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Intravenous versus Oral Rehydration: Physiological, Performance, and Legal Considerations

Casa, Douglas J.; Ganio, Matthew S.; Lopez, Rebecca M.; McDermott, Brendon P.; Armstrong, Lawrence E.; Maresh, Carl M.

Current Sports Medicine Reports: July-August 2008 - Volume 7 - Issue 4 - p S41-S49
doi: 10.1249/JSR.0b013e31817f3e85
Supplement-Sodium Balance and Exercise

The limited data to this point contradict conventional wisdom that intravenous (IV) rehydration is superior to oral rehydration for athletes who rehydrate between two bouts of exercise. Traditionally, it has been reasoned that the immediate entry of fluids into the bloodstream via IV offers a multitude of benefits for a healthy, dehydrated athlete looking to replace fluids rapidly. Our findings, however, indicate that the rehydration process is more complicated than simply the speed at which fluid enters the circulation and likely is influenced by physiological responses stimulated by oral fluid ingestion. Whether caused by oropharyngeal and thirst perturbations or other factors, given the speed in which fluids taken orally enter the circulation and become part of sweat, these responses should not be underestimated.

Human Performance Laboratory, Department of Kinesiology, Neag School of Education, University of Connecticut, Storrs, Connecticut

Address for correspondence: Douglas J. Casa, Ph.D., ATC, FACSM, FNATA, Director, Athletic Training Education, Research Associate, Human Performance Laboratory, Associate Professor, Department of Kinesiology, Neag School of Education, University of Connecticut, 2095 Hillside Road, U-1110, Storrs, CT 06269-1110 (E-mail: Douglas.Casa@Uconn.edu).

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INTRODUCTION

The practice of using intravenous (IV) fluids to rehydrate athletes appears to be a fairly common practice during training and competitive breaks and following participation across a wide spectrum of sporting events (1). There are unquestioned medical benefits for providing an IV to a dehydrated athlete who is semi- or unconscious or who cannot tolerate oral fluids (2). However, the benefit of IV rehydration over oral rehydration for an athlete simply needing to replace fluids rapidly (not for treatment of a specific medical condition) recently has been explored in a few controlled investigations. The desire for an athlete to enhance the rehydration process is not surprising given that dehydration equivalent to just 2%-3% of body weight can influence athletic performance negatively and compromise physiological function (3). The reasons why an athlete or sports medicine professional may choose to use an IV are multi-factorial, but some of the more common reasons include the following:

  • Water: rapid replacement of fluid to treat dehydration or heat illness
  • Sodium: rapid replacement of sodium for heat cramps, hyponatremia, heat exhaustion, or simply to aid absorption and retention of fluids
  • Energy: rapid replacement of simple sugars for an immediate energy source
  • Drugs: for example, caffeine for stimulation
  • Chilled fluids: to enhance body cooling while also rehydrating
  • Masking: aimed at diluting urine or to deliver substances aimed to obscure other substances

Concerning ergogenic issues, the notion among some sports health professionals are that an IV (1):

  • Is more effective than oral rehydration in rehydrating a dehydrated athlete.
  • Will provide a greater performance edge (over rehydrating orally) for an athlete between two bouts of exercise, such as the first and second halves of an athletic contest or between multiple daily heavy exercise bouts (e.g., decathlete or tennis player).

Some studies examining IV rehydration were not conducted to examine (as a priority) the mode of rehydration (IV vs. oral), but rather to explore the role of maintaining fluid balance on physiological function (i.e., what changes does dehydration impose, as compared to hyperthermia?) (4-7). These studies are important in furthering our understanding of fluid homeostasis on maintaining physiological function and maximizing performance. Few studies specifically have examined the differences between the route of delivery of fluids within the context of athletic performance. Hence, no review articles have focused on this question, although a few have had sections dealing with this issue (8-10). Over the past 10 yr, a series of studies were conducted at the University of Connecticut specifically to compare the efficacy of oral and IV modes of fluid delivery. These studies generally have shown that fluid taken orally (when in the same volume) has similar or advantageous thermoregulatory, cardiovascular, metabolic, perceptual, and health benefits compared with IV (11-19). These results are presented in later sections. This article builds upon a cursory examination first presented at the 2006 IAAF World Anti-Doping Symposium in Lausanne, Switzerland (20).

