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 and Carl M. Maresh
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 (
Lawrence E. Armstrong, Jorge A. Herrera Soto, Frank T. Hacker Jr., Douglas J. Casa, Stavros A. Kavouras and Carl M. Maresh
This investigation evaluated the validity and sensitivity of urine color (Ucol), specific gravity (Usg), and osmolality (Uosm) as indices of hydration status, by comparing them to changes in body water. Nine highly trained males underwent a 42-hr protocol involving dehydration to 3.7% of body mass (Day 1, −2.64 kg), cycling to exhaustion (Day 2, −5.2% of body mass, −3.68 kg), and oral rehydration for 21 hr. The ranges of mean (across time) blood and urine values were Ucol, 1-7; Usg, 1.004-1.029; U08m, 117-1,081 mOsm • kg−1; and plasma osmolality (Posm), 280-298 mOsm ⋅ kg−1. Urine color tracked changes in body water as effectively as (or better than) Uosm, Usg, urine volume, Posm, plasma sodium, and plasma total protein. We concluded that (a) Ucol, Uosm, and Usg are valid indices of hydration status, and (b) marked dehydration, exercise, and rehydration had little effect on the validity and sensitivity of these indices.
Sharon L. Miller, Carl M. Maresh, Lawrence E. Armstrong, Cara B. Ebbeling, Shannon Lennon and Nancy R. Rodriguez
The interaction of substrates and hormones in response to ingestion of intact proteins during endurance exercise is unknown. This study characterized substrate and hormone responses to supplementation during endurance exercise. Nine male runners participated in 3 trials in which a non-fat (MILK), carbohydrate (CHO), or placebo (PLA) drink was consumed during a 2-hour treadmill >· run at 65% V̇O2max. Circulating levels of insulin, glucagon, epinephrine, norepi-nephrine, growth hormone, testosterone, and cortisol were measured. Plasma substrates included glucose, lactate, free fatty acids, and select amino acids. Except for insulin and cortisol, hormones increased with exercise. While post-exercise insulin concentrations declined similarly in all 3 trials, the glucagon increase was greatest following MILK consumption. CHO blunted the post-exercise increase in growth hormone compared to levels in MILK. Free fatty acids and plasma amino acids also were responsive to nutritional supplementation with both CHO and MILK attenuating the rise in free fatty acids compared to the increase observed in PLA. Correspondingly, respiratory exchange ratio increased during CHO. Essential amino acids increased significantly only after MILK and were either unchanged or decreased in CHO. PLA was characterized by a decrease in branched-chain amino acid concentrations. Modest nutritional supplementation in this study altered the endocrine response as well as substrate availability and utilization following and during an endurance run, respectively.
Jennifer K. Ormerod, Tabatha A. Elliott, Timothy P. Scheett, Jaci L. VanHeest, Lawrence E. Armstrong and Carl M. Maresh
The purposes of this study were to characterize measures of fluid intake and perception of thirst in women over a 6-week period of exercise-heat acclimation and outdoor training and examine if this lengthy acclimation period would result in changes in fluid intake that differ from those previously reported in men utilizing a shorter acclimation protocol of 8–10 days. Voluntary water intake (11–17 °C) and perception of thirst were measured in a group of 5 women (21–26 yr) undergoing exercise-heat acclimation for 90 min/day, 3 days/wk (36 °C, rh 50–70%) and outdoor training 3 days/wk for 6 weeks. Decreased drinking during acclimation was characterized by a decrease in the number of drinks (35 ± 10 to 17 ± 5; p < .05), greater time to first drink (9.9 ± 2.0 to 23.1 ± 4.7 min; p < .05), and a decrease in total volume ingested per week (3310 ± 810 to 1849 ± 446 ml; p < .05) through the 6-week study. Mean perceived thirst measurements remained low and showed only slight variance (3 ± 0.4 to 5 ± 0.4). These observations support a psycho-physiological response pattern different than that previously observed during 8–10 day acclimation protocols in men.
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.
Brendon P. McDermott, Douglas J. Casa, Susan W. Yeargin, Matthew S. Ganio, Lawrence E. Armstrong and Carl M. Maresh
To describe the current scientific evidence of recovery and return to activity following exertional heat stroke (EHS).
Information was collected using MEDLINE and SPORTDiscus databases in English using combinations of key words, exertional heat stroke, recovery, rehabilitation, residual symptoms, heat tolerance, return to activity, and heat illness.
