Losing as little as 2% body mass (BM) through sweating has been shown to compromise physiological functioning during exercise (4,11,15,21). Cardiovascular and thermoregulatory responses, and ratings of perceived exertion (RPEs) were higher when dehydrated during exercise, contributing to premature fatigue at the same relative and absolute exercise intensity compared with a hydrated state (HYD) (4,11,15,21). Similarly, hypohydration before exercise exacerbates the rate of increase in heart rate, core temperature, and RPE during exercise compared with starting exercise hydrated (21,27). The severity of these effects is directly related to the degree of hypohydration (15,21,27).
The use of urine specific gravity (USG) is a simple, accurate and valid indicator of whether an athlete is hypohydrated before exercise (1–3,24). Armstrong et al. (4) were the first to report that USG and urine osmolality (Uosm) were the valid and reliable indicators of hydration status and could be used interchangeably to determine the hydration state during field testing. In support, Stover et al. (28) reported that USG measured with refractometry strongly correlated with Uosm (r = 0.995) and a USG of 1.020 correlated with a Uosm of approximately 800 mOsm·kg−1. Using refractometry, the National Athletic Trainer's Association (NATA) Position Statement on fluid replacement in athletes classifies hydration state into 4 conditions; well hydrated (USG < 1.010), minimal hypohydration (USG 1.010–1.020), significant hypohydration (USG ≥ 1.020), and serious hypohydration (USG > 1.030) (7). The published Position Stand on exercise and fluid replacement by the American College of Sports Medicine (ACSM) also classifies hypohydration as an USG exceeding 1.020 (8,20).
Several reports indicate that a high percentage of athletes arrive for training sessions and competitions mildly hypohydrated. For instance, Bergeron et al. (5) reported that mean USG was 1.025 ± 0.002 before a junior national tennis competition. Likewise, our laboratory reported a pretraining USG of 1.024 ± 0.001 in junior male hockey players (18) and that 43% of the same players arrived mildly hypohydrated before a hockey game (14). Similarly, Stover et al. (27) monitored the prepractice hydration status of high school football players over a 5-day period during twice a day practices and showed that 60–77% of the players were hypohydrated before practice each day with a USG >1.020, although the repeatability in morning USG was high within subjects. More recently, Higham et al. (12) compared daily hydration profiles of competitive adolescents over 4 consecutive days before training and reported mean USG to be 1.025 ± 0.001 with high repeatability between days. In summary, the majority of athletes arrived at training and competition sessions in a hypohydrated (HYPO) state and based on morning USG, athletes appear to be hypohydrated on a daily basis during training camps.
Shirreffs et al. (22) demonstrated that the amount of fluid retained after exercise is directly proportional to the [Na+] of the fluid consumed when acutely dehydrated by exercise. In fact, most studies reporting greater rehydration with an electrolyte containing solution have been conducted after exercise resulting in a BM loss of 1.5–2% and then rehydrating with a fluid volume of 150% of BM loss over a 3–4 hours recovery period (23,29,30). However, this work focused on a mild hypohydration situation (≤1% BM loss) that may be more representative of the normal hydration state before training for the average athlete. Based on previous research after exercise, we hypothesized that solutions containing electrolytes would be more effective than water alone at restoring hydration in HYPO subjects.
The goals of this study were to (a) monitor the day to day repeatability of morning USG over 5 consecutive days in adult subjects, (b) assess the time course of consuming 600 ml of water on the hydration status of euhydrated and HYPO subjects, and (c) assess whether 600 ml of combinations of carbohydrate and electrolyte solutions would be more effective than water at restoring euhydration in HYPO subjects. Based on previous research, we hypothesized that (a) morning USG would be repeatable for subjects between days, (b) hydration state (USG) would be significantly improved within 60 minutes of consuming 600 ml of water, and (c) drinking 600 ml of a solution containing electrolytes would increase fluid retention and hydration status of HYPO subjects to a greater extent than water.
Experimental Approach to the Problem
Two experimental protocols were adopted to (a) test the repeatability of morning USG and the effect of water intake on USG (study 1), and (b) determine the effects of 4 fluids: water only (W), salt water (SW, 40 mM Na+), and 2 carbohydrate-electrolyte solutions (CES) with varying carbohydrate content (3% carbohydrate, 20 mM Na+ [CES-L] and 6% carbohydrate, 20 mM Na+ [CES]) on USG (study 2). A randomized crossover design was used for both the experimental protocols.
Ten recreationally active individuals (6 women, 4 men) volunteered to participate in these experiments. Their mean (±SE) age and BM were 25.2 ± 0.9 years and 71.9 ± 4.6 kg. In study 2, 1 female subject withdrew from the study (n = 9). Each participant was engaged in physical activity at least 3 times per week. Female participants were taking oral contraceptives and were tested in the follicular phase of their menstrual cycle. All the participants were informed of the experimental protocol, both orally and in writing, before written informed consent was obtained. The study was approved by the Research Ethics Board at the University of Guelph.