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CARDIOVASCULAR/HEMODYNAMIC

Fluid replacement aids in the attenuation of cardiovascular deficits and impaired thermoregulation that occurs with exercise performed in a hypohydrated state (12,21,22). When 0.45% or 0.9% (sodium chloride, NaCl) IV solution is used as the rehydration method versus 0.45% oral rehydration, plasma volume (PV) increases more rapidly for either tonicity of IV rehydration (as compared with oral) when length of rehydration is 15-20 min (12,13,17,23). The differences are negated after 5-15 min of exercise that immediately follows the rapid rehydration period. A longer length of rehydration (45 min) results in similar PV levels between methods (15,17,19,24) and tonicities of IV used (14,17,24) during rehydration. These differences are most likely a result of fluid, via IV rehydration, immediately entering the systemic vasculature and increasing PV, whereas oral fluids must be emptied from the gut and absorbed before entering the vasculature and increasing PV (12,23). A 45-min rehydration period allows a longer time period for digestion and absorption of orally ingested fluids. Regardless of the length of the rehydration period, PV changes during a subsequent exercise bout in the heat are similar between modes and tonicities (12,15,19,23,24). It is likely that when rehydration occurs orally in 20 min, there is still substantial intestinal absorption occurring during exercise, resulting in similar PV changes as IV rehydration (Fig. 1), although the rate of this process will be influenced by intensity of exercise and other factors. The importance of PV changes must be balanced with the consideration that a majority of the fluid deficit occurs in intracellular and extracellular spaces exclusive of PV. Thus, the measures of PV changes between mode of rehydration do not capture fully the fluid compartments that most likely are influenced by the dehydration and restored by the rehydration. Additionally, while PV is an important consideration for cardiovascular hemodynamics, previous work has shown that PV restoration is not the primary cause for the attenuation of hyperthermia that is seen when an individual has an enhanced hydration status (typically ~0.2°C lower body temperature for every % body mass difference between a euhydrated and dehydrated state) (6,25,26).

Figure 1

Figure 1

Plasma Na+ increases as a result of hypotonic sweat loss during dehydration (14,24). Because PV is restored more quickly intravenously versus orally (with 0.45% Na+) over a 20-min rehydration period, subsequent plasma Na+ levels are lower with IV versus oral rehydration (12,19). A 45-min rehydration time period results in similar plasma Na+ levels between modes (17,19,24). Likewise, a short rehydration period (20 min) results in plasma Na+ levels that are lower during a subsequent exercise heat-stress when IV is used versus oral rehydration (12). Conversely, when a 45-min rehydration period is used, the increase in plasma Na+ levels during a subsequent exercise heat-stress is independent of rehydration method (19) and tonicity (24). It is possible that a more gradual change in plasma tonicity, as seen with oral rehydration, may be advantageous to more rapid changes seen with IV rehydration. Similarly, a more gradual change in PV may optimize fluid restoration long-term since less fluid may be filtered and stored in the bladder.

Plasma Na+ decreases when a 0.45% rehydration solution is used orally and intravenously (17,19,24). When a 0.9% NaCl IV concentration is used, plasma Na+ does not change during rehydration and thus remains greater than 0.45% IV (14,17,24) and oral (17,24) rehydration. This difference in plasma Na+ remains throughout a subsequent exercise bout performed in the heat (24). Although a 0.9% NaCl solution results in greater plasma Na+ levels, it is important to note that 0.45% NaCl rehydration does not result in hyponatremic blood Na+ levels (i.e., <130 mmol·L−1) (Fig. 2). Rehydration with an amount of fluid no greater than sweat lost during dehydration merely aids in restoring plasma Na+ to predehydration levels. Examining Na+ changes when hyperhydration occurs (with IV and oral rehydration modes) may provide insight into the benefits and risks that may result from that action.

Figure 2

Figure 2

The loss of hypotonic sweat during dehydration also results in plasma osmolality increases (15,17,23,24) that are reversed with rehydration (12,15,17,19,23,24). The restoration of plasma osmolality is independent of the mode of rehydration (IV versus oral) (12,15,17,19,23,24) and tonicity of IV (0.45 and 0.9%) (17,24). Likewise, plasma osmolality responses during a subsequent exercise heat-stress are similar between rehydration modes and tonicities (12,15,19,23,24).

