Susan M. Shirreffs

Post-exercise rehydration in man : effects of volume consumed and drink sodium content

The interaction between the volume and composition of fluids ingested was investigated in terms of rehydration effectiveness. Twelve male volunteers, dehydrated by 2.06 +/- 0.02% (mean +/- SE) of body mass by intermittent cycle exercise, consumed a different drink volume on four separate weeks; six subjects received drink L (23 mmol.l-1 Na+) in each trial and six were given drink H (61 mmol.l-1 Na+). Volumes consumed were equivalent to 50%, 100%, 150%, and 200% of body mass loss (trials A, B, C, and D, respectively). Blood and urine samples were obtained before exercise and for 7.5 h after exercise. Less urine was excreted following rehydration in trial A than in all other trials. Cumulative urine output (median ml) was less in trial B (493, range 181-731) than D (1361, range 1014-1984), which was not different from trial C (867, range 263-1191) in group L. In group H, the volume excreted in trial B (260, range 137-376) was less than trials C (602, range 350-994) and D (1001, range 714-1425), and the volume in trial C was less than in trial D. These results suggest that both sodium concentration and fluid volume consumed interact to affect the rehydration process. A drink volume greater than sweat loss during exercise must be ingested to restore fluid balance, but unless the sodium content of the beverage is sufficiently high this will merely result in an increased urinary output.

Errors in the estimation of hydration status from changes in body mass

Abstract Hydration status is not easily measured, but acute changes in hydration status are often estimated from body mass change. Changes in body mass are also often used as a proxy measure for sweat losses. There are, however, several sources of error that may give rise to misleading results, and our aim in this paper is to quantify these potential errors. Respiratory water losses can be substantial during hard work in dry environments. Mass loss also results from substrate oxidation, but this generates water of oxidation which is added to the body water pool, thus dissociating changes in body mass and hydration status: fat oxidation actually results in a net gain in body mass as the mass of carbon dioxide generated is less than the mass of oxygen consumed. Water stored with muscle glycogen is presumed to be made available as endogenous carbohydrate stores are oxidized. Fluid ingestion and sweat loss complicate the picture by altering body water distribution. Loss of hypotonic sweat results in increased osmolality of body fluids. Urine and faecal losses can be measured easily, but changes in the water content of the bladder and the gastrointestinal tract cannot. Body mass change is not always a reliable measure of changes in hydration status and substantial loss of mass may occur without an effective net negative fluid balance.

Urine osmolality and conductivity as indices of hydration status in athletes in the heat

PURPOSE The purpose of this study was to determine a quick and easy method for assessment of day-to-day hydration status in athletes in the heat. METHODS Measurement of the osmolality of the first urine sample of the day collected after wakening but before breakfast established a standardized collection procedure to allow day-to-day comparisons of individuals. RESULTS Laboratory measurements established that a difference in osmolality is found when individuals are dehydrated by a moderate extent in comparison with an euhydrated situation: the osmolality of the first morning urine sample of control subjects (N = 11) averaged over 5 d was 675 (+/- 232) mosmol.kg-1 (mean +/- SD). For subjects who were hypohydrated by exercise followed by fluid restriction, morning urine osmolality was 924 (+/- 99) mosmol.kg-1 (P < 0.001, N = 11, averaged over 7 d). Field measurements from 29 athletes undertaking warm weather training indicated that the athletes could, with appropriate feedback, maintain a satisfactory hydration status. Athletes in weight category sports tended to record a higher morning urine osmolality, reflecting their attempts to dehydrate: recorded values were 627 (+/- 186) mosmol.kg-1 (nonweight category sports, N = 8), 775 (+/- 263) mosmol.kg-1 (boxers, N = 15) and 777 (+/- 254) mosmol.kg-1 (wrestlers, N = 6). Results obtained with a hand-held portable conductivity were compared with those from measured osmolality. CONCLUSIONS The findings suggest that such an instrument could provide athletes with reliable information as to their hydration status from measurement of the first morning urine of the day and therefore provide a quick and easy method for achieving an approximation of hydration status from day-to-day.