Relevant peer-reviewed, military, and published text materials were reviewed.
Inclusion criteria were based on the article’s coverage of return to activity, residual symptoms, or testing for long-term treatment. Fifty-two out of the original 554 sources met these criteria and were included in data synthesis.
The recovery time following EHS is dependent on numerous factors, and recovery length is individually based and largely dependent on the initial care provided.
Future research should focus on developing a structured return-to-activity strategy following EHS.
David R. Hooper, William J. Kraemer, Rebecca L. Stearns, Brian R. Kupchak, Brittanie M. Volk, William H. DuPont, Carl M. Maresh and Douglas J. Casa
Purpose: Prior research has illustrated that high volumes of aerobic exercise result in a reduction in basal concentrations of testosterone in men. Those studies were mostly conducted on recreational runners and identified reduced testosterone, but not concentrations low enough to be considered pathological. Therefore, the purpose of this study was to assess the basal concentrations of testosterone and cortisol in elite triathletes, as well as the impact of a World Championship race, on the acute responses of these hormones. Methods: A total of 22 men (age 40.6 [11.5] y, height 179  cm, weight 77.0 [7.0] kg) who participated in the 2011 Ironman World Championships served as subjects. Resting blood samples were taken 2–4 d prior to provide a baseline (BL), as well as immediately, 1 d, and 2 d after the event and were later analyzed for total testosterone and cortisol concentrations. Results: At BL, 9 men had a normal testosterone concentration, whereas 9 men fell within a “gray zone” and 4 other men demonstrated concentrations suggestive of deficiency. Testosterone was significantly lower than BL at 1 d (95% confidence interval [CI] 0.10–0.34, P < .001, ES = 0.53) and 2 d (95% CI 0.01–0.21, P = .034, ES = 0.35) after the event. Cortisol was significantly different from BL at immediate post (95% CI 1.07–0.83, P < .001, ES = 8.0). There were significant correlations between time and age (R = .68, P = .001), as well as BL testosterone and cortisol (R = .51, P = .015). Conclusion: Elite ultraendurance athletes may demonstrate not only reduced testosterone but also sometimes clinically low concentrations that could be indicative of androgen deficiency.
Kate Sanders, Carl M. Maresh, Kevin D. Ballard, Brent C. Creighton, J. Luke Pryor, William J. Kraemer, Jeff S. Volek and Jeff M. Anderson
Compared with their physically active peers, overweight sedentary postmenopausal women demonstrate impaired vascular endothelial function (VEF), substantially increasing the risk for cardiovascular disease (CVD). Habitual exercise is associated with improved VEF and reduced CVD risk. The purpose of this study was to compare brachial artery flow mediated dilation (FMD), a measure of VEF, in overweight, postmenopausal women who were physically active (EX: n = 17, BMI: 29.3 ± 3.11 kg/m2) or sedentary (CON: n = 8, BMI: 30.3 ± 3.6 kg/m2). Anthropomorphic measures were similar in both groups (P > .05). FMD was significantly greater in EX (10.24 ± 2.36%) versus CON (6.60 ± 2.18%) (P < .002). FMD was not significantly correlated with estimated VO2max (EX: r = .17, P = .52; CON: r = .20, P = .60) but was negatively associated with percent body fat in EX group (EX: r = -.48, P = .05; CON: r = .41, P = .31). These results are consistent with the positive effects of habitual exercise on VEF in overweight postmenopausal women.
Sharon L. Miller, P. Courtney Gaine, Carl M. Maresh, Lawrence E. Armstrong, Cara B. Ebbeling, Linda S. Lamont and Nancy R. Rodriguez
This study determined the effect of nutritional supplementation throughout endurance exercise on whole-body leucine kinetics (leucine rate of appearance [Ra], oxidation [Ox], and nonoxidative leucine disposal [NOLD]) during recovery. Five trained men underwent a 2-h run at 65% VO2max, during which a carbohydrate (CHO), mixed protein-carbohydrate (milk), or placebo (PLA) drink was consumed. Leucine kinetics were assessed during recovery using a primed, continuous infusion of 1-13C leucine. Leucine Ra and NOLD were lower for milk than for PLA. Ox was higher after milk-supplemented exercise than after CHO or PLA. Although consuming milk during the run affected whole-body leucine kinetics, the benefits of such a practice for athletes remain unclear. Additional studies are needed to determine whether protein supplementation during exercise can optimize protein utilization during recovery.