Cardiovascular responses to exercise in the heat are influenced by PV changes (22). Because PV responses during an exercise-heat stress are similar between methods and tonicity of rehydration fluids (12,15,19,23,24), there are similar cardiovascular responses. There are little or no differences in respiratory rate, stroke volume, cardiac output, skin blood flow, and oxygen uptake between modes (12,15,19) and tonicities (14) of rehydration. Heart rate during an exercise heat stress preceded by rehydration is the same (12,13,19,23) or greater (15) with oral versus IV rehydration but not different between 0.9% and 0.45% IV concentration (14) (Fig. 3).

Figure 3

Figure 3

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FLUID REGULATORY HORMONES

The regulation of total body water (TBW) and Na+ occurs at the kidneys via neuroendocrine mechanisms (i.e., involving both the central nervous system and hormonal control). Aldosterone (ALD) and arginine vasopressin (AVP, the antidiuretic hormone) are the primary regulatory hormones. The release of ALD is stimulated by a decrease of blood Na+ concentration and a decreased PV (27). In contrast, the release of AVP is stimulated by an increase of plasma osmolality and body temperature (28), as well as a decrease of blood pressure (27). It is interesting to note that strenuous-prolonged exercise generates all of these stimuli. Thus the control of water and electrolyte balance by ALD and AVP is (a) complex and dynamic and (b) vital to optimal exercise performance and health. The examination of these parameters during different modes of rehydration offers potential explanations as to the mechanisms of stimulation of these responses.

Although other fluid-electrolyte hormones exist (i.e., atrial natriuretic peptide, ANP, which opposes the actions of ALD and AVP; vasoactive intestinal peptide, VIP; and urodilatin), they have not been studied in the context of this article. In fact, only two previous investigations have focused on the effects of ALD and AVP in controlled comparisons of oral versus IV rehydration. The first investigation (17) compared physiological responses with rehydration (i.e., after dehydration of −4% to −5%; fluid administered over a 45-min period) with two IV saline solutions (0.45% and 0.9% NaCl) to oral rehydration (0.45% NaCl) and no fluid. Immediately after rehydration, subjects stood for 55 min. Blood samples indicated that ALD and AVP concentrations, during resting recovery (0 to 100 min), were similar in all three rehydration experiments. This occurred despite the differences of Na+ concentrations in the normal saline (0.9% NaCl) and 1/2-normal saline (0.45% NaCl) intravenous IV solutions. However, the ALD and AVP levels in the no-fluid trial were significantly greater, indicating a greater stress on fluid-regulatory hormone mechanisms.

The second investigation (24) evaluated physiological responses to rehydration (i.e., after dehydration of −4%; fluid administered during a 75-min recovery period). The three rehydration solutions were identical to those used in the study above (17); a no-fluid experiment also was incorporated as a control. Subjects then walked for 90 min in the heat (36°C) at 50% VO2max. Blood samples indicated that ALD and AVP concentrations during rehydration (100 min) and exercise (goal: 90 min) were similar in the three rehydration trials. Similar to the study mentioned previously, the ALD and AVP levels in the no-fluid trial were significantly greater (P < 0.05) during exercise, indicating a greater stress on fluid-regulatory hormone mechanisms. However, no differences (i.e., among the rehydration tests) occurred during exercise for heart rate, rectal temperature, or skin temperature (Fig. 4). Remarkably, exercise performance time (min) during the no fluid (NF) trial averaged 24% less than the three rehydration tests (NF, 58 ± 8; 0.45 %IV, 77 ± 5; 0.9 %IV, 76+ 6; 0.45% oral, 84 ± 2 min; no statistical difference among rehydration modes).

Figure 4

Figure 4

Three practical applications result from these investigations. First, ALD and AVP respond similarly, regardless of the manner in which rehydration occurs (oral vs. IV solutions). This finding refers to a lengthy administration period (100 min) and moderate to severe dehydration (−4% to −5% body weight); it also agrees with published findings regarding thermoregulation and exercise performance (15). Second, these hormonal relationships were observed during post-dehydration resting recovery and exercise in the heat. Third, although Na+ in oral rehydration fluid promotes fluid retention better than water (29), the greater concentration of a 0.9% IV fluid (versus 0.45% IV) did not result in enhanced restoration of PV, enhanced heat tolerance, or reduced physiological strain. Therefore, the available literature indicates that IV rehydration offers no physiological, thermoregulatory, or fluid balance advantages over oral rehydration.

