Dehydration for all seasons:

Most of us are well aware of the risks of dehydration on blazing hot summer days, but what about the rest of the year? Dehydration is a significant issue in the winter as well. It’s not accompanied by the risk of heat exhaustion or heat stroke, so the medical risk is far lower, but you can bonk, and badly, if you don’t understand the risks.

The first risk is that of pure stupidity, combined with travel to warmer climates during the colder seasons. Allow me to illustrate using myself as an example. I traveled out to Phoenix recently to give a talk at a seminar featuring Bob Ballard, me and Lance Armstrong in succession. The talk was on Monday so I went out on Friday night to spend a weekend hiking. It was 88 degrees, sunny and dry. We had rented a mustang convertible, and set of for the 30 minute drive to the Superstition mountains with the top down. We planned a hard 10-mile hike, with a fair gain in elevation, so we prudently packed 4 liters of water for two people (a bit short of ideal, but plenty for a day hike). What we didn’t plan on is my somehow getting onto Interstate 10 to Tucson, insisting that I was right, and driving 100 miles out of our way before admitting my mistake. We turned around, drove back, and got to the trailhead after almost 3 hours in the car. I had also skipped bringing my heart monitor, as I planned to hike just for fun, and as much as I like the thing, I also enjoy the odd moment of freedom from gadgets.

The first hour of the hike went well, with a gain in elevation of about 1500 feet, but just before the last pitch I started to feel dizzy and nauseated. I had drunk about a full bottle of water so didn’t think of dehydration, and I didn’t have my heart monitor to warn me about my pace. In the end I had to rest in the shade of a boulder for about half an hour before starting down, and though I felt much better, my legs were like rubber for the entire descent. In fact it took me until that evening to figure out how dehydrated I was, and it took through most of the next day to get back to feeling 100%.

In retrospect I had combined the dehydration of a 5 hour plane trip with 3 hours in a convertible, top down under the mid-day sun. Because of the low humidity and the wind coming through the car I felt great, even as every last drop of moisture was sucked out of me. It wasn’t the hike, it was the events leading up to it that got to me. I have talked a lot about pre-hydrating in previous articles, and just didn’t think to follow my own advice!

So when you go to sunny climes during the winter, remember that you are NOT heat adapted, that the flight and simply walking around in the heat, or sitting by the pool will dehydrate you, and drink accordingly.

The next two causes of winter dehydration are actually caused by the season, and not by stupidity. The first, which most people are aware of, is that air holds much less moisture when it is cold than when it is hot. When you take cool, or cold air in from outside and heat it, the relative humidity plummets. In fact, the 70 degree air inside most buildings in New England in January is drier than the air in the Sahara desert. Of course, you are not losing fluid by sweating in this situation, but you lose tremendous amounts of moisture by breathing.

Here’s how it works. Your body absorbs oxygen in a network of extraordinarily delicate tissues called alveoli, which are deep, deep inside your lungs. The walls of the alveoli are only one or two cells thick, and unless the air inside your lungs is kept close to 100% humidity, the alveoli will collapse. When you breathe dry air in through your nose or mouth, your body uses water to humidify it so that it comes close to 100% by the time it reaches the bottom of your lungs. When you exhale, your body is able to reclaim some of this water, but not all of it. Under normal circumstances, you lose 1-2 liters a day of fluid this way, but when the air is extraordinarily dry, as it is in EVERY heated building in the world, this can rise 2-4 fold. The same thing is true for airplanes, cars and anywhere else air is heated. Since the process is reversed with air conditioning, building air has a much higher humidity in the summer than the winter, so the net effect is that most people actually have a higher fluid requirement during the winter than the summer.

The final pathway to cold-weather dehydration happens when you are actually outside, breathing in the cold stuff. It’s the same principle as above. Let’s say you breathe in a lungful of 20-degree air. Your alveoli are just as sensitive to temperature as they are to humidity, and your body will heat the air to 98.6 degrees. This is just like heating the air anywhere else, the relative humidity will plummet, but your body still has to make it 100% humid. As an aside, when you can see your breath on a cold winter day, that’s the excess fluid you are losing with each breath condensing into a vapor trail when it hits the cold air. When you are exercising, you almost 2000 breaths an hour. That’s a lot of fluid you are pumping out into the atmosphere.

One last pathway that’s worth mentioning is that when you are exposed to cold air, the arteries in your skin contract, shunting blood away from your skin, where it’s cold, and deep into your body to preserve the warmth. This has the side effect of forcing more blood through your kidneys, which therefore produce more urine. This is called cold-induced diuresis, and while not as significant a cause of dehydration as breathing, it adds further insult to the process.

The key to dealing with all of these is to pre-hydrate. Stay on top of your fluid requirements all day long, and cut back on alcohol and caffeine during ski trips or other outdoor exercise.