Morning Hydration Status Over 5 Consecutive Days
The participants collected a midstream urine sample upon waking on 5 consecutive weekdays. After voiding their bladder, BM was determined on a Zenith scale (Zenith LG Electronics Canada, Mississauga, Ontario, Canada). Every morning urine sample was refrigerated at 4°C and later analyzed for USG (Atago USA, Inc., Bellevue, Washington, D.C., USA). The Participants were instructed to weigh themselves at the same time and with the same clothing each day and record their BM and daily fluid intake on a data sheet provided. The participants were asked to drink normally the night before each trial and were instructed to maintain normal sleep patterns and nutrition.
Effect of Fluid Consumption on Hydration State
On 4 weekday afternoons over a 2-week period, the participants arrived at the laboratory at 3 PM for an experimental trial. The participants restrained from consuming caffeine the day of each trial and dietary intake was replicated for each trial day. On 2 of the 4 randomized occasions, the participants were allowed to drink ad libitum before the trial to arrive in an HYD state (USG < 1.020). For the other 2 experimental trials, the subjects drank ad libitum until 11 AM and then were asked not to drink any fluid up until the time of their trial (3 PM) to ensure the participants arrived in a HYPO state (USG > 1.020). The 4-hour fluid restriction was chosen to simulate what players do as they approach practice time.
Upon arriving at the laboratory the participants provided a small midstream urine sample for the determination of pretrial USG, voided their bladder and were weighed to determine pretrial BM (Figure 1). Each subject then drank 300 ml of water at 0 minutes and another 300 ml of water at 15 minutes for a total of 600 ml. This volume was chosen as the ACSM and NATA Position Stands recommend that approximately 600 ml should be ingested in the 60–90 minutes before strenuous exercise. The practical significance is that this is a reasonable amount of fluid that can be consumed and ensure that the athlete is well hydrated when their training session or game begins.
The subjects were then asked to void their bladder at 30, 45, and 60 minutes into a measuring cup to determine USG and urine volume, and were then weighed.
The effect of carbohydrate and electrolyte solutions on hydration status is as follows: On 4 weekday mornings (once per week), the participants arrived at 9 AM to the laboratory for an experimental trial with explicit instructions not to drink any fluid upon waking. The participants replicated the same experimental protocol as in Figure 1; however, an additional urine sample was collected, and a BM measurement was taken at 15 minutes. The participants completed 4 randomized trials which varied in the type of fluid ingested; (a) water (W), (b) an SW solution (40 mM Na+), (c) a commercially available CES solution with light carbohydrate content (CES-L, G2, 20 mM Na+, 3% carbohydrate), (d) a commercially available CES with normal carbohydrate content (CES, Gatorade, 20 mM Na+, 6% carbohydrate).
The USG was determined for each urine sample using a hand-held pocket refractometer (Atago USA Inc.) The refractometer was calibrated with distilled water before measuring each urine sample. USG values <1.020 indicated an HYD state and USG ≥ 1.020 signified hypohydration. BM was measurement using a Zenith scale (LG Electronics Canada, Mississauga, Ontario, Canada) accurate to ±0.1 kg. Urine volume was collected in measuring cups accurate to ±25 ml.
All data were tested for normality of distribution and presented as the mean ± SE. Time vs. trial data were assessed using a 2-way analysis of variance (ANOVA). A 1-way ANOVA was used to compare differences between fluid types. The significance of the measured differences between 2 trials was assessed using a paired t test. Statistical significance was accepted as p ≤ 0.05. Associations between variables were investigated using Pearson's correlation analysis. The coefficient of variation (CV) was calculated using the SD divided by the mean, multiplied by 100.
Study 1: Morning USG
Morning USG was consistent over consecutive days (M 1.015 ± 0.002, Tu 1.017 ± 0.004, W 1.019 ± 0.002, Th 1.018 ± 0.003, F 1.017 ± 0.002). All correlations were significant between any 2 days (sample correlation in Figure 2), and there was no significant difference in USG between days. The mean CV for individual USGs between days was 0.2 ± 0.1%. Three participants had a mean morning USG of ≥1.020, 6 participants had USGs between 1.012 and 1.019, and 1 participant had a mean USG < 1.008. Body mass was also consistent between days (M 72.5 ± 5.0, Tu 72.1 ± 4.9, W 71.9 ± 4.9, Th 71.9 ± 4.9, F 71.8 ± 4.8 kg) with a mean individual CV of 0.3 ± 0.4%. Mean daily fluid intake was 3495 ± 511 ml with low variability within participants, but large variability between subjects (1,538–7,299 ml). All the participants consumed between 1,500 and 5,000 ml of fluid per day with the exception of one participant who drank between 6,000 and 7,300 ml·d−1.