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STRESS HORMONES

The stress hormone response after various modes of rehydration offers a window to the overall physiological stress the athlete is experiencing during the bout of exercise and during the rehydration period. It allows the researcher to examine the cumulative response that is being experienced. Ultimately, it offers another physiological measure to potentially corroborate findings within other physiological systems, in that one mode of rehydration may offer a decreased cumulative stress than another. Because stress hormones influence athletes in a variety of ways (i.e., cardiovascular changes, performance anxiety, and substrate utilization), it is important to understand whether oral and IV rehydration influence the hypothalamic-pituitary axis similarly. It is possible that the invasive nature of IV placement alone may cause a stress response. Regardless, there is limited research regarding stress hormone responses to subsequent exercise after rehydration with IV fluid. All known studies have examined these responses following exercise-induced dehydration to −4% body mass loss, rehydration, and subsequent exercise in the heat (13-15). This protocol has been repeated with varying methods (IV and oral), NaCl concentrations (0.45% vs. 0.9%), and amounts (complete or partial restoration of amount of fluid lost) of rehydration (13-15).

When rehydration was compared between oral and IV replacement of the same concentration (0.45% NaCl) with subsequent 90-min exercise in the heat, plasma norepinephrine (NE) was significantly greater in the oral replacement group when compared with IV at 45 min of exercise only (30). However, no significant differences were found between rehydration methods in epinephrine (EPI), adrenocorticotropic hormone (ACTH), or cortisol (CORT) throughout exercise (15). Overall, no clinical significance can be gleaned from the results of this study implicating oral or IV replacement as a superior rehydration method.

Partial rehydration (restoring 50% of losses) comparing IV and oral fluid replacement using exercise to exhaustion has shown no difference in stress hormone changes throughout exercise between groups. Similar responses were found with ACTH, CORT, and NE through this exercise protocol (13).

Researchers have hypothesized that ACTH and CORT demonstrate similar increases in response to changes in PV and osmolality, regardless of Na+ concentration of IV fluid (13). NE responses have shown a proportional increase (sympathetic) in response to increased rectal temperatures (13-15). Currently, the stress hormone evidence does not indicate any systemic differences in the overall extent of the stress of the activity and rehydration process between oral and IV rehydration.

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Perceptual

Factors that influence ratings of perceived exertion (RPE) and sensations of thirst and thermal stress in response to exercise or environmental heat are of interest because these perceptual responses are linked closely to physiological perturbations. Both central and peripheral physiological cues can serve as important mediators of RPE (31), thirst and drinking behavior, and thermal sensations (18,19). Furthermore, thermal sensations, thermal comfort (18,19,31), and thirst (19), per se, may influence RPE during exercise in environmental extremes. Because rehydration plays a pivotal role in reducing the physiological stress associated with dehydration during exercise-heat stress, perceptual responses to both oral and IV rehydration under these circumstances have gained added focus and attention (18,19,23).

Riebe et al. (19) were the first to compare the effects of oral (0.45% saline) and IV (0.45% saline) rehydration, and no rehydration (control), upon RPE and thirst. Male subjects underwent these three randomly assigned treatments after a 2- to 4-h exercise-induced dehydration bout to reduce body weight by 4%. After 45 min of rehydration and rest (total of 2 h), subjects walked at 50% V˙O2max for 90 min in 36°C. Oral rehydration resulted in reduced RPE compared with IV and control. Thirst was higher during control than IV and oral, and lower during oral than IV, suggesting that thirst may be an underlying cue for the RPE response.

In a follow-up study, Maresh et al. (18) examined 0.45% saline oral and IV rehydration, and control, on RPE, thirst, and thermal sensations during cycling exercise (74% V˙O2max) until volitional exhaustion in 37°C. On the day before testing, eight cyclists were dehydrated by −4% body weight through a combination of exercise and fluid restriction. On the following morning, immediately before exercise, the subjects were rehydrated over a 20-min period back to −2% (body weight). Despite no differences in the exercise time to exhaustion between the oral and IV treatments, RPE and thirst responses were lower (P < 0.05) during oral compared with IV; thermal sensations were higher (P < 0.05) during IV versus oral and were significantly correlated with RPE responses in all treatments (Fig. 5).

