Cold Weather Increases Risk
UNH Study Gives Insight Into Why
We Feel Less Thirsty
Contact: Sharon Keeler
UNH Media Relations
Jan. 28, 2005
DURHAM, N.H. -- Frostbite and hypothermia are not the only
health hazards associated with frigidly cold temperatures. Cold
weather studies at the University of New Hampshire show increased
risk for dehydration, a condition more commonly associated with
“People just don’t feel as thirsty when the weather
is cold,” says Robert Kenefick, UNH associate professor of
kinesiology. “When they don’t feel thirsty, they don’t
drink as much, and this can cause dehydration.”
We lose a great deal of water from our bodies in the winter due
to respiratory fluid loss through breathing. Our bodies also are
working harder under the weight of extra clothing, and sweat evaporates
quickly in cold, dry air.
The body is about two-thirds water, and when the total water level
drops by only a few percent, we can become dehydrated. Kenefick
says fluid deficits of 3 to 8 percent of body mass have been reported
in individuals working in cold environments, and dehydration is
a major problem with exercise in the cold.
Yet the loss of fluid from our bodies, which triggers thirst in
warmer weather, does not elicit the same response when the temperatures
dip. It’s not simply because we don’t feel hot, Kenefick
says. His recent study, published in the journal Medicine &
Science in Sports &
Exercise, shows that cold actually alters thirst sensation.
“Fluid balance in our bodies often relies on the stimulation
of thirst, resulting in voluntary fluid intake, as well as the kidneys
conserving or excreting water,” Kenefick says. “This
process is mediated by fluid-regulating hormones such as plasma
argentine vasopressin (AVP).”
There are two factors that trigger the response of this fluid-regulating
hormone. As our bodies lose water, sodium levels in the blood increase.
Overall blood volume also decreases. These two responses trigger
the hypothalamus to secrete AVP, which causes the kidneys to slow
down their production of urine. This restores body fluid. At the
same time, the hypothalamus signals the brain's cortex to create
a thirst drive to force the increased water intake needed to restore
the normal salt level.
To find out why the body reacts differently in the cold, Kenefick
subjected individuals to the cold chambers at UNH, where they both
exercised on a treadmill and rested. During cold exposure, he explains,
vasoconstriction takes place – the body decreases blood flow
to the periphery of the body to decrease heat loss.
What he also discovered was that, because blood volume at the body’s
core increases, the brain does not detect blood volume decrease.
Thus, the hormone AVP is not secreted at the same increased rate,
despite elevated blood sodium. The kidneys get a diminished signal
to conserve fluid, and thirst sensation is reduced by up to 40 percent.
“It’s a trade off – maintaining the body’s
core temperature becomes more important than fluid balance,”
Kenefick says. “Humans don’t naturally hydrate themselves
properly, and they can become very dehydrated in cold weather because
there is little physiological stimulus to drink.”
Interestingly, animals like rats and dogs also show the same decreased
thirst sensation in cold weather that human do, even though they
will normally drink back all the fluid they have lost. They
also experience a rise in central blood volume due to cold induced
Kenefick offers the same advice he tenders during the heat. Drink
plenty of water, especially when exercising or working outdoors.
A good way to monitor proper hydration is to examine urine output
– the color should be nearly clear.