Hypohydration, or a body water deficit, is a common occurrence in athletes and recreational exercisers following the completion of an exercise session. For those who will undertake a further exercise session that day, it is important to replace water losses to avoid beginning the next exercise session hypohydrated and the potential detrimental effects on performance that this may lead to. The aim of this review is to provide an overview of the research related to factors that may affect postexercise rehydration. Research in this area has focused on the volume of fluid to be ingested, the rate of fluid ingestion, and fluid composition. Volume replacement during recovery should exceed that lost during exercise to allow for ongoing water loss; however, ingestion of large volumes of plain water results in a prompt diuresis, effectively preventing longer-term maintenance of water balance. Addition of sodium to a rehydration solution is beneficial for maintenance of fluid balance due to its effect on extracellular fluid osmolality and volume. The addition of macronutrients such as carbohydrate and protein can promote maintenance of hydration by influencing absorption and distribution of ingested water, which in turn effects extracellular fluid osmolality and volume. Alcohol is commonly consumed in the postexercise period and may influence postexercise rehydration, as will the coingestion of food. Future research in this area should focus on providing information related to optimal rates of fluid ingestion, advisable solutions to ingest during different duration recovery periods, and confirmation of mechanistic explanations for the observations outlined.
The change in blood and plasma volume following ingestion of glucose solutions of varying concentrations was estimated in twelve healthy male volunteers. Subjects consumed, within a 5 min period, 600 ml of a solution containing 0, 2, 5 or 10 % glucose with osmolalities of 0 (SD 0), 111 (SD 1), 266 (SD 7) and 565 (SD 5) mOsm/kg, respectively. Blood samples were collected over the course of 1 h after ingestion at intervals of 10 min. After ingestion of the 2 % glucose solution, plasma volume increased from baseline levels at 20 min. Plasma volume decreased from baseline levels at 10 and 60 min after ingestion of the 10 % glucose solution. Heart rate was elevated at 10 and 60 min after ingestion of the 10 % glucose solution and decreased at 30 and 40 min after ingestion of the 2 % glucose solution relative to the average heart rate recorded before drinking. It is concluded that ingestion of hypertonic, energy-dense glucose solutions results in a decrease in plasma and extracellular fluid volume, most likely due to the net secretion of water into the intestinal lumen. The osmolality of an ingested solution determines the osmotic gradient that is the driving force behind the movement of water across the intestinal wall and, therefore, is an important factor determining the direction and rate of water flux in the small intestine. Both rat and human models have suggested that intestinal water absorption is influenced by luminal osmolality, but the relationship will depend on the nature of the added solute. Shi et al. (1) demonstrated that perfusion of a hypotonic carbohydrate solution with an osmolality of 186 mOsm/kg resulted in a 17 % increase in the rate of water absorption relative to a hypertonic (403 mOsm/kg) carbohydrate solution. Similarly, Gisolfi et al. (2) reported that a hypertonic 8 % carbohydrate solution (osmolality approximately 460 mOsm/kg) containing glucose resulted in reduced net water absorption relative to a hypotonic solution (osmolality approximately 270 mOsm/kg). These findings are supported by other segmental perfusion studies (3,4) . Perfusion of hypertonic solutions can result in net secretion of water into the lumen. Leiper & Maughan (5) reported that perfusion of a hypertonic solution, with an osmolality of 488 (SD 53) mOsm/kg after transit through the 15 cm mixing segment of a multilumen tube, resulted in net efflux of water and electrolytes into the lumen over the 30 cm test segment of the intestine, whereas perfusion of an isotonic glucose-electrolyte solution promoted water and electrolyte uptake. Most intestinal absorption studies have assessed the net rate of water absorption using segmental perfusion techniques. Lambert et al. (6) showed that it is possible to investigate the intestinal absorption characteristics of an orally ingested solution if gastric emptying rate is kept relatively constant. However, investigations that directly perfuse the intestine may not accurately represent fluid absorption characteristics of ingested solutions due to changes in the composition of the sol...
