Hypothermia vs Performance


By William Misner, Ph.D.

William Misner, Ph.D.
William Misner, Ph.D.
From 1996 until his retirement in 2006, Dr. Bill worked full-time as Director of Research & Development at Hammer Nutrition. Among his many accomplishments, both academically and athletically, he is an AAMA Board Certified Alternative Medicine Practitioner and the author of "What Should I Eat? A Food-Endowed Prescription For Well Being".

Does Elevated Core Body Temperature & Dehydration Influence Endurance Performance?


Over the course of the past 7 years, reports from endurance athletes competing in hot or hot and humid events substantiate that there are times when fluids, fuels, and electrolytes are consumed at recommended ideal dose do not result in unsuccessful performance failure. The human body has great difficulty adapting to heat imposed when dehydration exceeds 4% loss of body weight and rectal temperature exceeds 39 °C, 102 °F. Fluids bodyweight loss of 4% occurs from evaporative sweating during aerobic exercise at 60% VO2 Max rate in 95 °F within 2 hours 11-19 minutes time [1]. The first symptom of dehydration is dry mouth, which occurs when approximately -2% of the bodyweight fluid stores is lost. At -4% or more dehydration dramatically increases core temperatures giving rise to dizziness, lack of clear thinking, sense of overheating, high shallow heart rate, loss of motivation, depression, muscle cramping, and the beginning of gastric stress which may present bloating, flatulence, diarrhea resulting in remarkable deterioration of pace rate. It is a combination of too much heat and the lack of evaporative sweat from excessive body fluid loss that produces an intolerable body core temperature. It is this combination which completely shuts down the stomach portal entry for resolving fluids repletion. When this occurs in an endurance event, the athlete needs to reduce body core temperatures rapidly in order to "reopen" the gastric channel for fluids, fuels, and electrolytes. Halting exercise-induced core body heating, cooling the skin, especially the face, trunk, armpits, groin, and leg muscles with fluids, and drinking an icy-cold hypocaloric fluids with Endurolytes during a walk break may be the only resolution left when facing this predicament. There are several adaptive mechanisms that may forestall the advent of a performance-ending hyperthermic dehydration experience. It is with that in mind, this article is presented. Understanding how severe overheating influences performance may be the best method for avoiding it.


Body weight fluid loss in the marathon ranges between 1.5%-6.2%, indicating that sweat loss exceeded fluid intake. These losses were greater in men than in women (average man -3.4% BW and average woman -2.6% BW). GI complaints were not associated with larger drink intakes. In contrast, dehydration above a certain limit appears to increase the frequency of gastrointestinal disorders. In the marathon, 80% of the runners who lost greater than -4% BW had gastrointestinal problems. It is possible that reduced blood flow to the GI region is compromised by exercise and reduced blood volume, which may disrupt normal secretion/absorption of the digestive tract. It may also be that a rising core body temperature, associated with decreased sweating at high levels of dehydration, may be related to GI dysfunction [1, 2].

It has generally been considered that decrease in performance occurs when hypohydration exceeds a -2% fluid loss of body weight; that performance decrements become substantial when fluid losses exceed -5% of body weight; and that when fluid losses approach 6-10% of body weight, heat stroke and heat exhaustion are life-threatening...Extreme -4% body fluid weight loss may adversely affect gastrointestinal function [3]. Previous investigations demonstrate that dehydration by -4% Body Weight [BW] or hypohydration of -5% Body Weight [BW], when combined with elevated core temperatures (~102 °F), IMPAIR GASTRIC EMPTYING OF INGESTED FLUIDS [i.e., 7% carbohydrate-electrolyte solution (CES) or water] during moderate-intensity [50-60% maximal oxygen consumption (O2 max)] treadmill exercise performed in a cool (64 °F) or warm (86-95 °F) environment [3, 4]. These findings suggest that AN EXCESSIVE LOSS OF BODY WATER NOT ONLY CAN ENHANCE BODY HEAT STORAGE DURING EXERCISE BUT ALSO CAN IMPAIR AN INDIVIDUAL'S ABILITY TO REPLENISH NEEDED FLUIDS AND CARBOHYDRATES.

Athletes commonly report gastrointestinal distress after ingestion of a beverage due to that ingestion occurs after dehydration has taken place. The high prevalence of GI disorders in marathon runners who have lost greater than or equal to -4% body weight supports this theory. Dehydration-exercise resulted in slower Gastric Emptying [GE] than in all other treatments (P less than 0.05). ANOVA revealed significant effects of dehydration and exercise, these two effects being additive in delaying Gastric Emptying [GE]. GI complaints were reported by 37.5% of the subjects during dehydrationexercise experiments [5, 6, and 7].

Sawaka [8] indicates that HEAT ACCLIMATION ADAPTIVELY LOWERS THE METABOLISM elicited by exercise. They observed that percent decreases lowered metabolism in the hot (-3%) as compared with the cool (-5%) test environments.


