Recreational enthusiasts and athletes often are advised to abstain from consuming caffeinated beverages (CB). The dual purposes of this review are to (a) critique controlled investigations regarding the effects of caffeine on dehydration and exercise performance, and (b) ascertain whether abstaining from CB is scientifically and physiologically justifiable. The literature indicates that caffeine consumption stimulates a mild diuresis similar to water, but there is no evidence of a fluid-electrolyte imbalance that is detrimental to exercise performance or health. Investigations comparing caffeine (100-680 mg) to water or placebo seldom found a statistical difference in urine volume. In the 10 studies reviewed, consumption of a CB resulted in 0-84% retention of the initial volume ingested, whereas consumption of water resulted in 0-81% retention. Further, tolerance to caffeine reduces the likelihood that a detrimental fluid-electrolyte imbalance will occur. The scientific literature suggests that athletes and recreational enthusiasts will not incur detrimental fluid-electrolyte imbalances if they consume CB in moderation and eat a typical U.S. diet. Sedentary members of the general public should be at less risk than athletes because their fluid losses via sweating are smaller.
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Lawrence E. Armstrong
Yoram Epstein and Lawrence E. Armstrong
Body water and electrolyte balance are essential to optimal physiological function and health. During exercise, work, or high temperatures, a significant level of dehydration can develop, and the ratio of extracellular to intracellular fluid can change, despite an ample supply of water. Physical and cognitive performance are impaired at 1-2% dehydration, and the body can collapse when water loss approaches 7%. Because fluid needs and intakes vary, formulating one general guideline for fluid replacement is difficult. Knowing the amount of water lost in sweat may enable predicting fluid needs via mathematical models for industrial, athletic, and military scenarios. Sodium imbalance might result from excessive Na+ loss or from gross o verity dration. In most work or exercise lasting < 3-4 hr, the major concern is that fluid be available to prevent heat-related illnesses, which can be prevented if fluid and electrolyte losses are balanced with intake, using the recommendations presented.
Lawrence E. Armstrong and Rebecca M. Lopez
Exertional heat exhaustion (HEX) is the most common form of heat illness experienced by athletes, laborers, and military personnel. Both dehydration stemming from a water and/or salt deficiency and a high ambient temperature must exist for HEX to occur. In the field, appropriate therapy can reduce recovery time.
This manuscript provides clinical guidance regarding return to activity. The primary focus of this paper is to describe the evaluation of residual effects and the underlying personal characteristics that initially predispose the athlete to HEX. Attention to these factors will reduce the risk of future episodes.
Lawrence E. Armstrong and Carl M. Maresh
Exercise-heat tolerance (EHT) in children is influenced by many physiological factors, including sweat gland activity, cardiac output, exercise economy, ability to acclimate to heat, and maturation of organ systems. It is generally believed that children cannot tolerate hot environments as well as adults, although some children exhibit EHT that is superior to that of adults. There has been no research showing large exercise-induced differences between the core body temperatures of children versus adults, but differences in the time to onset of syncope and fatigue have been observed. This suggests that the greatest risk of heat illness for children is heat exhaustion (i.e., cardiovascular instability) and not heat stroke (i.e., hyperthermia). Therefore this review (a) examines the conclusions of previous studies to clarify misinterpretations of data, and (b) identifies research questions that require future study.
Lawrence E. Armstrong, Evan C. Johnson, Amy L. McKenzie, Lindsay A. Ellis, and Keith H. Williamson
This field investigation assessed differences (e.g., drinking behavior, hydration status, perceptual ratings) between female and male endurance cyclists who completed a 164-km event in a hot environment (35 °C mean dry bulb) to inform rehydration recommendations for athletes. Three years of data were pooled to create 2 groups of cyclists: women (n = 15) and men (n = 88). Women were significantly smaller (p < .001) than men in height (166 ± 5 vs. 179 ± 7 cm), body mass (64.6 ± 7.3 vs. 86.4 ± 12.3 kg), and body mass index (BMI; 23.3 ± 1.8 vs. 26.9 ± 3.4) and had lower preevent urinary indices of hydration status, but were similar to men in age (43 ± 7 years vs. 44 ± 9 years) and exercise time (7.77 ± 1.24 hr vs. 7.23 ± 1.75 hr). During the 164-km ride, women lost less body mass (−0.7 ± 1.0 vs. −1.7 ± 1.5 kg; −1.1 ± 1.6% vs. −1.9 ± 1.8% of body weight; p < .005) and consumed less fluid than men (4.80 ± 1.28 L vs. 5.59 ± 2.13 L; p < .005). Women consumed a similar volume of fluid as men, relative to body mass (milliliters/kilogram). To control for performance and anthropomorphic characteristics, 15 women were pair-matched with 15 men on the basis of exercise time on the course and BMI; urine-specific gravity, urine color, and body mass change (kilograms and percentage) were different (p < .05) in 4 of 6 comparisons. No gender differences were observed for ratings of thirst, thermal sensation, or perceived exertion. In conclusion, differences in relative fluid volume consumed and hydration indices suggest that professional sports medicine organizations should consider gender and individualized drinking plans when formulating pronouncements regarding rehydration during exercise.
