lengths of the limbs were measured using a nonelastic tape measure (cm) (KaWe CE; Kirchner und Welhelm, Asperg, Germany) to the nearest 0.1 cm. The circumference of the upper arm was measured at mid upper arm; the circumference of the thigh was taken at mid-thigh, and the circumference of the calf was measured at mid-calf. The skinfold data were obtained using a skinfold calliper (GPM-Hautfaltenmessgerät; Siber & Hegner, Zurich, Switzerland) and recorded to the nearest 0.2 mm. The skinfold measurements were taken following International Society for the Advancement of Kinanthropometry standard once for all 4 skinfolds, and then, the procedure was repeated twice more by the same investigator; the mean of the 3 times was then used for the analyses. The timing of the taking of the skinfold measurements was standardized to ensure reliability. According to Becque et al. (5), readings were performed 4 seconds after applying the calliper. One trained investigator took all the skinfold measurements as intertester variability is a major source of error in skinfold measurements. An intratester reliability check was conducted on 27 male athletes before testing. The intraclass correlation (ICC) within the 2 measurers was excellent for all anatomical measurement sites and various summary measurements of skinfold thicknesses (ICC > 0.9). Agreement tended to be higher within measurers than between measurers but still reached excellent reliability (ICC > 0.9) for the summary measurements of skinfold thicknesses (28). For the sum of 8 skinfolds for measurer 1, bias (average difference between measures 1 and 2) was −0.515, SD of the average difference was 1.492; and 95% limits of agreement were between −3.439 and 2.409. Fat mass was estimated using the equation following Stewart and Hannan (63) for male athletes: Fat mass (g) = 331.5 × abdominal skinfold thickness + 356.2 × thigh skinfold thickness + 111.9 * body mass − 9,108. The coefficient of determination was 0.82, and the SEE was 1843 g, equivalent to 2.4% for a typical athlete in the sample. Skeletal muscle mass (kg) was estimated using the anthropometric equation of Lee et al. (40) with skeletal muscle mass = Ht * (0.00744 * CAG2 + 0.00088 * CTG2 + 0.00441 * CCG2) + 2.4 * sex − 0.048 * age + race + 7.8, where Ht = height; CAG = skinfold-corrected upper arm girth; CTG = skinfold-corrected thigh girth; CCG = skinfold-corrected calf girth; sex = 1 for male; age is in years; and race = 0 for white men and 1 for black men. This equation was validated using magnetic resonance imagining (MRI) to determine skeletal muscle mass. There was a high correlation between the predicted skeletal muscle mass and the MRI-measured skeletal muscle mass (r2 = 0.83, p < 0.0001, SEE = 2.9 kg). The correlation between the measured and the predicted skeletal muscle mass difference and the measured skeletal muscle mass was significant (r2 = 0.90, p = 0.009).
After the anthropometric measurements, venous blood samples were drawn and urine samples were collected. Two Sarstedt S-Monovettes (plasma gel, 7.5 ml) for chemical and one Sarstedt S-Monovette (EDTA, 2.7 ml) (Sarstedt, Nümbrecht, Germany) for hematological analysis were drawn. Monovettes for plasma were centrifuged at 3,000g for 10 minutes at 4°C. Plasma was collected and stored on ice. Urine was collected in Sarstedt monovettes for urine (10 ml). Blood and urine samples were transported immediately after collection to the laboratory and were analyzed within 6 hours. Immediately after arrival at the finish line, identical measurements were applied.