Endurance-trained cyclists exercising in the heat were observed to lose up to 4-5% of their body weight in evaporative sweat in 1.5-2 hours (J Appl Physiol. 1997 Apr;82(4):1229-36). During exercise, sweat output often exceeds water intake, producing a remarkable body fluid deficit (hypohydration). During situations of stress and prolonged high sweat loss, athletes may dehydrate by 2.8% Body Weight Loss [BWL]. The main cardiovascular consequences of combined dehydration and hyperthermia [per 1 degree C higher core temperature and 3-4 kg or -4% body weight loss] result in significant reductions in cardiac output (3 l/min), muscle blood flow, skin blood flow and blood pressure [9]. Hypohydration (poor/lessor replacement of water loss) increases heat storage by reducing sweating rate and skin blood flow responses for a given core temperature. Hypertonicity (excess muscle tone or serum/solution osmotic pressure) and hypovolemia (low blood volume) both contribute to reduced heat loss and increased heat storage [10]. A fluid volume deficit LOWERS BOTH INTRACELLULAR AND EXTRACELLULAR VOLUME and also results in plasma hypertonicity and hypovolemia. When sweat output exceeds water intake, resulting in hypohydration, a body fluid deficit occurs. This fluid deficit is comprised of water loss from both the intracellular and extracellular fluid compartments. Hypohydration during exercise causes greater heat storage and reduces endurance in comparison with euhydration levels. The GREATER HEAT STORAGE is attributed to a decreased sweating rate (evaporative heat loss) as well as a decreased cutaneous blood flow (dry heat loss). These response decrements have been attributed to both plasma hyperosmolality and a plasma hypovolemia. Subject gender, acclimation state, and aerobic fitness do not alter the increased heat storage when hypohydrated. Hyperhydration, or body fluid excess, does not seem to provide a clear advantage during exercise-heat stress, but will delay the development of hypohydration [11]. As an interesting secondary finding, researchers observed that carbohydrate ingestion INCREASED heat production, final core temperature, and whole body-sweating rate. They concluded that, during prolonged moderate-intensity exercise in a warm environment, ingestion of Water attenuates the decline in Pmax. Furthermore, INGESTION OF WATER + CARBOHYDRATES attenuates the decline in maximal power more than does Water alone, and ingestion of Carbohydrate alone does not attenuate the decline in Pmax compared with placebo [12]. Aerobic exercise is likely to be adversely affected by HEAT STRESS + HYPOHYDRATION; the warmer the climate the greater the potential for performance decrements. Hypohydration increases heat storage and reduces a person's ability to tolerate heat strain. The INCREASED HEAT STORAGE is mediated by a LOWER SWEATING RATE (evaporative heat loss) and REDUCED SKIN BLOOD FLOW (dry heat loss) for a given core temperature. Heat-acclimated persons need to pay particular attention to fluid replacement because heat acclimation increases sweat losses, and hypohydration negates the thermoregulatory advantages conferred by acclimation [13].


Hypohydration reduces the GASTRIC EMPTYING RATE of ingested fluids during exercise in the heat [14, 15]. Neufer et al [14], for example, found a reduction of .25% in the gastric emptying rate when their subjects were hypohydrated (-5% body weight) that was related to increased core temperature [10, 13, 14, 15].

Dehydration amounting to about -10% of body weight was induced in adult MALE RATS by exposure to a hot, dry environment (D.B.T., 36 degrees C; R.H., 20%) over 6 to 8 hr. (A 10% fluid loss in humans may result in death.)

SKIN 30%
BONE 14%

*The G.I. tract had the highest tendency to lose water while the brain and liver showed the least [16].


To prevent thermal injuries during distance running, the American College of Sports Medicine proposes that between 28-56 fluid ounces water should be ingested each hour during prolonged exercise. YET SUCH HIGH RATES OF FLUID INTAKE (28-56 fluid ounces) have been reported to cause WATER INTOXICATION.

To establish the freely-chosen rates of fluid intake during prolonged competitive exercise, Noakes et al measured fluid intake during, body weight before and after, and rectal temperature after competition in a total of 102 runners and 91 canoeists competing in events lasting from 170-340 min (nearly 3-6 hour events). Fluid intakes during competition ranged from 10-21.0 fluid ounces/hour; rates of water loss ranged from 23-43 fluid ounces per hour in the runners; values were lower in the canoeists. Mean post-race rectal temperatures ranged from 100-102 degrees F. There was no relationship between the degree of dehydration and post-race rectal temperature. They concluded that hyperthermia is uncommon in prolonged competitive events held in mild environmental conditions, and that exercise intensity, not the level of dehydration, is probably the most important factor determining the postexercise rectal temperature. During prolonged exercise in mild environmental conditions, a fluid intake of 17-34 fluid ounces per hour will prevent significant dehydration in the majority of athletes. The mismatch of fluid intake and fluid losses may lead to a body water deficit. It has generally been considered that decreases in performance become apparent when hypohydration exceeds -2% of body weight; that performance decrements become substantial when fluid losses exceed -5% of body weight; and that when fluid losses approach 6-10% of body weight, heat stroke and heat exhaustion become life-threatening [17].

