Dehydration, when sufficiently severe, impairs both mental and physical performance. It degrades decision-making, increases perceived effort, causes early fatigue, and can compromise safety on the mountain. For mountain and endurance athletes, the challenge is that access to fluids is often limited, the weight of water constrains what you can carry, and at high altitude, fluid loss increases significantly due to dry air and elevated breathing rates.
In Training for the New Alpinism, we advocate starting climbing days well hydrated and carrying less water than you might think necessary. This article expands on that advice with practical hydration strategies for before, during, and after long efforts—including situations where fluid access is restricted and you need to make the most of what you carry.
Is Water Enough for Long Efforts?
If you have free access to fluids and can drink to thirst throughout the day, water alone should be sufficient to offset any significant level of dehydration that would impair performance. However, in situations where fluid access is limited—carrying a fixed amount on a long climb, limited opportunities to refill—choosing a drink with higher energy and sodium content will promote longer-term fluid retention.
Research on the beverage hydration index has shown that drinks with higher energy and sodium content help the body retain more fluid compared to water alone. Milk, orange juice, oral rehydration solutions, and even full-sugar cola all showed significantly higher fluid retention two hours after ingestion compared to the same volume of water. The practical implication: when you know your fluid intake will be compromised, these drinks can help you retain more of what you consume.
Anecdotally, mountaineers I work with have reported consuming as little as 500 milliliters of water per day and carrying a maximum of around 2 liters at any one time while on the mountain. If this describes your situation, the strategies below become especially important.
What Affects How Much Fluid You Lose?
Factors that affect fluid loss through sweating include your sweat rate, your level of fitness, the intensity of physical exertion, direct sunlight exposure, glare from snow, and your layering system. At high altitude, water loss is significantly increased due to the dry air and your increased rate of ventilation to maintain tissue oxygenation.
Fluid needs are highly individual, and the best way to understand your own requirements is through experience. Practice your hydration strategy during training before heading out on a major objective. Consider the environment you will be in, how long you will be out, and your access to fluids.
How Should You Hydrate Before a Mountain Day?
In the days leading up to a mountain trip, aim to maintain hydration by sipping on fluids throughout the day and monitoring your urine output and color. Your urine should be a pale straw color. This is not about loading up with excess water the night before—it is about arriving at the start already in a good hydration state rather than playing catch-up from the first hour.
On the morning of your effort, include a glass of milk or orange juice with your breakfast, or have a milky coffee. These drinks have a higher beverage hydration index than water, meaning they help you retain more fluid and reduce the need for a bathroom stop before setting off. If you start dehydrated, you are already on the back foot and face an increased likelihood of severe dehydration that impairs both physical and mental performance.
What Is the Best Strategy for Drinking During a Long Effort?
Consider your scenario. Will you be able to drink freely, or will fluid intake be limited to opportune breaks? The answer shapes your entire hydration strategy for the day.
Drink to thirst when possible. Relying on thirst and your habitual drive to drink may be sufficient in many situations. But at altitude, or during efforts where fluid access is restricted, having a planned hydration strategy—being deliberate about taking a mouthful of fluid during opportune breaks—can go a long way toward offsetting severe dehydration.
Choose your drink strategically. Consider premixing a drink with higher energy and sodium content: 200 milliliters of fresh fruit juice, 300 milliliters of water, and a pinch of salt. If you carry two soft flasks, fill one with water and the other with a sports drink. Always practice your hydration strategy before your main event.
Eat and drink together. Whenever you eat something, take a mouthful of water to wash it down. The energy and salt content of your food will help you retain more of the water you drink.
Swish water around your mouth. When fluid supplies are low and you want to preserve water for the rest of the day, take a mouthful, swirl it around your mouth, then swallow. This sensation of fluid in the mouth has a positive sensory effect that can reduce the perception of thirst and buy you time.
How Should You Rehydrate After a Big Day?
Rehydration is especially important when you have a short turnaround before your next effort. Immediately after finishing, drink a glass of milk, a recovery shake, a hot chocolate, or even a cola—all of which have a higher beverage hydration index than water alone. Have a glass of fruit juice or milk with your next meal. Sip regularly on water until approximately 30 minutes before bedtime.
If you feel extremely dehydrated after a long day with minimal fluid intake, or if you have a short turnaround until your next effort, consider an oral rehydration solution. These are specifically formulated to maximize fluid absorption and can help restore hydration status faster than water or standard beverages.
What Hydration System Should You Use?
Use whatever system encourages you to actually drink and is easy to carry. Nalgene bottles and soft flasks are popular choices among mountain athletes. The best hydration system is the one you will use consistently, not the one with the most features. If carrying two soft flasks (one water, one sports drink) works for your activity, that setup gives you flexibility without adding significant weight.
What Should You Take Away from This?
When you are going to have limited access to fluids and a tight window for rehydration, make the most of what you drink. Start hydrated, choose beverages that promote fluid retention, eat and drink together, and rehydrate aggressively when you get the chance. And above all, practice your hydration strategy during training before testing it on a major objective. Your fluid needs are individual, and the only way to understand them is through experience.
References
Benton, D., et al. “Executive summary and conclusions from the European Hydration Institute expert conference on human hydration, health, and performance.” Nutrition Reviews 2015 Sept;73(suppl 2):148–150.
Hoffman, M.D., et al. “Considerations for ultra-endurance activities: part 2 – hydration.” Research in Sports Medicine 2018 July;27(2):182–194.Maughan, R.J., et al. “A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index.” The American Journal of Clinical Nutrition 2016 Mar;103(3):717–723.