A water tale for all seasons: When it comes to hydration and exercise, the system works

For over 20 years, the U.S. Army Research Institute of Environmental Medicine has studied the effect of temperature and the environment on physical performance. According to Michael Sawka, chief of USARIEM's Thermal and Mountain Medicine Division, "we're filling in the data gaps regarding the interaction of temperature and hydration on physical performance so we can set guidelines to optimize results relevant not just to soldiers or navy divers, but to athletes, firefighters and hunters – anyone who's in extreme environments without access to food or water for long periods."

Several recent USARIEM studies in the Journal of Applied Physiology describe experiments in both warm and cold temperatures. One report showed that dehydration reduces physical performance, in this case cycling, 8% in temperate/cool air (68 degrees Fahrenheit), but only 3% in a cold 36 degrees F. Furthermore it found that cold weather itself had an insignificant impact on physical performance, irrespective of hydration level.

A second USARIEM-generated study found that ingesting glycerol, a sweetish syrup, was an effective hyperhydration agent, causing "nearly twice as much fluid" to be retained after four hours of cold-air exposure (CAE) compared with water ingestion alone. "This study also demonstrates that hyperhydration doesn't modify cardiovascular or thermoregulatory 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 for prolonged outdoor exposure when rehydration is not possible. "Because glycerol is freely distributed in body water, hyperhydration with GI (glycerol ingestion) may better preserve the extravascular fluid volume, accounting for the improved TBW (total body water), compared with water alone. This extravascular 'reserve' could later be called on during exercise or heat stress, when hydration becomes important to performance 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 for soldiers on short-range patrol with inadequate access to rehydration." The paper noted that the experiments supported earlier findings "suggesting that glycerol induced hyperhydration through renal reabsorption of water and glycerol. Finally, this study provides insight into the hormonal mechanisms of cold-induced diuresis and fluid shifts due to hyperhydration."

Next steps

"Whether the degree of hyperhydration" in the current study "is sufficient to improve physical performance in the cold or thermoregulation during subsequent body warming due to exercise or heat exposure remains to be demonstrated," the paper noted.

In addition, O'Brien said: "We learned previously that hydration doesn't seem to affect susceptibility to frostbite. But soldiers and outdoorsmen are more affected by their hands and fingers getting stiff. We're going to look at how physical performance such as manual dexterity 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 the temperature falls from hot to cool. It is also known that dehydration worsens performance in the heat, but its effect in milder environments is not well understood. A USARIEM team led by Samuel N. Cheuvront found that dehydration by 3% of body weight had little adverse impact on cycling performance in the cold (36F), but markedly reduced performance in temperate air (68F).

"We induced a 3% body weight loss because that's about how much water the average marathon runner loses," Cheuvront noted. The team found that while this much dehydration produced only a minor negative affect at 36F, at 68F it made a significant 8% cut in performance. "We measured performance as work performed (in kilojoules), but the real indicator is time: 8% over the course of a marathon is the difference between finishing in 2 hours 30 minutes or 2 hours 42 minutes – and that's a big difference!" Cheuvront said.

He added a quick note of realism, though: "Remember that although we're testing healthy and fit Army recruits, the average competitive runner's performance might not drop as drastically." The other important finding in the experiment was that with hydration kept steady, 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, even though there was a major difference between their actual performances.

Another measure they used is called the "zone of indifference," which can indicate not just whether a finding is or is not "statistically significant, but if it's biologically important or meaningful," Cheuvront said. "In this case the results were both statistically significant 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 for the exercise sciences by Dr. William G. Hopkins," the paper noted.

Next steps: "The preservation of endurance performance in cold air when hypohydrated may be explained by differences in cardiovascular function and oxygen uptake dynamics," the paper said. "Although the present experiment was not designed to assess the mechanism behind performance changes, the explanation is reasonable based on the work of others," it added.

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.

Source: American Physiological Society