Hydration for Hikers

By Howard E. Friedman, DPM

Item #1 on the Trail Conference’s list of 10 Hiking Essentials is Water—generally, “two quarts per day per person in every season.”

“Fluid loss is heightened in winter as well as summer,” the advice continues. “Don't put yourself in the position of having to end your hike early because you have run out of water.”

I would add health and safety considerations to this advice. Under-hydrate and risk weakness, muscle cramps, increased cardiovascular strain and, in severe cases, the need for intravenous fluids or worse. Over-hydrate and risk headache, vomiting, fatigue, confusion, and in the worst case, death. Runners have died in both the Boston and London Marathons from over-hydration.

Yet experts disagree on the best indicators for staying properly hydrated. One leading sports physician group advises endurance athletes to calculate their fluid needs and schedule hydration methodically. Another group advises reliance on our built-in thirst mechanism to signal the need to hydrate. No specific guidelines exist for hikers, but many of the principles outlined for runners can be applied to a long day of hiking.

What the Experts Say

The Institute of Medicine of the National Academies suggests that total water intake for men is about 3.7 liters/day while women need about 2.7 liters, with more required during prolonged physical activity. (One liter is equal to about one quart.)Total water includes all beverages, water, and water contained in food. Several physician associations have published hydration guidelines specifically for marathon runners.

The American College of Sports Medicine downplays the role of thirst and recommends that marathon runners estimate the amount of fluid they will need and drink measured doses throughout the day. They note that thirst is a late sign indicating the body is already dehydrated.

To calculate hourly fluid loss, experts recommend weighing oneself first thing in the morning naked and after urinating, running (after dressing) at race speed for one hour, weighing oneself afterward while naked, and adding in the amount of fluid consumed during the run. The difference in weight can be used to calculate a “sweat rate,” the approximate volume of water one will need to replace each hour.         

The International Marathon Medical Director’s Association (IMMDA), on the other hand, states emphatically that thirst is the best way of knowing when to drink. The IMDDA feels that turning off the thirst mechanism by drinking consistently, even at pre-measured amounts, can lead to dangerous over-hydration.

Fluid Loss

Sweating is the primary mechanism by which our bodies prevent overheating during physical activity. As sweat evaporates from the skin it creates a cooling effect. People can lose between one half to two liters of sweat per hour during intense exercise, with a loss of one liter per hour common for runners.

Athletes are advised to stay within their “hydration zone,” losing no more than about 2% of their body weight from fluid loss. Sweating one liter equals losing about two pounds of body weight. A 150-pound person running for four hours could lose 6% of body weight if she does not replenish any fluids, resulting in severe dehydration.

Determining if one is well hydrated is not readily obvious and one might not know how much fluid is lost while running or hiking. A normal color of urine, neither too dark nor too clear is an indicator of proper hydration. And thirst is a clear indicator of the need to drink. Ultra-marathon runners covering 50-100 miles get weighed several times during their races to be sure they have neither lost nor gained too much weight from inappropriate hydration.

What about hiking?

Marathon runners log 26.2 miles. Some backpackers and even day hikers could cover a similar distance. But high-mileage, arduous hiking differs from endurance running in important ways that affect fluid needs. Runners rarely stop but do slow down to drink; hikers can stop at will to drink and eat. Runners lose almost all their body fluid through sweat; hikers out all day will lose fluids from sweat and probably urine. Marathoners can rehydrate at water stations; hikers need to either carry all water their water or find it along the way. Runners travel faster than hikers.

Nonetheless, basic principles for runners apply to hikers too. They should begin the day well hydrated, drinking in the morning. If urine appears dark at the start of the day, then drink some more. Several long-distance hikers routinely drink a liter at the start of their day to minimize the amount of water they need to carry.

Body weight, air temperature, humidity, trail difficulty, and the weight of one’s backpack all factor into personal hydration needs, as does age—older people tend to sweat less than younger ones. So too do the foods one eats: salty foods and carbohydrates may negate the need for a sports drink; water should be sufficient. Fruits and vegetables can also count toward fluid intake.

In Ultimate Hiker’s Gear Guide (National Geographic 2012), author Andrew Skurka recounts how he drank about a third of a liter each hour while on a 70-mile, mostly night-time desert trek across Joshua Tree National Park. Furthermore he estimates that his daily fluid needs range between 4 to 10 liters a day depending on where and when he is hiking. Mr. Skurka wrote in an email that 10 liters would be an extreme upper limit. He recalled one day’s hike along the Pacific Crest Trail in hot weather when he was drinking two quarts an hour and still felt dehydrated.

Carrying a water container with volume measurement markings is helpful to track just how much one is actually drinking.

Hikers should be attuned to how much water they have drunk during the day, be familiar with symptoms of dehydration, and understand the risk of over-hydration as well. Remaining hydrated is important even if it means, in extenuating circumstances, drinking water from an untreated source.

Howard E. Friedman, DPM, is an avid hiker, a podiatrist in Suffern, NY, and a frequent contributor to Trail Walker.