The effects of fluid restriction on hydration status and subjective feelings in man

Hydration status and the effects of hypohydration have been the topic of much public and scientific debate in recent years. While many physiological responses to hypohydration have been studied extensively, the subjective responses to hypohydration have largely been ignored. The present investigation was designed to investigate the physiological responses and subjective feelings resulting from 13, 24 and 37 h of fluid restriction (FR) and to compare these with a euhydration (EU) trial of the same duration in fifteen healthy volunteers. The volunteers were nine men and six women of mean age 30 (sd 12) years and body mass 71.5 (sd 13.4) kg. Urine and blood samples were collected and subjective feelings recorded on a 100 mm verbally anchored questionnaire at intervals throughout the investigation. In the EU trial the subjects maintained their normal diet. Body mass decreased by 2.7 (sd 0.6) % at 37 h in the FR trial and did not change significantly in the EU trial. Food intake in the FR trial (n 10) provided an estimated water intake of 487 (sd 335) ml and urinary losses (n 15) amounted to 1.37 (sd 0.39) litres. This is in comparison with an estimated water intake of 3168 (sd 1167) ml and a urinary loss of 2.76 (sd 1.11) litres in the EU trial. Plasma osmolality and angiotensin II concentrations increased from 0-37 h with FR. Plasma volume decreased linearly throughout the FR trial amounting to a 6.2 (sd 5.1) % reduction by 37 h. Thirst increased from 0-13 h of FR then did not increase further (P>0.05). The subjects reported feelings of headache during the FR trial and also that their ability to concentrate and their alertness were reduced.

Fluid and electrolyte needs for preparation and recovery from training and competition

For a person undertaking regular exercise, any fluid deficit that is incurred during one exercise session can potentially compromise the next exercise session if adequate fluid replacement does not occur. Fluid replacement after exercise can, therefore, frequently be thought of as hydration before the next exercise bout. The importance of ensuring euhydration before exercise and the potential benefits of temporary hyperhydration with sodium salts or glycerol solutions are also important issues. Post-exercise restoration of fluid balance after sweat-induced dehydration avoids the detrimental effects of a body water deficit on physiological function and subsequent exercise performance. For effective restoration of fluid balance, the consumption of a volume of fluid in excess of the sweat loss and replacement of electrolyte, particularly sodium, losses are essential. Intravenous fluid replacement after exercise has been investigated to a lesser extent and its role for fluid replacement in the dehydrated but otherwise well athlete remains equivocal.

POST-EXERCISE REHYDRATION IN MAN : EFFECTS OF ELECTROLYTE ADDITION TO INGESTED FLUIDS

This study examined the effects on water balance of adding electrolytes to fluids ingested after exercise-induced dehydration. Eight healthy male volunteers were dehydrated by approximately 2% of body mass by intermittent cycle exercise. Over a 30-min period after exercise, subjects ingested one of the four test drinks of a volume equivalent to their body mass loss. Drink A was a 90 mmol·l−1 glucose solution; drink B contained 60 mmol·l−1 sodium chloride; drink C contained 25 mmol·l−1 potassium chloride; drink D contained 90 mmol·l−1 glucose, 60 mmol·l−1 sodium chloride and 25 mmol·l−1 potassium chloride. Treatment order was randomised. Blood and urine samples were obtained at intervals throughout the study; subjects remained fasted throughout. Plasma volume increased to the same extent after the rehydration period on all treatments. Serum electrolyte (Na+, K+ and Cl−) concentrations fell initially after rehydration before returning to their pre-exercise levels. Cumulative urine output was greater after ingestion of drink A than after ingestion of any of the other drinks. On the morning following the trial, subjects were in greater net negative fluid balance [mean (SEM);P<0.02] on trial A [745 (130) ml] than on trials B [405 (51) ml], C [467 (87) ml] or D [407 (34) ml]. There were no differences at any time between the three electrolyte-containing solutions in urine output or net fluid balance. One hour after the end of the rehydration period, urine osmolality had fallen, with a significant treatment effect (P=0.016); urine osmolality was lowest after ingestion of drink A. On the morning after the test, subjects were in greater net negative sodium balance (P<0.001) after trials A and C than after trials B and D. Negative potassium balance was greater (P<0.001) after trials A and B than after C and D. Chloride balance was positive after drink D and a smaller negative balance (P<0.001) was observed after drink B than after A and C. These results suggest that although the measured blood parameters were similar for all trials, better whole body water and electrolyte balance resulted from the ingestion of electrolyte-containing drinks. There appeared, however, to be no additive effect of including both sodium and potassium under the conditions of this experiment.