In the venous blood samples, hemoglobin, hematocrit, [Na+], [K+], creatinine, urea, glucose, aldosterone, and osmolality were measured. Hematologic parameters were determined using ADVIA 120 (Siemens Healthcare Diagnostics, Deerfield, IL). Plasma parameters were measured using COBAS INTEGRA 800 (Roche, Mannheim, Germany). The concentration of aldosterone was measured using Radioimmuno assay with Gamma-Counter 1277 (DRG Instruments GmbH, Marburg, Germany). In the urine samples, creatinine, urea, [Na+], [K+], urine-specific gravity, and osmolality were determined. Urine-specific gravity was analyzed using Clinitek Atlas–Automated Urine Chemistry Analyzer (Siemens Healthcare Diagnostics). Creatinine and urea in urine were measured using COBAS INTEGRA 800. Electrolytes in urine were determined using ISE IL 943 Flame Photometer (GMI, Inc, Ramsey, MN). The osmolality of plasma and urine was determined using Fiske Modell 210 Mikro-Osmometer (IG Instrumenten-Gesellschaft AG, Zurich, Switzerland). Fractional sodium excretion was calculated using the equation fractional excretion of sodium = ([sodiumurine * creatinineplasma]/[sodiumplasma * creatinineurine]) * 100 according to Steiner (62). Fractional urea excretion was calculated using the equation fractional urea excretion = ([ureaurine * creatinineplasma] /[ureaplasma * creatinineurine]) * 100 following Dole (15). Transtubular potassium gradient was calculated using the equation transtubular potassium gradient = ((potassiumurine * osmolalityplasma)/(potassiumplasma * osmolalityurine) according to West et al. (68). Creatinine clearance was calculated according Gault et al. (22). Percentage change in plasma volume was determined following Strauss et al. (64).
Between the pre- and the postrace measurements, all athletes recorded their intake of solid food and fluids using paper and pencil. The investigator provided a prepared paper with all aid stations and the food and drinks provided there. At each aid station, they marked the kind and the amount of food and fluid ingested. At these aid stations, liquids and food were prepared in a standardized manner, that is, beverages and food were provided in standardized size portions. The drinking cups were filled to 0.2 L, and the energy bars and the fruits were halved. The athletes had only to mark at which station they consumed liquids and food. They also recorded additional food and fluid intake provided by the support crew and the intake of salt tablets and other supplements. The composition of fluids and solid food was determined according to the reports of the athletes using a food table (26).
Data are presented as mean and SD. Pre- and postrace results were compared using a paired t-test. Pearson's correlation analyses were used to check for associations between parameters with statistically significant changes between pre- and postrace results. Analysis of variance (1 way) was used to determine differences between groups. A Tukey's post hoc test was performed when the overall F value of the model was significant to detect differences. Statistical significance was accepted with p ≤ 0.05 (2-sided hypothesis).
Pre race, the 50 subjects reported 11.8 (7.9) years of experience as long-distance runners. During training, they were running for 8.6 (10.8) hours per week, completed 66.5 (27.6) weekly running kilometres, and were running at a speed of 10.7 (1.3) km·h−1. The finishers completed the 100-km ultramarathon within 734 (119) minutes, running at a speed of 8.4 (1.2) km·h−1. The weekly running kilometres (r = −0.28; p = 0.0473), the running speed during training (r = −0.58; p < 0.0001), the personal best time in a marathon (r = 0.61, p < 0.0001), and the personal best time in a 100-km ultramarathon (r = 0.80; p < 0.0001) were related to their 100-km race time.
During the run, the subjects lost 1.9 (1.4) kg of body mass (p < 0.0001) and 0.7 (2.0) kg of skeletal muscle mass (p < 0.05), whereas fat mass remained unchanged (p > 0.05) (Table 2). Expressed in percentage, the athletes lost 2.6 (1.8) % in body mass. The change in body mass varied from a loss of −8% body mass to an increase in 1.3% body mass (Figure 1). The decrease in body mass was not related to the decrease in skeletal muscle mass (p > 0.05). However, the change in body mass was associated with the change in fat mass (r = 0.44, p = 0.0014). Race time was significantly and positively related to the change in body mass; faster runners lost more body mass than slower runners (Figure 2).
Hemoglobin, hematocrit, serum glucose, and serum [K+] remained unchanged; calculated plasma volume increased by 1.0 (7.8) %. The concentration of aldosterone, serum [Na+], serum creatinine, serum urea, and plasma osmolality increased (p < 0.0001) (Table 2). In urine, urine-specific gravity and urine osmolality increased (p < 0.0001). Fractional sodium excretion, fractional urea excretion, and creatinine clearance decreased (p < 0.0001); fractional potassium excretion, transtubular potassium gradient, and the potassium to sodium ratio in urine increased (p < 0.0001).