Lawrence E. Armstrong, Roger W. Hubbard, E. Wayne Askew, Jane P. De Luca, Catherine O'Brien, Angela Pasqualicchio, and Ralph P. Francesconi
This investigation examined whether low sodium (Na+) (LNA; 68 mEq Na+·d-1) or moderate Na+ (MNA; 137 mEq Na+.d-1) intake allowed humans to maintain health, exercise, and physiologic function during 10 days of prolonged exercise-heat acclimation (HA). Seventeen volunteers, ages 19 to 21, consumed either LNA (n=8) or MNA (n=9) during HA (41°C, 21% RH; treadmill walking for 30 min.h-1, 8 h·d-1 at 5.6 kmh-l, 5% grade), which resulted in significantly reduced heart rate, rectal temperature, and urine Na+ for both groups. There were few between-diet differences in any variables measured. Mean plasma volume in LNA expanded significantly less than in MNA by Days 11 and 15, but reached the MNA level on Day 17 (+12.3 vs. +12.4%). The absence of heat illness, the presence of normal physiologic responses, and the total distance walked indicated successful and similar HA with both levels of dietary Na+.
Lawrence E. Armstrong, Carl M. Maresh, John W. Castellani, Michael F. Bergeron, Robert W. Kenefick, Kent E. LaGasse, and Deborah Riebe
Athletes and researchers could benefit from a simple and universally accepted technique to determine whether humans are well-hydrated, euhydrated, or hypohydrated. Two laboratory studies (A, B) and one field study (C) were conducted to determine if urine color (
Michael F. Bergeron, Carl M. Maresh, Lawrence E. Armstrong, Joseph F. Signorile, John W. Castellani, Robert W. Kenefick, Kent E. LaGasse, and Deborah A. Riebe
Twenty (12 male and 8 female) tennis players from two Division I university tennis teams performed three days of round-robin tournament play (i.e., two singles tennis matches followed by one doubles match per day) in a hot environment (32.2 ±
Douglas J. Casa, Carl M. Maresh, Lawrence E. Armstrong, Stavros A. Kavouras, Jorge A. Herrera-Soto, Frank T. Hacker Jr., Timothy P. Scheett, and James Stoppani
The purpose of this study was to determine if intravenous fluid rehydration, versus oral rehydration. during a brief period (20 min) differentially affects plasma ACTH, cortisol, and norepinephrine concentrations during subsequent exhaustive exercise in the heat. Following dehydration (DHY) to −4% of body weight, 8 nonacclimated highly trained males (age = 23.5 ± 1.2 years, V̇O2peak = 61.4±0.8 ml · kg · min−1, % body fat = 13.5±0.6%) cycled to exhaustion at 74% V̇O2peak in 36.8 °C on three different occasions. These included: (a) no fluid (NF), where no fluid was provided during the rehydration period; (b) DRINK, where oral rehydration (0.45% NaCl) was provided equal to 50% of the prior DHY; and (c) IV, where intravenous infusion (0.45% NaCl) was provided equal to 50%’ of the prior DHY. Exercise time to exhaustion was not different p = .07) between the DRINK (34.86 ±4.01) and IV (29.48 ± 3.50) trials, but both were significantly p < .05) longer than the NF (18.95 ± 2.73) trial. No differences (p > .05) were found for any of the hormone measures among trials. The endocrine responses at exhaustion were similar regardless of hydration state and mode of rehydration, but rehydration prolonged the exercise time to exhaustion.
Marcos Echegaray, Lawrence E. Armstrong, Carl M. Maresh, Deborah Riebe, Robert W. Kenefick, John W. Castellani, Stavros Kavouras, and Douglas Casa
This study assessed the plasma glucose (PG) and hormonal responses to carbohydrate ingestion, prior to exercise in the heat, in a hypohydrated state versus partial rehydration with intravenous solutions. On separate days, 8 subjects (21.0 ± 1.8 years; 57.3 ± 3.7 ml · kg−1 · min−1) exercised at 50% V̇O2maxin a 33 °C environment until a 4% body weight loss was achieved. Following this, subjects were rehydrated (25 ml · kg−1) with either: 0.45% IV saline (45IV), 0.9% IV saline (9IV), or no fluid (NF). Subjects then ingested 1 g · kg−1 of carbohydrate and underwent an exercise test (treadmill walking, 50% V̇O2max, 36 °C) for up to 90 min. Compared to pre-exercise level (294 mg · dl−1), PG increased significantly (>124 mg · dl−1) at 15 min of the exercise test in all trials and remained significantly elevated for 75 min in NF, 30 min more than in the 2 rehydration trials. Although serum Insulin increased significantly at 15 min of exercise in the 45IV trial (7.2 ± 1.2 vs. 23.7 ± 4.7 μIU · ml−1) no significant differences between trials were observed. Peak plasma norepinephrine was significantly higher in NF (640 ± 66 pg · ml−1) compared to the 45IV and 9IV trials (472 ± 55 and 474 ± 52 pg · ml−1, respectively). In conclusion, ingestion of a small solid carbohydrate load prior to exercise in the 4% hypohydration level resulted in prolonged high PG concentration compared to partial IV rehydration.