Study 1: Effect of Water Consumption on USG
Test-retest data revealed that USG and urine volume were repeatable between HYD and HYPO trials (Table 1). The mean data of the 2 HYD and the 2 HYPO trials were therefore presented below. A significant moderate relationship was found between pretrial USG and total urine volume (r = 0.54).
Pretrial USG indicated that all the participants were well hydrated before the trial (1.012 ± 0.002, Table 1) and USG was significantly lower than in the HYPO trials (p = 0.01). The USG was significantly lower at 45 and 60 minutes than the pretrial USG (Figure 3A, p = 0.02). Mean total urine volume over the trial was 356 ± 7 ml, such that 41% of the fluid consumed was retained. Average pretrial BM was 73.2 ± 5.1 and was unchanged after the trial at 73.4 ± 5.0 kg (p > 0.05).
Pretrial USG indicated that the participants were hypohydrated before the trial (1.022 ± 0.004, Table 1). The USG was lower than pretrial USG and within the euhydrated range at 45 (1.013 ± 0.003) and 60 (1.010 ± 0.002) minutes after ingestion of 600 ml of water (Figure 3A). Mean total urine volume was 167 ± 3 ml and significantly lower than the HYD trial (Figure 3B, p = 0.009), such that participants retained 72% of the fluid consumed. The BM increased from a pretrial mass of 73.6 ± 5.1 to 73.9 ± 5.1 kg after 60 minutes.
Study 2: Impact of Water, Salt, and Carbohydrate Solutions on USG
There was no significant difference between pretrial BM and USG in the 4 trials. All the participants were hypohydrated before all the trials (Table 2). Consuming 600 ml of W, SW, CES-L, or CES decreased USG below the pretrial USG and <1.020 at 45 and 60 minutes (p = 0.03) with no significant differences between trials (Figure 4A). There were also no significant differences for total urine volume (W 192 ± 49, SW 166 ± 38, CES-L 193 ± 30, CES 147 ± 30 ml, Figure 4B, p > 0.05) or fluid retention (W 68%, SW 72%, CES-L 68%, CES 76%, p > 0.05) between trials.
This study examined the reliability of hydration status over 5 consecutive days and how USG was affected by consuming 600 ml of fluid in HYPO and HYD subjects. The main findings of this study were (a) individual morning USG was highly repeatable over 5 consecutive days, (b) the USG and fluid retention responses to ingesting 600 ml of water when euhydrated or hypohydrated were also repeatable, (c) consuming 600 ml of water when hypohydrated significantly decreased USG to within the HYD range within 45 minutes, and (d) there was no differences in the type of fluid consumed (W vs. SW vs. CES) in terms of reducing USG into the HYD range in the hour after consumption.
In this study, average morning USG over 5 consecutive days (1.017 ± 0.002) demonstrated that adult participants were hydrated upon waking. However, 19 out of the 50 individual responses (38%) were >1.020. The mean USG was lower than previously reported over 4–5 consecutive days with adolescent football players in 2 separate studies (Stover et al.  1.023 ± 0.001; Godek et al.  1.023 ± 0.002) and adolescent swimmers (Higham et al.  1.025 ± 0.001). The higher mean morning USGs in these studies may have been because of greater sweat losses during training camp without proper rehydration, as compared with that in this study with recreationally active participants, who were not involved in a training camp over the 5 days of testing. Another possibility is the age of the participants, as this study employed adult subjects vs. the adolescents studied in the previous studies. A consistent result between all studies is the strong individual repeatability in morning USG between days. This reflects the reliability of using morning USG as an indicator of hydration status in adults over consecutive days (25). When investigating the relationship between morning BM and USG, Lew et al. (13) reported a significant inverse correlation, such that when USG was ≥1.020, a 0.003 increase in USG reflected a 1% loss in BM. The data in the present study along with that of Armstrong et al. (1) and Lew et al. (13) suggest that monitoring morning BM is also a useful indicator of HYD status when USG is not available.