The changes in body mass were significantly and negatively related to postrace serum [Na+] (Figure 3). The increase in plasma osmolality was highly significantly associated with the increase in both serum urea (r = 0.71, p < 0.0001) and serum [Na+] (r = 0.51, p < 0.0001). For urine, the increase in osmolality was highly significantly related to the increase in urea (r = 0.77, p < 0.0001) and to the decrease in [Na+] (r = 0.57, p < 0.0001).
While running, the athletes consumed 7.2 (2.2) L of fluids, equal to 0.60 (0.20) L·h−1. Considering the intake of electrolytes, they ingested 4.2 (3.7) g of sodium and 0.6 (0.3) g of potassium, respectively. This was equal to 353 (319) mg of sodium per race hour and 52 (30) mg of potassium per race hour, respectively. Fluid intake was significantly and positively related to the change in body mass (r = 0.40, p = 0.004) (Figure 4). Furthermore, fluid intake was significantly and positively related to running speed (r = 0.33, p = 0.0182) where faster runners were drinking more fluids per hour (Figure 5). Additionally, fluid intake was significantly and negatively related to both postrace serum [Na+] (r = −0.34, p = 0.0142) and the change in serum [Na+] (r = −0.38, p = 0.0072). Sodium intake was related neither to postrace serum [Na+] nor to the change in serum [Na+] (p > 0.05).
The concentration of aldosterone increased highly significantly (p < 0.0001). The change in aldosterone concentration was highly significantly and positively associated with the change in transtubular potassium gradient (r = 0.37, p = 0.0078) and the change in potassium to sodium ratio in urine (r = 0.68, p < 0.0001). Postrace aldosterone concentration was not related to postrace serum [Na+] (p > 0.05); also, the change in aldosterone concentration was not associated with the change in serum [Na+] (p > 0.05). Postrace aldosterone concentration (Figure 6) and the change in aldosterone concentration (Figure 7) were significantly and positively related to running speed. The faster the athletes were running the more the aldosterone concentration increased. Furthermore, the change in aldosterone concentration was significantly and negatively associated with the increase in plasma volume (r = −0.40, p = 0.0037). Fluid intake was related neither to postrace aldosterone concentration nor to the change in aldosterone (p > 0.05). Also, fluid intake correlated neither to post-race aldosterone concentration nor to the change in aldosterone concentration (p > 0.05).
The subjects were divided into athletes with >2% loss of body mass and athletes with ≤2% of body mass (Table 3). Of the 50 subjects, 31 finishers (62%) showed a loss of >2% body mass and 19 finishers (38%) a loss of ≤2% body mass. The finishers with a loss of >2% body mass completed the race within 729 (109) minutes and were not slower compared to the finishers with a loss of ≤2% body mass, finishing within 744 (137) minutes (p > 0.05). The finishers with >2% loss of body mass showed a higher increase in aldosterone concentration (p < 0.001). In addition, they showed a higher increase in fractional potassium excretion, in transtubular potassium gradient, and in the potassium to sodium ratio in urine (p < 0.05).
Furthermore, we divided the subjects into 3 groups (Table 4): athletes with ≤2% loss of body mass (group 1), athletes with a decrease between 2 and 4% of body mass (group 2), and athletes with a loss of > 4% body mass (group 3). Race time was not different between the 3 groups (p > 0.05). The athletes in group 1 consumed significantly more fluids than the athletes in the other groups (p < 0.05). The finishers with the highest decrease in body mass (group 3) showed a significantly greater change in aldosterone concentration, in hemoglobin, in hematocrit, in serum [Na+], in serum urea, in plasma osmolality, and in fractional urea excretion compared with the other groups (p < 0.05).
This study aimed to investigate whether (a) ultramarathoners in a 100-km ultramarathon undergo a decrease in body mass, (b) an eventual decrease in body mass was because of dehydration, and (c) race performance was impaired in case of a decrease in body mass. We hypothesized that (a) body mass would decrease and (b) a decrease in body mass would be related to an impaired race performance.