Figure 5

Figure 5

In contrast to Riebe et al. (19), who provided rehydration and rest over an extended time period before exercise, and Maresh et al. (18), who imposed dehydration on the day before rehydration and exercise, Kenefick and coworkers (23) examined the effect of rapid 0.45% saline IV and oral rehydration immediately after dehydration upon perceptual responses during subsequent exercise in 37°C. The results indicated no overall differences during this exercise in RPE, thermal sensations, or exercise time between IV and oral. However, thirst sensations were lower in oral compared with IV and control (Fig. 6).

Figure 6

Figure 6

Collectively, and despite differences in research design, it appears that no differences exist between IV and oral regarding perceptual responses, or quite possibly oral rehydration provides more favorable RPE, thirst, and thermal responses to subsequent exercise in the heat. In general, these perceptual responses are similar to results reported for cardiovascular and thermoregulatory measures (11,15,23) and suggest that oropharyngeal factors associated with oral rehydration may contribute strongly to these results.

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EXERCISE HEAT TOLERANCE/PERFORMANCE/HEALTH

To our knowledge, only five studies (6,12,15,23,24) have compared exercise heat tolerance after IV or oral rehydration. In one of these studies (6), both oral and IV rehydration occurred during the exercise session, thus limiting application to real-world athletic scenarios. Additionally, that study (6) used different types of fluids for ingestion (sports drink) and IV (6% dextran in saline), and reported enhanced exercise heat tolerance with oral as compared with IV (increased skin blood flow and lower rectal temperatures in oral). This study was important because it was the first to show that oral fluid ingestion influences skin blood flow independent of increases in blood volume, setting the stage for the examination of oropharyngeal reflexes and the influence of sodium/osmolality upon thermoregulatory control. The remaining studies generally found no differences (15,23,24) in exercise heat tolerance (as measured by rectal temperature and mean weighted skin temperature, Fig. 4) or reported a minimal benefit derived from oral (12) as compared with IV (lower rectal temperature). An enhanced exercise heat tolerance (12) may have been confounded by differences in the temperatures of the fluids (oral = 10°C vs IV = 22°C). When specific heat calculations were completed to isolate the temperature effect, a 0.28°C lower body temperature was predicted in oral, an impact that likely would have made the body temperatures similar if the rehydration temperatures were the same (oral/rectal temperature was actually about 0.3°C less than IV). One study (24) compared IVs of different tonicities (0.45% saline and 0.9% saline) with oral fluid ingestion (0.45% saline) and found no differences among the trials in exercise heat tolerance regardless of the IV Na+ concentration. A study by Kenefick et al. (23) made an attempt to mimic closely a real-world sports scenario by dehydrating subjects via exercise, then rapidly rehydrating them via oral or IV after exercise (with same temperature, amount, and timing of fluid), followed by an intense exercise session. No differences were found in exercise heat tolerance between the two modes of rehydration.

Only four studies (12,15,23,24) have compared exercise performance between oral and IV rehydration. Those studies all showed no differences in exercise performances. The studies did, however, show an enhanced performance response with rehydration (either mode) versus no fluid (12,23) or when oral was compared with no fluid (15,24), thus showing the benefit of mitigating just a small difference in hydration status upon exercise performance. It is interesting to note that exercise time was 5 min longer (P = 0.07) with oral rehydration, 14% better (12) in one study and 7 min longer (not significant), and 8% better (15,24) in another (when comparing oral and 0.45% IV). The key point to emphasize is that performance was never enhanced in the IV rehydration trials, regardless of the study design or type of IV solution.

To date, only one study (11) has examined the influence of oral or IV rehydration upon health status when the rehydration immediately preceded an intense exercise session. No difference was found between the mode of rehydration on a postexercise environmental symptoms questionnaire, a subjective assessment of extent of personal discomfort associated with the activity. The authors did note a more rapid onset of symptoms in a no-fluid trial, emphasizing the impact of hydration itself. We conclude that IV rehydration does not enhance exercise heat tolerance, exercise performance, or health status as compared with oral rehydration when key variables are controlled, including temperature of the rehydration fluid, the amount of fluid given, and the timing of administration.

