The goal of prehydrating is to start the physical activity euhydrated and with normal plasma electrolyte levels. If sufficient beverages are consumed with meals and a protracted recovery period (8–12 h) has elapsed since the last exercise session, then the person should already be close to being euhydrated. However, if the person has suffered substantial fluid deficits and has not had adequate time or fluids/electrolytes volumes to reestablish euhydration, then an aggressive prehydration program may be merited. The prehydration program will help ensure that any previously incurred fluid-electrolyte deficit is corrected prior to initiating the exercise task.
When hydrating prior to exercise the individual should slowly drink beverages (for example, ~5–7 mL·kg−1 per body weight) at least 4 h before the exercise task. If the individual does not produce urine, or the urine is dark or highly concentrated, s/he should slowly drink more beverage (for example, another ~3–5 mL·kg−1) about 2 h before the event. By hydrating several hours prior to exercise there is sufficient time for urine output to return towards normal before starting the event. Consuming beverages with sodium (20–50 mEq·L−1) and/or small amounts of salted snacks or sodium-containing foods at meals will help to stimulate thirst and retain the consumed fluids.[88,112,128]
Attempting to hyperhydrate with fluids that expand of the extra- and intracellular spaces (e.g., water and glycerol solutions) will greatly increase the risk of having to void during competition[58,107] and provides no clear physiologic or performance advantage over euhydration.[77,79,80] In addition, hyperhydration can substantially dilute and lower plasma sodium[58,107] before starting exercise and therefore increase the risk of dilutional hyponatremia, if fluids are aggressively replaced during exercise.
Enhancing palatability of the ingested fluid is one way to help promote fluid consumption, before, during, or after exercise. Fluid palatability is influenced by several factors including temperature, sodium content and flavoring. The preferred water temperature is often between 15 and 21°C, but this and flavor preference varies greatly between individuals and cultures.
Recommendations. Prehydrating with beverages, if needed, should be initiated at least several hours before the exercise task to enable fluid absorption and allow urine output to return toward normal levels. Consuming beverages with sodium and/or salted snacks or small meals with beverages can help stimulate thirst and retain needed fluids.
The goal of drinking during exercise is to prevent excessive dehydration (>2% BW loss from water deficit) and excessive changes in electrolyte balance to avert compromised exercise performance. The amount and rate of fluid replacement depends upon the individual sweating rate, exercise duration, and opportunities to drink. Individuals should periodically drink (as opportunities allow) during exercise, if it is expected they will become excessively dehydrated. Care should be taken in determining fluid replacement rates, particularly in prolonged exercise lasting greater than 3 h. The longer the exercise duration the greater the cumulative effects of slight mismatches between fluid needs and replacement, which can excessive dehydration or dilutional hyponatremia.
It is difficult to recommend a specific fluid and electrolyte replacement schedule because of different exercise tasks (metabolic requirements, duration, clothing, equipment), weather conditions, and other factors (e.g., genetic predisposition, heat acclimatization and training status) influencing a person's sweating rate and sweat electrolyte concentrations. Table 4 provides approximate sweating rates for individuals of different sizes, running at different speeds in cool/temperate and warm weather conditions. These predicted sweating rates range from ~0.4 to ~1.8 L·h−1 and individual sweating rates for any of these conditions probably have a normal distribution with unknown variance. Therefore, it is recommended that individuals should monitor body weight changes during training/competition sessions to estimate their sweat lost during a particular exercise task with respect to the weather conditions. This allows customized fluid replacement programs to be developed for each person's particular needs; however, this may not always be practical. Fluid and electrolyte replacement strategies will be vastly different for a large football player in early season summer practice when contrasted with a petite marathoner running at a 6-h pace.
A possible starting point suggested for marathon runners (who are euhydrated at the start) is they drink ad libitum from 0.4 to 0.8 L·h−1, with the higher rates for faster, heavier individuals competing in warm environments and the lower rates for the slower, lighter persons competing in cooler environments.Table 5 provides the predicted body weight changes (from under- or overconsumption of fluids) during a 42-km marathon for persons of different sizes running at different speeds in cool/temperate weather. The analysis employed the sweating rates provided in Table 4 and three fluid replacement rates (0.4, 0.6, 0.8 L·h−1). For smaller runners, drinking at 0.8 L·h−1 resulted in over-consumption (weight gain, light shaded areas), and for larger runners, drinking at 0.4 L·h−1 resulted in excessive dehydration (3% body weight loss, dark shaded areas). Clearly, this table demonstrates that it is inappropriate to use a single fluid replacement rate for all runners; however, the use of activity specific caveats can broaden the applicability of general guidance. For example, a mathematical analysis to estimate plasma sodium levels for the conditions in Table 5, predicted that if the above caveats for runner size, speed and environmental conditions are followed, the 0.4–0.8 L·h−1 guidelines are probably satisfactory for individuals participating in marathon length events. However, longer durations or different types of physical activity, more extreme weather and unique populations may have considerably different fluid replacement needs. For example, some American football players (often with very large body weights) wearing full equipment in hot weather are reported to have sweating losses of >8 L·d−1, these individuals will require much larger fluid volumes to maintain euhydration on a day to day basis compared to the runners Table 6.