A main finding in these male 100-km ultramarathoners was that the faster runners lost more body mass than the slower runners did (Figure 2). One might assume that the decrease in body mass would be related to a decreased fluid intake. However, the faster runners were drinking more than the slower runners (Figure 5). Although the faster runners were drinking more each hour, they were losing more body mass during the race presumably because of more sweating but they were still drinking appropriately at a relatively lesser rate than were the slower runners. The goal of drinking during endurance exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance. Because there is a considerable variability in sweating rates and sweat electrolyte content between individuals, customized fluid replacement programs are recommended (14,52). However, this behavior may lead to fluid overload (7). When athletes drink too much to prevent dehydration, fluid overload with consequent exercise-associated hyponatremia may occur (60). Instead of a full replacement of body weight loss through fluid ingestion, drinking to thirst seems to prevent from fluid overload (46). Therefore, ad libitum fluid ingestion is optimal as it prevents athletes from ingesting too little or too much fluid (16).
In an actual study of Zouhal et al. (73) using 643 marathoners and investigating the association between a loss in body mass and race performance, the degree in the loss of body mass was inversely and linearly related to the marathon race time. Fifty-five percent of their subjects lost >2% of body mass during the marathon, and runners with that extent of body mass loss ran significantly faster than those runners who lost less body mass. Their findings confirmed recent findings where a loss of body mass seems to be “ergogenic” regarding an endurance performance. In marathon runners, (73), in cyclists (17), in ironman triathletes (56,69), in 24-hour ultramarathoners (24), and in ultrarunners, in the Marathon des Sables (74), an inverse relationship between body mass changes and race performance has been demonstrated.
Regarding the association between a loss of body mass and race performance in ultraendurance athletes, Kao et al. (24) investigated both 12- and 24-hour ultramarathoners. Their ultrarunners were competing at a temperature between 11.5 and 14.6°C. In the 18 participants in the 12-hour run, body mass change was not related to running performance.
However, in the 23 runners in the 24-hour run, the loss in body mass was positively associated with the completed kilometres. All runners with a loss of >7% body mass ran more than 200 km during the 24 hours. Their findings were in accordance to studies in Ironman triathletes where athletes exhibiting the most dramatic changes in body mass during an Ironman were among the fastest to finish (55,56). High levels of body mass losses also did not affect performance in the Marathon des Sables (74). These findings that faster athletes lost more body mass during an endurance performance compared with slower athletes were, however, no new findings. Already the early studies on marathoners of both Pugh et al. (50) and Wyndham and Strydom (71) showed that the winners were those who lost the most body mass and usually showed the highest postrace core body temperatures (9,50,71). In the study of Pugh et al. (50) using 77 marathon runners, the winner finished the marathon race with a core temperature of 41.1°C, having lost 6.7% of body mass. The winner of the 1970 Commonwealth Games Marathon in near world record time did not drink during the race and lost 3.9% of body mass (44).
We must, however, be aware that the change in body mass is not always a reliable measure of the change in hydration status (41,47), although a recent laboratory study concluded that measuring pre to postexercise changes in body mass was an accurate and reliable method to assess the change in total body water (2). In 181 male Ironman triathletes, plasma volume and serum [Na+] were maintained despite a significant loss of 5% body mass. It was concluded that body mass was not an accurate surrogate of body fluid homeostasis during prolonged endurance performance (23). In addition, ultraendurance performance may lead to a decrease in solid masses such as fat mass (32,33,36–38,74) and skeletal muscle mass (27,32–36). In the present ultramarathoners, skeletal muscle mass was significantly reduced and the change in body mass was related to the change in fat mass. The change in body mass is only one potential variable to determine a change in hydration status among other different possibilities (25,57,58). Armstrong (1) recently presented a list of 13 different methods to assess hydration status. A major problem is that a single gold standard, including plasma osmolality, is not possible for all hydration assessment requirements. For a field study such as an ultraendurance race, techniques to assess hydration status should be easy-to-use, safe, portable, and inexpensive such as body mass changes, urine-specific gravity, 24-hour urine volume, urine color, and thirst (1). Therefore, we also determined, apart from the body mass changes, the change in urine-specific gravity. The decrease of 2.6% body mass means minimal dehydration and the postrace urine-specific gravity of 1.026 g/ml significant dehydration, following Kavouras (25). Presumably, the combination of blood indices such as plasma osmolality, hemoglobin concentration, and hematocrit together with urine indices such as urine osmolality and urine-specific gravity would be the best method to define dehydration instead of using only body mass changes as a single parameter of hydration status (25,57,58). Clinical signs thought to indicate dehydration such as altered skin turgor, dry oral mucous membranes, sunken eyes, an inability to spit, and the sensation of thirst were not able to identify runners with a total body mass loss of >3% at the end of a marathon (43).