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PRACTICAL IV USAGE

IV rehydration may be necessary in instances when oral fluid replacement is impossible or impractical (2,8). However, if oral fluid or Na+ replacement is possible, the psychological and physiological benefits outweigh risks associated with IV administration. The instances in which IV fluid administration may be necessary include, but are not limited to, a) severe cramping, b) severe loss in body mass (>5%), c) nausea, vomiting, and diarrhea associated with illness, d) hypernatremia, e) hyponatremia, f) moderate to severe orthostatic hypotension, g) abnormally low blood glucose levels, and h) altered consciousness (i.e., exertional heat stroke) (2,8). Potential risks exist with IV infusion. An IV infusion done incorrectly can introduce bubbles and possible embolisms. Nonsterile techniques can introduce pathogens, and repeated venipunctures can cause thrombitis and injury to the veins. Additionally, some individuals may be salt-sensitive, which may induce bouts of nausea or possibly diarrhea soon after a taking a salt-loading solution.

Physicians may choose to administer fluid replacement in a preventive fashion for some individuals if they have a recent consistent history of severe and uncontrollable cramping. Some athletes do not tolerate high Na+ diets or fluids in conjunction with exercise, and thus may require Na+ replacement via IV when exercising in the heat. Athletes practicing multiple times a day who are unable to replace fluids and Na+ losses by oral means before the next practice may require IV rehydration before subsequent exercise to avoid medical consequences. Athletes and soldiers should be encouraged to attempt oral rehydration (if possible) in conjunction with IV rehydration if IV is necessary (8).

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LEGAL/ETHICAL CONSIDERATIONS

The ethical considerations of IV rehydration have important implications. An athlete who uses IV simply to enhance performance and not for a noted medical condition may be crossing the line of ethical practice and may violate anti-doping policies. The three basic questions specific to IV use when deciding if it might be considered unethical/illegal are as follows (32,33):

  1. Does IV rehydration have the potential to enhance sports performance? (Possibly, in some circumstances in which oral rehydration might not be feasible.)
  2. Does IV rehydration represent an actual or potential health risk to the athlete? (In most circumstances there is no health risk, and ironically, it may enhance health for individuals with certain medical conditions. Obviously, an improper technique during the IV process could pose a risk, but is not likely in the hands of trained professionals.)
  3. Does IV rehydration violate the "spirit of sports"? (When used as an ergogenic aid, then yes; when used for medical treatment, no.)

The World Anti-Doping Agency (WADA) recently added infusions as a prohibited ergogenic method. Specifically, the 2006 Prohibited List of the World Anti-Doping Code states in section M2 (Chemical and Physical Manipulation) part b that "Intravenous infusions are prohibited, except as legitimate acute medical treatment" (32). This is a method that is "prohibited at all times," which means it applies "in and out-of-competition" (32). The 2007 version of the code was modified to remove the word "acute" from the statement (33).

A great deal of grey area exists when considering this policy. Consider for example an athlete who is dehydrated to −4% of body weight during day 2 of a decathlon competition. He is not experiencing any notable signs or symptoms associated with the dehydration. Does this degree of dehydration, in the absence of any signs and symptoms, constitute a legitimate medical need and warrant (allow) IV rehydration? What if he does have some mild signs and symptoms related to the dehydration? In the first case, the medical treatment theoretically could be preventing a future problem, but also could be assisting the athlete from a performance perspective. In the second case, does some level of medical intervention in any way jeopardize future participation on that day? While it is clear that IV infusion is needed in more emergent conditions, the range for interpretation is notable when considering various levels of dehydration or very mild medical conditions. This rule imposed by WADA concerning infusions is well intentioned and necessary, but may need to be refined to limit the scope in which it could be arbitrarily interpreted. Corresponding to and in addition to the WADA guidelines, the question must be addressed: Should IV infusion preclude continued participation in competition on that day (or for some time period like 4-6 h) due to ethical and legal ramifications? Future deliberations and conversations should address this and other important considerations pertaining to IV rehydration.

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REHYDRATION GUIDELINES

Given the legal and physiological considerations noted previously regarding IV rehydration, the following guidelines may address some unique aspects when implementing oral rehydration/electrolyte replacement practices in situations where IV options were previously considered/implemented. These are not meant to be an exhaustive overview of the rehydration process, which is extensively covered elsewhere (3,34).