Restoration of fluid balance after exercise-induced dehydration: effects of food and fluid intake

This study investigated the effects of post-exercise rehydration with fluid alone or with a meal plus fluid. Eight healthy volunteers (five men, three women) were dehydrated by a mean of 2.1 (SEM 0.0)% of body mass by intermittent cycle exercise in a warm [34 (SEM 0)°C], humid [55 (SEM 1)% relative humidity] environment. Over 60 min beginning 30 min after exercise, the subjects ingested a commercially-available sports drink (21 mmol · l−1 Na+, 3.4 mmol · l− K+, 12 mmol · l−1 Cl−1) on trials A and B; on trial C a standard meal [63 kJ · kg−1 body mass (53% CHO, 28%fat,19%protein; 0.118 mmol · kJ−1 Na+, 0.061 mmol · kJ−1 K+)] plus drink (1 mmol · l−1 Na+, 0.4 mmol · l−1 K+, 1 mmol · l−1 Cl−) were consumed. Water intake (in millilitres) was 150% of the mass loss (in grams). The trials took place after an overnight fast and were separated by 7 days. Blood and urine samples were collected at intervals throughout the study. Blood was analysed for haematocrit, haemoglobin concentration, serum osmolality, Na+, Ku+ and Cl− concentrations and plasma angiotensin II concentration. Urine volume, osmolality and electrolyte concentrations were measured. Dehydration resulted in a mean 5.2 (SEM 1.3)% reduction in plasma volume. With the exception of serum osmolality, which was higher on trial B than A at the end of the rehydration period, no differences were recorded for any of the measured parameters between trials A and B. Cumulative urine output following rehydration was lower (P < 0.01) on trial C [median 665 (range 396–1190) ml] than on trial B [median 934 (range 550–1403) ml] which was not different (P = 0.44) from trial A [median 954 (range 474–1501) ml]. Less urine was produced over the 1-h period ending 2 h after rehydration on trial C than on B (P = 0.01). On trials A and B the subjects were in net negative fluid balance by 337 (range 779-minus 306) ml and 373 (range 680-minus 173) ml, respectively (P < 0.01): on trial C the subjects were no different from their initial euhydrated state [median minus 29 (range minus 421−137) ml] 6 h after the end of rehydration (P = 1.00). A larger fraction of total water intake was retained when the standard meal plus drink was consumed. This may have been due to the larger quantities of Na+ and K+ ingested with the meal [mean 63 (SEM 4)mmol Na+, 21.3 (SEM 1.3)mmol K+] than with the drink [mean 42 (SEM 2) mmol Na+, 6.8 (SEM 0.4) mmol K+]. There was no difference between trials B and C in any of the measured blood parameters, but urinary Na+ and K+ excretion were both higher on trial C than B. These results suggest that post-exercise fluid replacement can be achieved by ingestion of water if consumed in sufficient volume together with a meal providing significant amounts of electrolytes.

Fluid and electrolyte balance in elite male football (soccer) players training in a cool environment

There are few data in the published literature on sweat loss and drinking behaviour in athletes training in a cool environment. Sweat loss and fluid intake were measured in 17 first-team members of an elite soccer team training for 90 min in a cool (5°C, 81% relative humidity) environment. Sweat loss was assessed from the change in body mass after correction for the volume of fluid consumed. Sweat electrolyte content was measured from absorbent patches applied at four skin sites. Mean ( ± s) sweat loss during training was 1.69 ± 0.45 l (range 1.06 - 2.65 l). Mean fluid intake during training was 423 ± 215 ml (44 - 951 ml). There was no apparent relationship between the amount of sweat lost and the volume of fluid consumed during training (r 2  =  0.013, P  =  0.665). Mean sweat sodium concentration was 42.5 ± 13.0 mmol · l−1 and mean sweat potassium concentration was 4.2 ± 1.0 mmol · l−1. Total salt (NaCl) loss during training was 4.3 ± 1.8 g. The sweat loss data are similar to those recorded in elite players undergoing a similar training session in warm environments, but the volume of fluid ingested is less.