A further important finding was that plasma [Na+] increased with an increasing loss of body mass. The group with >4% loss of body mass showed an increase in plasma [Na+] of 3.75 (2.86) mmol/L (p < 0.05). In ironman triathletes, Sharwood et al. (56) described that serum [Na+] was significantly higher in athletes with the greatest percentage of body mass loss. This increase in plasma [Na+] might be becuase of endocrine regulation during dehydration (35). In a case study in a multistage ultratriathlon with one ironman triathlon per day for 5 consecutive days, body mass decreased and plasma [Na+] increased after each stage (29). In marathoners, Whiting et al. (70) showed a decrease in body mass and an increase in plasma [Na+]. The increase in plasma [Na+] in marathoners might not be because of dehydration but rather because of endocrine regulation by hormones such as aldosterone, renin, and atrial natriuretic peptide, which are increased after a marathon (49). After an endurance performance, sodium excretion in urine is reduced (21,49), presumably because of the increased activity of aldosterone (21). An endocrine regulation of plasma [Na+] is quite possible because fluid intake was related neither to the change in body mass nor to the change in plasma [Na+] in the present ultramarathoners. In the present ultramarathoners, both postrace aldosterone concentration and the change in aldosterone concentration were significantly and negatively related to race time; the faster the athlete, the higher the aldosterone concentration. Wade et al. (66) described chronically elevated plasma aldosterone levels in multistage runners with a decreased urine excretion rate. This might explain why plasma [Na+] increased in the present 100-km ultramarathoners.
An important finding was that the faster runners were drinking more than the slower runners (Figure 5). Although the faster runners were drinking more each hour, they were losing more body mass during the race (Figure 2). Also, drinking more while running was associated with an increase in body mass (Figure 4). The faster runners—although drinking more—lost more body mass during the race presumably because of more sweating, but they were still drinking less than were the slower runners. We assume that the faster runners had a support crew to provide drinks also between the aid stations in contrast to the slower runners with no support crew. Most probably, the faster runners were not stopping at the aid stations to get food and drinks. We must also be aware that intensity drives sweat rate (51). The fact that they were more dehydrated is likely because of the greater sweat rate at a higher intensity more so than it being ergogenic. The better performance in the faster runners is also because of numerous reasons where one main reason is certainly the motivation to achieve a fast race time (31).
This study is limited because of the design. We do not know how the athletes would have consumed fluids with an enhanced hydration plan. Two field studies using a randomized cross-over design showed that dehydration impaired performance during a 12-km trailrun in the heat (11,61). These studies involved the same 17 subjects (9 men and 8 women) with repeated measures and controlled numerous factors except the level of hydration. The conclusions of these 2 well-controlled studies were consistent that dehydration impaired performance (11,61). An ultramarathon of 100 km will produce a substantial protein catabolism, leading to a decrease in skeletal muscle mass (36,38) and thus leading to an increase in urine urea concentration, which will increase urine-specific gravity independent of a changing water fraction. Therefore, the hydration parameter urine-specific gravity for fluid and hydration status is limited under the present circumstances. The faster ultramarathoners were running through the night at rather low temperatures and finished early in the morning. The slower ultramarathoners had to continue on the second day while the ambient temperature started to rise. This change in temperature from the cool night to the hot day for the slower runners might have influenced our results. Future studies may investigate whether ultramarathoners competing for hours or days run faster during the night with lower temperatures than during the day with higher temperatures and how they consume fluids depending on the ambient temperature.
This field study showed that recreational, male, 100-km ultramarathoners dehydrated as evidenced by the decrease in 2.6% body mass and the increase in urine-specific gravity. Race performance, however, was not impaired because of the loss of body mass. In contrast, faster athletes lost more body mass compared with slower athletes. The concept that a loss of >2% body mass leads to dehydration and consequently impairs endurance performance must be questioned for ultraendurance athletes competing in the field. For practical application, a loss in body mass was associated with a faster running speed during a 100-km ultramarathon in recreational male ultrarunners.
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