The composition of the consumed fluids can be important. The Institute of Medicine provided general guidance for composition of "sports beverages" for persons performing prolonged physical activity in hot weather. They recommend that these types of fluid replacement beverages might contain ~20–30 meq·L−1 sodium (chloride as the anion), ~2–5 meq·L−1 potassium and ~5–10% carbohydrate. The need for these different components (carbohydrate and electrolytes) will depend on the specific exercise task (e.g., intensity and duration) and weather conditions. The sodium and potassium are to help replace sweat electrolyte losses, while sodium also helps to stimulate thirst, and carbohydrate provides energy. These components also can be consumed by nonfluid sources such as gels, energy bars, and other foods.
Carbohydrate consumption can be beneficial to sustain exercise intensity during high-intensity exercise events of ~1 h or longer, as well as less intense exercise events sustained for longer periods.[13,43,44,76,146] Carbohydrate-based sports beverages are sometimes used to meet carbohydrate needs, while attempting to replace sweat water and electrolyte losses. Carbohydrate consumption at a rate of ~30–60 g·h−1 has been demonstrated to maintain blood glucose levels and sustain exercise performance.[43,44] For example, to achieve a carbohydrate intake sufficient to sustain performance, an individual could ingest one-half to one liter of a conventional sports drink each hour (assuming 6–8% carbohydrate, which would provide 30–80 g·h−1 of carbohydrate) along with sufficient water to avoid excessive dehydration. The greatest rates of carbohydrate delivery are achieved with a mixture of sugars (e.g., glucose, sucrose, fructose, maltodextrine). If both fluid replacement and carbohydrate delivery are going to be met with a single beverage, the carbohydrate concentration should not exceed 8%, or even be slightly less, as highly concentrated carbohydrate beverages reduce gastric emptying.[75,145] Finally, caffeine consumption might help to sustain exercise performance and likely will not alter hydration status during exercise.[44,72,147]
Recommendations. Individuals should develop customized fluid replacement programs that prevent excessive (< 2% body weight reductions from baseline body weight) dehydration. The routine measurement of pre- and postexercise body weights is useful for determining sweat rates and customized fluid replacement programs. Consumption of beverages containing electrolytes and carbohydrates can help sustain fluid-electrolyte balance and exercise performance.
After exercise, the goal is to fully replace any fluid and electrolyte deficit. The aggressiveness to be taken depends on the speed that rehydration must be accomplished and the magnitude of the fluid-electrolyte deficit. If recovery time and opportunities permit, consumption of normal meals and snacks with a sufficient volume of plain water will restore euhydration, provided the food contains sufficient sodium to replace sweat losses. If dehydration is substantial with a relatively short recovery periods (< 12 h) then aggressive rehydration programs may be merited.[87,88,128]
Failure to sufficiently replace sodium losses will prevent the return to euhydrated state and stimulate excessive urine production.[87,105,127] Consuming sodium during the recovery period will help retain ingested fluids and help stimulate thirst. Sodium losses are more difficult to assess than water losses, and it is well known that individuals lose sweat electrolytes at vastly different rates. Drinks containing sodium such as sports beverages may be helpful, but many foods can supply the needed electrolytes. A little extra salt may usefully be added to meals and recovery fluids when sweat sodium losses are high.
Individuals looking to achieve rapid and complete recovery from dehydration should drink ~1.5 L of fluid for each kilogram of body weight lost. The additional volume is needed to compensate for the increased urine production accompanying the rapid consumption of large volumes of fluid. Therefore, when possible, fluids should be consumed over time (and with sufficient electrolytes) rather than being ingested in large boluses to maximize fluid retention.[78,148]
Intravenous fluid replacement after exercise may be warranted in individuals with severe dehydration (>7% body weight loss), with nausea, vomiting, or diarrhea, or who for some reason cannot ingest oral fluids. For most situations, intravenous fluid replacement does not provide an advantage over drinking in replacing fluid and electrolyte deficits.
Recommendations. If time permits, consumption of normal meals and beverages will restore euhydration. Individuals needing rapid and complete recovery from excessive dehydration can drink ~1.5 L of fluid for each kilogram of body weight lost. Consuming beverages and snacks with sodium will help expedite rapid and complete recovery by stimulating thirst and fluid retention. Intravenous fluid replacement is generally not adventagous, unless medically merited.
Cite this: Exercise and Fluid Replacement - Medscape - Feb 01, 2007.