Cold Drink Ingestion Improves Exercise Endurance Capacity in the Heat

PURPOSE To investigate the effect of drink temperature on cycling capacity in the heat. METHODS On two separate trials, eight males cycled at 66 +/- 2% VO2peak (mean +/- SD) to exhaustion in hot (35.0 +/- 0.2 degrees C) and humid (60 +/- 1%) environments. Participants ingested three 300-mL aliquots of either a cold (4 degrees C) or a warm (37 degrees C) drink during 30 min of seated rest before exercise and 100 mL of the same drink every 10 min during exercise. Rectal and skin temperatures, heart rate, and sweat rate were recorded. Ratings of thermal sensation and perceived exertion were assessed. RESULTS Exercise time was longer (P < 0.001) with the cold drink (63.8 +/- 4.3 min) than with the warm drink (52.0 +/- 4.1 min). Rectal temperature fell by 0.5 +/- 0.1 degrees C (P < 0.001) at the end of the resting period after ingestion of the cold drinks. There was no effect of drink temperature on mean skin temperature at rest (P = 0.870), but mean skin temperature was lower from 20 min during exercise with ingestion of the cold drink than with the warm drink (P < 0.05). Heart rate was lower before exercise and for the first 35 min of exercise with ingestion of the cold drink than with the warm drink (P < 0.05). Drink temperature influenced sweat rate (1.22 +/- 0.34 and 1.40 +/- 0.41 L x h(-1) for the cold and the warm drink, respectively; P < 0.05). Ratings of thermal sensation and perceived exertion (P < 0.01) during exercise were lower when the cold drink was ingested. CONCLUSION Compared with a drink at 37 degrees C, the ingestion of a cold drink before and during exercise in the heat reduced physiological strain (reduced heat accumulation) during exercise, leading to an improved endurance capacity (23 +/- 6%).

Volume repletion after exercise-induced volume depletion in humans: replacement of water and sodium losses

Sodium and water loss during, and replacement after, exercise-induced volume depletion was investigated in six volunteers volume depleted by 1.89 ± 0.17% (SD) of body mass by intermittent exercise in a warm, humid environment. Subjects exercised in a large, open plastic bag, allowing collection of all sweat secreted during exercise. For over 60 min beginning 40 min after the end of exercise, subjects ingested drinks containing 0, 25, 50, or 100 mmol/l sodium ( trials 0, 25, 50, and 100) in a volume (ml) equivalent to 150% of the mass lost (g) by volume depletion. Body mass loss and sweat electrolyte (Na+, K+, and Cl-) loss were the same on each trial. The measured sweat sodium concentration was 49.2 ± 18.5 mmol/l, and the total loss (63.9 ± 38.7 mmol) was greater than that ingested on trials 0 and 25. Urine production over the 6-h recovery period was inversely related to the amount of sodium ingested. Subjects were in whole body negative sodium balance on trials 0 (-104 ± 48 mmol) and 25 (-65 ± 30 mmol) and essentially in balance on trial 50(-13 ± 29 mmol) but were in positive sodium balance on trial 100 (75 ± 40 mmol). Only on trial 100 were subjects in positive fluid balance at the end of the study. There was a large urinary loss of potassium over the recovery period on trial 100, despite a negligible intake during volume repletion. These results confirm the importance of replacement of sodium as well as water for volume repletion after sweat loss. The sodium intake on trial 100 was appropriate for acute fluid balance restoration, but its consequences for potassium levels must be considered to be undesirable in terms of whole body electrolyte homeostasis for anything other than the short term.Sodium and water loss during, and replacement after, exercise-induced volume depletion was investigated in six volunteers volume depleted by 1.89 +/- 0.17% (SD) of body mass by intermittent exercise in a warm, humid environment. Subjects exercised in a large, open plastic bag, allowing collection of all sweat secreted during exercise. For over 60 min beginning 40 min after the end of exercise, subjects ingested drinks containing 0, 25, 50, or 100 mmol/l sodium (trials 0, 25, 50, and 100) in a volume (ml) equivalent to 150% of the mass lost (g) by volume depletion. Body mass loss and sweat electrolyte (Na+, K+, and Cl-) loss were the same on each trial. The measured sweat sodium concentration was 49.2 +/- 18.5 mmol/l, and the total loss (63.9 +/- 38.7 mmol) was greater than that ingested on trials 0 and 25. Urine production over the 6-h recovery period was inversely related to the amount of sodium ingested. Subjects were in whole body negative sodium balance on trials 0 (-104 +/- 48 mmol) and 25 (-65 +/- 30 mmol) and essentially in balance on trial 50 (-13 +/- 29 mmol) but were in positive sodium balance on trial 100 (75 +/- 40 mmol). Only on trial 100 were subjects in positive fluid balance at the end of the study. There was a large urinary loss of potassium over the recovery period on trial 100, despite a negligible intake during volume repletion. These results confirm the importance of replacement of sodium as well as water for volume repletion after sweat loss. The sodium intake on trial 100 was appropriate for acute fluid balance restoration, but its consequences for potassium levels must be considered to be undesirable in terms of whole body electrolyte homeostasis for anything other than the short term.