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What effect does hydration have on body temperature?
Abstract
Fluid balance in the body is the balance between water input (from foods, beverages , and a small amount generated by metabolism) a nd water output (urine, insensible losses, sweat, and fecal loss). During conditions of physiological stress (ie, exercise), the ability to maintain optimal fluid balance (ie, euhydration) has significant implications with regard to body temperature regulation. The body's response to increased body temperature during exercise is to increase skin blood flow and sweating. The ability of these thermoregulatory responses to optimally attenuate increases in body temperature is affected by hydration status. For example, a deficit in fluid balance (ie, dehydration) results in a higher core body temperature than if euhydration is maintained. A higher core body temperature can negatively affect athletic performance, mood state, and cognitive function and can increase one's risk for heat illness. This risk is further exacerbated when extreme environmental conditions place additional stress on thermoregulatory function (ie, hot/dry or hot/ humid conditions; temperature ≥ 90°F, 32°C). This chapter will focus on and provide the scientific evidence for the relationship between hydration status and core body temperature during exercise. C o p y r i g h t e d m a t e r i a l. N o t f o r d i s t r i b u t i o n .
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Authors of most field studies have not observed decrements in physiologic function and performance with increases in dehydration, although authors of well-controlled laboratory studies have consistently reported this relationship. Investigators in these field studies did not control exercise intensity, a known modulator of body core temperature.
To directly examine the effect of moderate water deficit on the physiologic responses to various exercise intensities in a warm outdoor setting.
Semirandomized, crossover design.
Field setting. Patients or Other
Seventeen distance runners (9 men, 8 women; age = 27 +/- 7 years, height = 171 +/- 9 cm, mass = 64.2 +/- 9.0 kg, body fat = 14.6% +/- 5.5%).
Participants completed four 12-km runs (consisting of three 4-km loops) in the heat (average wet bulb globe temperature = 26.5 degrees C): (1) a hydrated, race trial (HYR), (2) a dehydrated, race trial (DYR), (3) a hydrated, submaximal trial (HYS), and (4) a dehydrated, submaximal trial (DYS). Main Outcome Measure(s): For DYR and DYS trials, dehydration was measured by body mass loss. In the submaximal trials, participants ran at a moderate pace that was matched by having them speed up or slow down based on pace feedback provided by researchers. Intestinal temperature was recorded using ingestible thermistors, and participants wore heart rate monitors to measure heart rate.
Body mass loss in relation to a 3-day baseline was greater for the DYR (-4.30% +/- 1.25%) and DYS trials (-4.59% +/- 1.32%) than for the HYR (-2.05% +/- 1.09%) and HYS (-2.0% +/- 1.24%) trials postrun (P < .001). Participants ran faster for the HYR (53.15 +/- 6.05 minutes) than for the DYR (55.7 +/- 7.45 minutes; P < .01), but speed was similar for HYS (59.57 +/- 5.31 minutes) and DYS (59.44 +/- 5.44 minutes; P > .05). Intestinal temperature immediately postrun was greater for DYR than for HYR (P < .05), the only significant difference. Intestinal temperature was greater for DYS than for HYS postloop 2, postrun, and at 10 and 20 minutes postrun (all: P < .001). Intestinal temperature and heart rate were 0.22 degrees C and 6 beats/min higher, respectively, for every additional 1% body mass loss during the DYS trial compared with the HYS trial.
A small decrement in hydration status impaired physiologic function and performance while trail running in the heat.
We studied the effects of graded hypohydration levels on thermoregulatory and blood responses during exercise in the heat. Eight heat-acclimated male subjects attempted four heat-stress tests (HSTs). One HST was attempted during euhydration, and three HSTs were attempted while the subjects were hypohydrated by 3, 5, and 7% of their body weight. Hypohydration was achieved by an exercise-heat regimen on the day prior to each HST. After 30 min of rest in a 20 degrees C antechamber the HST consisted of a 140-min exposure (4 repeats of 10 min rest and 25 min treadmill walking) in a hot-dry (49 degrees C, 20% relative humidity) environment. The following observations were made: 1) a low-to-moderate hypohydration level primarily reduced plasma volume with little effect on plasma osmolality, whereas a more severe hypohydration level resulted in no further plasma volume reduction but a large increment in plasma osmolality; 2) core temperature and heart rate responses increased with severity of hypohydration; 3) sweating rate responses for a given rectal temperature were systematically decreased with severity of hypohydration; and 4) the reduction in sweating rate was more strongly associated with plasma hyperosmolality than hypovolemia. In conclusion, an individual's thermal strain increases linearly with the severity of hypohydration during exercise in the heat, and plasma hyperosmolality influences the reduction in sweating more profoundly than hypovolemia.