Heart rate was measured in 79 young male soccer players during training in the third week of Ramadan. Forty-eight players were practising Ramadan fasting, while the other 31 players were eating normally. All participants trained for 60-80 min at an ambient temperature of 25-28 degrees C and relative humidity of 50-53%. Heart rate, which was measured throughout the training session, was marginally higher in the fasting (mean 144 beats . min(-1), s = 25) than in the non-fasting (139 beats . min(-1), s = 23) group (P < 0.001). When assessed as the percentage of heart rate reserve utilized, however, the training load was similar for both groups (62%, s = 8). No difference was detected in training intensity for the fasting and non-fasting groups when quantified by either training impulse (253, s = 139 and 253, s = 108, respectively) or training load indicator (222, s = 123 and 179, s = 49, respectively). The overall subjective rating of perceived exertion of the training session reported 20 min after finishing training was similar for the fasting (12, range 6-17) and non-fasting (12, range 7-17) groups, which was comparable (P = 0.16) to the mean value for the entire week (13, range 8-16). A similar finding was observed in the players' subjective appraisal of the difficulty of training of the individual session and for the whole week's training. Overall exercise load measures indicated that there was no biologically significant difference between the fasting and non-fasting groups during training in the third week of Ramadan.
The present study investigated the relationship between the milk protein content of a rehydration solution and fluid balance after exerciseinduced dehydration. On three occasions, eight healthy males were dehydrated to an identical degree of body mass loss (BML, approximately 1·8 %) by intermittent cycling in the heat, rehydrating with 150 % of their BML over 1 h with either a 60 g/l carbohydrate solution (C), a 40 g/l carbohydrate, 20 g/l milk protein solution (CP20) or a 20 g/l carbohydrate, 40 g/l milk protein solution (CP40). Urine samples were collected pre-exercise, post-exercise, post-rehydration and for a further 4 h. Subjects produced less urine after ingesting the CP20 or CP40 drink compared with the C drink (P,0·01), and at the end of the study, more of the CP20 (59 (SD 12) %) and CP40 (64 (SD 6) %) drinks had been retained compared with the C drink (46 (SD 9) %) (P,0·01). At the end of the study, whole-body net fluid balance was more negative for trial C (2470 (SD 154) ml) compared with both trials CP20 (2 181 (SD 280) ml) and CP40 (2107 (SD 126) ml) (P,0·01). At 2 and 3 h after drink ingestion, urine osmolality was greater for trials CP20 and CP40 compared with trial C (P, 0·05). The present study further demonstrates that after exercise-induced dehydration, a carbohydrate-milk protein solution is better retained than a carbohydrate solution. The results also suggest that high concentrations of milk protein are not more beneficial in terms of fluid retention than low concentrations of milk protein following exercise-induced dehydration.Key words: Milk: Protein: Rehydration: Water balance: Maltodextrin Exercise results in an increase in energy expenditure, heat production and the initiation of the sweat response to help dissipate some of the heat produced. It has been commonly reported that during exercise, athletes lose more fluid through sweat than they gain through drink ingestion, and thus they finish exercise in a hypohydrated state (1) . In situations where two exercise bouts are completed in close proximity, effective and rapid rehydration after the first bout of exercise will be required if performance in the second bout is not to be affected (2) . As long as a sufficient volume of a rehydration solution is consumed (3) , the main factors that determine how much of the solution is retained are the rate at which it is consumed (4) and its composition (5 -14) . It is likely that these factors exert their effects on rehydration via the inclusion of osmotic substances that enhance water retention or by influencing the rate of appearance in the peripheral circulation, thus attenuating serum osmolality and arginine vasopressin responses.It has been shown that increasing the Na (6,9,10) or carbohydrate (7,11) concentration of a rehydration solution consumed after exercise increases the fraction of the ingested solution that is retained. It has also been shown that low-fat milk is better retained than a carbohydrate -electrolyte sports drink (12,13) ; however, the number of compositional dif...
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