When dehydration is superimposed on hyperthermia, the reductions in mediated Stroke Volume (SV) were significantly greater (26 mean, 23-29 ml/beat), and Cardiac Output Declined 13% (2.8 +/- 0.3 l/min). Furthermore, Mean Arterial Pressure declined 5%, and Systemic Vascular Resistance increased 10%. When hyperthermia was prevented, the entire decline in mediated Stroke Volume with dehydration was due to REDUCED BLOOD VOLUME (approximately 200 ml). These results demonstrate that the superimposition of DEHYDRATION + HYPERTHERMIA during exercise in the heat causes an inability to maintain cardiac output and blood pressure that makes the dehydrated athlete less able to cope with hyperthermia [18].

When endurance-trained athletes exercised at 70-72% VO2 max, hyperthermia (when subjects are by normally hydrated = euhydrated during exercise in the heat) and dehydration (when hyperthermia was prevented during exercise in the cold) each lowered stroke volume 7-8% and increased heart rate sufficiently to prevent a significant decline in cardiac output. However, when dehydration was allowed to cause hyperthermia during exercise in the heat, the decline in stroke volume was greater (20%) and cardiac output declined synergistically (13%). The resulting cardiac output appears to be the highest possible by the stressed cardiovascular system, yet it was insufficient for maintaining arterial blood pressure and a low vascular resistance during exercise. Clearly, the superimposition of dehydration on hyperthermia during exercise in the heat causes greater reductions in stroke volume and cardiovascular function that make the dehydrated athlete much less able to cope with hyperthermia [19].


  1. Starting pace rate should be modified if the heat index (heat + humidity) is higher than previous training exposure.
  2. Hydrate the 3 hours prior to the event 8-16 fluid ounces per hour.
  3. Hydrate during the event: 20-28 fluid ounces in divided dose each hour.
  4. Fuel between 240-280 calories per hour - Less (240 calories) may be better absorbed when core temperatures are excessive.
  5. Electrolyte dose should be utilized prior to the event in training and dose determined between 2-6 Endurolytes per hour in divided dose taken with fuel and fluids.
  6. If overheating occurs resulting in dry mouth, gastric stress, or muscle cramps, slow pace accordingly, or take walk breaks frequently. Consume as cold/icy fluids as tolerated in small dose often. Immersing upper body and leg musculature in cool water will increase evaporative cooling.
  7. If heat stress is not resolved or modified, cease exercise and avoid serious life-threatening heat injury. No event is worth putting your life in harm's way.


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    [2] Rehrer NJ, Janssen GM, Brouns F, Saris WH. Fluid intake and gastrointestinal problems in runners competing in a 25-km race and a marathon. Int J Sports Med. 1989 May;10 Suppl 1:S22-5.

    [3] Ryan AJ, Lambert GP, Shi X, Chang RT, Summers RW, Gisolfi CV. Effect of hypohydration on gastric emptying and intestinal absorption during exercise. J Appl Physiol. 1998 May;84(5):1581-8.

    [4] Armstrong, L. E., R. P. Francesconi, W. J. Kraemer, N. Leva, J. P. DeLuca, and R. W. Hubbard. Plasma cortisol, renin, and aldosterone during an intense heat acclimation program. Int. J. Sports Med. 10: 38-42, 1989. pubmed&list_uids=2649446&dopt=Abstract

    [5] Barr, S. I., D. L. Costill, and W. J. Fink. Fluid replacement during prolonged exercise: effects of water, saline, or no fluid. Med. Sci. Sports Exerc. 23: 811-817, 1991. cmd=retrieve&db=pubmed&list_uids=1921673&dopt=Abstract

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    [7] Rehrer, N. J., E. J. Beckers, F. Brouns, F. T. Hoor, and W. H. Saris. Effects of dehydration on gastric emptying and gastrointestinal distress while running. Med. Sci. Sports Exerc. 22: 790-795, 1990.

    [8] Sawka MN, Pandolf KB, Avellini BA, Shapiro Y. Does heat acclimation lower the rate of metabolism elicited by muscular exercise? Aviat Space Environ Med. 1983 Jan;54(1):27-31. [9] Gonzalez-Alonso J. Separate and combined influences of dehydration and hyperthermia on cardiovascular responses to exercise. Int J Sports Med. 1998 Jun;19 Suppl 2:S111-4.

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    [13] Sawka MN, Montain SJ. Fluid and electrolyte supplementation for exercise heat stress. Am J Clin Nutr. 2000 Aug;72(2 Suppl):564S-72S. Review.

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    [18] Naghii MR. The significance of water in sport and weight control. Nutr Health. 2000;14(2):127- 32. Review.

    [19] Gonzalez-Alonso J, Mora-Rodriguez R, Below PR, Coyle EF. Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise. J Appl Physiol. 1997 Apr;82(4):1229-36.

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