Adapting the gut for exercise should also be considered when developing an athlete's individualized oral hydration protocol. Gastrointestinal (GI) distress has been associated with exercise and the interaction it may have with fluid intake for some individuals. However, some have speculated that the gut can be adapted to meet the demands of vigorous exercise (35). Since the gut is involved in gastric emptying and nutrient intake during exercise, proper training and nutrition may help minimize GI discomfort. Adapting the gut to certain types and amounts of fluids during exercise may lead to an increased rate of gastric emptying and the absorption of nutrients and fluids, thereby minimizing GI distress (35).

Developing an individualized hydration protocol is both simple and inexpensive. The duration and intensity of exercise, the individual's preferences, as well as sweat rate should be incorporated into the development of the hydration protocol. An athlete's sweat rate can be calculated by using published guidelines (9). Additionally, simply measuring body weight before and after each practice can provide important insights. If an athlete's post-exercise weight is less, he or she should attempt to drink more next time. Conversely, if the athlete weighs more post-exercise, he or she should drink less next time. Over time, this behavior can be honed to individualize the response to match fluid losses closely. We recommend during exercise to try and match fluid/electrolyte consumption with fluid/electrolyte losses, with the general guideline to not lose beyond 2% of pre-activity weight, and to never drink more than fluid losses. While complete restoration of fluids may not be possible or necessary in some circumstances, it does provide an element of flexibility for forthcoming points in the activity when fluid access is limited or the ability to consume fluids is not as optimal.

Aside from the amount of fluid needed for rehydration, some individuals may need to also increase Na+ intake. Some athletes may lose an excessive amount of Na+ via sweating (sometimes referred to as "salty sweaters"; these athletes may experience a higher incidence of heat cramps or exertional heat illness and should replace lost Na+ via their diet or sports drinks). Athletes involved in ultra-endurance events may also be Na+ deficient as a result of not replacing lost Na+ or by ingesting excessive amounts of water throughout the event (10). Although sports drinks already contain Na+, individuals requiring additional Na+ may dilute salt into their sports drinks to increase the osmolality of their beverage. For every tsp of salt (590 mg of Na+) that is added to 1 L of sports drink, the osmolality of the beverage is raised approximately 50 mOsm (Michael Bergeron, personal communication, 2007). The supplemental sodium could be added to some of the rehydration beverages consumed during the course of an event or practice (based on needs), not to all fluid that is consumed. The palatability of the beverage will be compromised if the sodium level is too high, so the balance between fluid restoration and electrolyte replacement must be carefully considered and balanced. Another viable option for restoration of some of the salt is to consume salty snacks during competition or breaks.

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CONCLUSION

The limited data to this point seem to contradict conventional wisdom with regards to IV rehydration. It had been long thought that the immediate entry of fluids into the bloodstream would offer a multitude of benefits for an otherwise healthy, dehydrated athlete looking to replace fluids rapidly. The evidence indicates that when equal volumes of the same concentration of fluid are used for rehydration intravenously or orally, no performance or physiological advantage exists to justify the use of IV rehydration. IV treatment is invasive, requires trained medical staff, must be given off the field, and increases risk of infection and bruising. Thus, an oral rehydration protocol is usually a more efficacious and safe hydration approach. Nevertheless, the fact remains that greater volumes can be tolerated (from a gastrointestinal perspective) when the fluid is administered intravenously. However, the oral route of fluid replacement seems to hold benefits that likely stem from the "natural" mode of delivery that enhances the physiological and psychological response surrounding the rehydration process. Whether this is due to the oropharyngeal reflexes and thirst perturbations or other factors, the surprising speed in which oral fluids can enter the blood stream and become part of the sweat, the importance of these responses should not be underestimated (30,36,37).

We recommend that oral rehydration be seriously considered. This mode of rehydration should be preferentially used whenever feasible. Consider the importance of prehydration, partial rehydration, and other practical implications to maximize the rehydration process (3), and highly encourage the "training" of oral rehydration so that the body becomes accustomed to "handling" the fluids during the stress of training and competition. An additional advantage of an effective oral rehydration protocol is that it encourages athletes to take an active role in rehydrating themselves, thus avoiding psychological dependence on IV fluids. IV rehydration will continue to play an important role in the medical treatment of athletes who cannot ingest fluids (e.g., nauseous, vomiting, or altered consciousness) or for those who may have GI distress with large volumes of fluid or salt. Additionally, when IV infusion is deemed necessary but the athlete can also handle fluids orally, consider splitting the amount between the two modes so as to glean the benefits of both. This combination will also involve the athlete with the process and may enhance future rehydration behavior.

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