BETHESDA, Md. (Sept. 4, 2005) -- For over 20 years, the U.S. ArmyResearch Institute of Environmental Medicine has studied the effect oftemperature and the environment on physical performance. According toMichael Sawka, chief of USARIEM's Thermal and Mountain MedicineDivision, "we're filling in the data gaps regarding the interaction oftemperature and hydration on physical performance so we can setguidelines to optimize results relevant not just to soldiers or navydivers, but to athletes, firefighters and hunters -- anyone who's inextreme environments without access to food or water for long periods."
Several recent USARIEM studies in the Journal of Applied Physiologydescribe experiments in both warm and cold temperatures. One reportshowed that dehydration reduces physical performance, in this casecycling, 8% in temperate/cool air (68 degrees Fahrenheit), but only 3%in a cold 36 degrees F. Furthermore it found that cold weather itselfhad an insignificant impact on physical performance, irrespective ofhydration level.
A second USARIEM-generated study found that ingesting glycerol, asweetish syrup, was an effective hyperhydration agent, causing "nearlytwice as much fluid" to be retained after four hours of cold-airexposure (CAE) compared with water ingestion alone. "This study alsodemonstrates that hyperhydration doesn't modify cardiovascular orthermoregulatory responses during resting CAE," the reported added.
How glycerol may hold water 'in reserve' in body for use later
The implications of the second study are particularly interesting forprolonged outdoor exposure when rehydration is not possible. "Becauseglycerol is freely distributed in body water, hyperhydration with GI(glycerol ingestion) may better preserve the extravascular fluidvolume, accounting for the improved TBW (total body water), comparedwith water alone. This extravascular 'reserve' could later be called onduring exercise or heat stress, when hydration becomes important toperformance and thermoregulation," the paper noted.
Catherine O'Brien, lead author of the glycerol study, said"there's a window of two to six hours where GI could be beneficial.That's a narrow niche where it might be useful for instance forsoldiers on short-range patrol with inadequate access to rehydration."The paper noted that the experiments supported earlier findings"suggesting that glycerol induced hyperhydration through renalreabsorption of water and glycerol. Finally, this study providesinsight into the hormonal mechanisms of cold-induced diuresis and fluidshifts due to hyperhydration."
"Whether the degree of hyperhydration" in the current study"is sufficient to improve physical performance in the cold orthermoregulation during subsequent body warming due to exercise or heatexposure remains to be demonstrated," the paper noted.
In addition, O'Brien said: "We learned previously that hydrationdoesn't seem to affect susceptibility to frostbite. But soldiers andoutdoorsmen are more affected by their hands and fingers getting stiff.We're going to look at how physical performance such as manualdexterity can better be maintained in the cold."
Some dehydration shows no performance effect in cold, but does as temperature rises
It's well recognized that athletes perform progressively better as thetemperature falls from hot to cool. It is also known that dehydrationworsens performance in the heat, but its effect in milder environmentsis not well understood. A USARIEM team led by Samuel N. Cheuvront foundthat dehydration by 3% of body weight had little adverse impact oncycling performance in the cold (36F), but markedly reduced performancein temperate air (68F).
"We induced a 3% body weight loss because that's about how much waterthe average marathon runner loses," Cheuvront noted. The team foundthat while this much dehydration produced only a minor negative affectat 36F, at 68F it made a significant 8% cut in performance. "Wemeasured performance as work performed (in kilojoules), but the realindicator is time: 8% over the course of a marathon is the differencebetween finishing in 2 hours 30 minutes or 2 hours 42 minutes -- andthat's a big difference!" Cheuvront said.
He added a quick note of realism, though: "Remember thatalthough we're testing healthy and fit Army recruits, the averagecompetitive runner's performance might not drop as drastically." Theother important finding in the experiment was that with hydration keptsteady, cold in and of itself did not negatively impact performance.
Some elegant measures of "importance" and exertion
Interestingly, the researchers found that during exercise the subjects"thought" they were working at exactly the same rate of exertion, eventhough there was a major difference between their actual performances.
Another measure they used is called the "zone of indifference," whichcan indicate not just whether a finding is or is not "statisticallysignificant, but if it's biologically important or meaningful,"Cheuvront said. "In this case the results were both statisticallysignificant and meaningful," he added. The "spirit of this approach,most closely related to equivalence testing in the clinical sciences,has recently been championed as a performance interpretation tool forthe exercise sciences by Dr. William G. Hopkins," the paper noted.
Next steps: "The preservation of endurance performance in cold air whenhypohydrated may be explained by differences in cardiovascular functionand oxygen uptake dynamics," the paper said. "Although the presentexperiment was not designed to assess the mechanism behind performancechanges, the explanation is reasonable based on the work of others," itadded.
Some 'common-sense' tips on hydration
USARIEM and the American Physiological Society
Source: Two USARIEM studies in 'Journal of Applied Physiology'
The two studies from the USARIEM laboratory appear in the Journal ofApplied Physiology, published by the American Physiological Society."Hypohydration impairs endurance exercise performance in temperate butnot cold air," available online, is by Samuel N. Cheuvront, RobertCarter III, John W. Castellani and Michael N. Sawka.
The second paper, "Glycerol hyperhydration: physiological responsesduring cold-air exposure," is by Catherine O'Brien, Beau J. Freund,Andrew J. Young and Michael N. Sawka, and appears in the August issueof JAP.
All researchers for both papers are at the U.S. Army Research Instituteof Environmental Medicine, Thermal and Mountain Medicine Division,Natick, Mass.
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