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
- The Boy Scout adage still holds: "Check urine color. It should be relatively clear. If it's dark, you need to drink more," O'Brien said.
- "Although the 8-by-8 rule of drinking eight 8-ounce glasses of water a day is well recognized, is has almost no scientific basis. The recent Institute of Medicine report on water and electrolytes established an Adequate Intake (AI) for water of 3.7 liters/day for a normal adult male, but there is wide variation. Importantly, that 3.7 liters includes water from food and drink, including beverages like coffee or tea," Cheuvront noted.
- Exercise fluid intakes should result in neither weight gain nor excessive weight loss (more than 2% of body weight). "Weighing oneself nude before and after exercise is the best way to gauge success around this recommendation," Cheuvront added.
- Don't drink too much, even in the heat: "We have this mistaken belief that more water is better. Not true. The Army has actually reduced the amount of water it gives in the heat," Sawka said.
- Even in the cold, other recent USARIEM studies showed that "reduced body water levels (hypohydration) does not increase the risk of hypothermia or peripheral cold injury" such as frostbite, the Cheuvront paper reported.
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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