This is the first study to examine the impact of consuming 600 ml of water in HYD and HYPO states on hydration status over a 60-minute period and confirms that USG is highly repeatable for participants between trials. It is well established that many athletes arrive at training or competition in a HYPO state (>1.020), which may exacerbate cardiovascular and thermoregulatory responses at the beginning of exercise (6,9,10,12,17,27,28). Our results demonstrate that if an athlete arrives at training or competition in a HYPO state and consumes 600 ml of water, the athlete will move into an HYD state (USG < 1.020) within 45 minutes. This will help minimize any “dehydration during exercise-related” detriments to performance. In 2 studies from the same laboratory on collegiate wrestlers after acute hypohydration of approximately 3% BM, consuming water to replace 150% of BM loses (3.6–3.8 L) in the hour before competition was effective in significantly lowering USG into the HYD range, and lowering urine osmolality (Uosm) and plasma osmolality (Posm) (29,30). Our results are novel in that only 600 ml of fluid consumed was effective at significantly lowering USG within 45 minutes compared with the USG change seen in the previous studies after rehydrating with 150% of BM losses amounting to 3.6–3.8 L (29,30). Ultimately, our results are relevant for normal exercise situations with mild hypohydration, as our participants were not dehydrated from previous exercise before testing as compared with that in the latter 2 studies.
In addition, 2 studies reported that both USG and Uosm lagged behind Posm in accurately identifying changes in the HYD state produced by acute exercise dehydration after rehydrating with 100% BM losses (16,19). Although Uosm and Posm were not measured in this study we would predict based on the data of the above studies that the significant lowering in USG after consuming 600 ml suggests that Posm was also likely lowered to a value consistent with effective rehydration. In addition, we showed that when subjects arrived hydrated to the trial, 41% of the 600 ml of water consumed was retained, whereas 72% of the ingested water was retained when participants arrived in a HYPO state. As might be expected, a moderate relationship was found between pretrial USG and total Uvol, such that greater dehydration lead to greater fluid retention.
In this study, we expected to see greater fluid retention after consuming 600 ml of a salt and salt and carbohydrate solution (SW, CES-L, CES). However, our results did not demonstrate a significant difference between trials in fluid retention measured by urine output. This is in contrast to other studies where dehydration was induced by exercise and was more severe and the following rehydration amounted to approximately 150% of BM losses (25). Valiente et al. (30) and Shirreffs and Maughan (25) reported that a greater amount of fluid was retained after exercise when rehydration occurred with a CES compared with that of water. The authors attributed these results in part to the increased osmolality of the CES to promote greater fluid retention and less acute diuresis. Shirreffs et al. (22,23,26) also suggested that the amount of fluid retained was directly proportional to the [Na+] of the fluid consumed. However, the hypohydration in this study was not exercise-induced and quite mild, and we reported no difference of the type of fluid consumed on USG response or total Uvol over a 1-hour period after consuming 600 ml of fluid. There was a trend for fluid retention to be slightly greater in the CES (76% retention) and SW (72%) trials vs. the W and CES-L trials (68%); however, the differences were not significant. Given the effectiveness of plain water in rehydrating the mildly HYPO participants (68% retention), it may not have been realistic to expect a greater effect with the combinations of carbohydrate and electrolyte solutions.
Because many athletes are instructed to drink 6–8 ml·kg−1 BM of a Na+ containing fluid in the 1–2 hours before exercise (2,7,8,20), we adopted this strategy as our subjects did not dehydrate themselves with previous exercise. Instead we focused on a mild hypohydration situation (≤1% BM loss) that appears to occur during normal living before training and competition. We believe that this situation is more representative of the normal situation for the average athlete. If the correlation reported by Lew et al. (13) between BM and USG is used (∼0.003 increase in USG above 1.020 reflects a 1% loss in BM) then our participants replaced approximately 41% of fluid loss by consuming 600 ml of fluid. Ultimately, our results demonstrate that if an individual is mildly hypohydrated before exercise, consuming 600 ml of any fluid (W, SW, CES-L, or CES) will reverse the hypohydration and put an athlete in an HYD state (USG > 1.020) within 45 minutes before exercise.
In conclusion, this study confirms the use of monitoring morning BM and USG over consecutive days in adults as a reliable indication of daily hydration status. Monitoring morning HYD state (USG or BM) is valuable for athletes to ensure they are replenishing daily sweat losses over subsequent training days. Our results were also highly repeatable on a test-retest basis when the participants arrived for testing in an HYD or HYPO condition and consumed 600 ml of water. Lastly, this study demonstrated that the consumption of either 600 ml of water, water with electrolytes, or CES solutions with varying carbohydrate content, 45 minutes before training or competition, is sufficient to ensure that an athlete begins the exercise in an HYD condition
Morning BM and USG are reliable and valuable measurements that an athlete can use to monitor body water or hydration status over consecutive days. If an athlete arrives at a training session or competition in a HYPO state (USG > 1.020), they can consume 600 ml of water or a CES solution over 15 minutes, and they will retain about 75% of the ingested fluid over the following hour. The importance of the ingested fluid is that it will move the athlete into an HYD state as assessed by USG measures (USG < 1.020) within 30–45 minutes from their arrival. This will minimize the possibility that preexercise hypohydration will exacerbate any exercise-induced sweat losses and decrease the likelihood that fluid loss-related performance decrements will occur.
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