The purpose of this study was to examine the thermoregulatory sweating control parameters of threshold temperature and sensitivity to determine whether 1) these variables were altered by hypohydration level and exercise intensity and 2) these alterations, if present, were additive and independent. Nine heat-acclimated men completed a matrix of nine trials: three exercise intensities of 25, 45, and 65% maximal O2 uptake and three hydration levels, i.e., euhydration and hypohydration (Hy) at 3 and 5% of body weight. During each trial, subjects attempted 50 min of treadmill exercise in a warm room (30 degrees C dry bulb, 50% relative humidity) while esophageal temperature and upper arm sweating rate were continuously measured. Hypohydration was achieved by exercise and fluid restriction the day preceding the trials. The following new findings were made: 1) threshold temperature increased in graded manner with hypohydration level (approximately 0.06 degree C/% Hy); 2) sensitivity decreased in a graded manner with hypohydration level (approximately 0.06 units/%Hy); 3) threshold temperature was not altered by exercise intensity; and 4) sensitivity increased from low- to moderate- and high-intensity exercise. We conclude that both hypohydration level and exercise intensity produce independent effects on control of thermoregulatory sweating.
This investigation determined the effect of different rates of dehydration, induced by ingesting different volumes of fluid during prolonged exercise, on hyperthermia, heart rate (HR), and stroke volume (SV). On four different occasions, eight endurance-trained cyclists [age 23 +/- 3 (SD) yr, body wt 71.9 +/- 11.6 kg, maximal O2 consumption 4.72 +/- 0.33 l/min] cycled at a power output equal to 62-67% maximal O2 consumption for 2 h in a warm environment (33 degrees C dry bulb, 50% relative humidity, wind speed 2.5 m/s). During exercise, they randomly received no fluid (NF) or ingested a small (SF), moderate (MF), or large (LF) volume of fluid that replaced 20 +/- 1, 48 +/- 1, and 81 +/- 2%, respectively, of the fluid lost in sweat during exercise. The protocol resulted in graded magnitudes of dehydration as body weight declined 4.2 +/- 0.1, 3.4 +/- 0.1, 2.3 +/- 0.1, and 1.1 +/- 0.1%, respectively, during NF, SF, MF, and LF. After 2 h of exercise, esophageal temperature (Tes), HR, and SV were significantly different among the four trials (P < 0.05), with the exception of NF and SF. The magnitude of dehydration accrued after 2 h of exercise in the four trials was linearly related with the increase in Tes (r = 0.98, P < 0.02), the increase in HR (r = 0.99, P < 0.01), and the decline in SV (r = 0.99, P < 0.01). LF attenuated hyperthermia, apparently because of higher skin blood flow, inasmuch as forearm blood flow was 20-22% higher than during SF and NF at 105 min (P < 0.05). There were no differences in sweat rate among the four trials. In each subject, the increase in Tes from 20 to 120 min of exercise was highly correlated to the increase in serum osmolality (r = 0.81-0.98, P < 0.02-0.19) and the increase in serum sodium concentration (r = 0.87-0.99, P < 0.01-0.13) from 5 to 120 min of exercise. In summary, the magnitude of increase in core temperature and HR and the decline in SV are graded in proportion to the amount of dehydration accrued during exercise.
Work in the heat as affected by intake of water, salt and glucose Figure 37-1. Schematic illustrating differences in body temperature response during exer-cise related to hydration status
- Johnson R Consolazio
Pitts G, Johnson R, Consolazio F. Work in the heat as affected by intake of water, salt and glucose. Am J Physiol. 1944;142. Figure 37-1. Schematic illustrating differences in body temperature response during exer-cise related to hydration status.