“Change and fluidity of living organisms is constant. An organism, whether at rest or in motion, is constantly adapting to its environment.” D. W. Wheatley

Water, Hydration, and Dehydration

Cellular life is a continuous dance of water and molecules, and when these dancers are out of sync, the molecular ballet can transform into cellular dysfunction. Cellular imbalance is more likely during strenuous physical activity, particularly outdoor activity, when water and sodium—the two lead dancers—are most likely to evaporate. 

The superbly tuned metabolic processes that maintain cellular homeostasis require an optimal temperature and precise water-to-ion concentrations. Intracellularly, water balance is maintained through osmotic forces, sodium-potassium ion pump activity, and augmented protein folding. Extracellular water balance is regulated across the entire organism, by osmoreceptors in the hypothalamus influencing kidney functions of fluid filtration, water reabsorption, and fluid excretion. Physical activity can upset the critical balance between intracellular and extracellular water and ion concentrations, pushing an organism’s water dance to its limits.

Figure 1. The Hydration Dance. Generated using DALL·E. Dewaeert, D.

During physical exertion, you breathe quickly, increase oxygen demand, and your muscles generate heat. As your muscles work, capillaries open, requiring a robust extracellular water volume to fill this increased vascular capacity. At the same time, your body is dissipating heat through evaporative sweating, leading to water and salt loss from the extracellular fluid—namely from the plasma. If you’re not replacing the lost water and ions through proper hydration, the body will compensate by pulling water and sodium out of cells and into the extracellular plasma space—water and sodium that cells need to maintain proper functioning. Thus, orally replacing lost fluid and ions avoids dehydration and cellular damage by restoring the circulating blood volume needed for optimal oxygen delivery and cellular function. Crucially, the ever-shifting outdoor environment, particularly fluctuating temperature and humidity, can exacerbate fluid and ion losses caused by physical exertion. 

In general, the average adequate daily water intake is about 2.5 L for adult men and 2.0 L for adult women, and these requirements vary among individuals and can increase due to physical activity and environmental conditions. “Hydration” during outdoor activity is more than simply having a drink of water; what, how, and when we drink and eat have a direct impact on water and electrolyte balance, physical performance, and outdoor safety. In preparing for an outdoor adventure, pre-activity fluid intake is important to prepare the body for fluid loss during exertion. But bolus drinking and hyperhydration trigger baroreceptors that reduce kidney sodium reabsorption, causing excess water to be lost through urination. This means that single, high-volume pre-exercise fluid intake is not a sufficient way to prepare for a day of exertion, and no evidence exists to suggest that pre-exercise excess water consumption (aka, “camel-up”) can improve performance or mitigate dehydration. Instead, sustained activity calls for sustained hydration—before, during, and after activity. Note that while plain water is rapidly absorbed in the small intestine, combining water and electrolyte-containing fluids while also eating water- and mineral-rich foods provides a slower, sustained hydration that can be absorbed along the entire intestine. In addition to regular water and electrolyte-containing fluid intake, eating snacks with >90% water content, such as fresh vegetables and fruits, is optimal. Finally, when the day is done and activity has ended, drinking plain water and having a meal containing sufficient sodium should adequately replace lost fluids and salts.

Figure 2.  The Hydration Cycle: Exercise diminishes extracellular fluid stores. This will be replenished by intracellular fluid unless it is replaced orally.  Created in BioRender. Adams, H.

Rehydration when Dehydration Occurs

Modest levels of dehydration are generally well-tolerated, and a body weight loss of 2% to 3% indicates only moderate dehydration. But serious dehydration can occur, particularly in hot and humid environments that promote increased sweating. Counterintuitively, heat-related dehydration can lead to impaired sweating, which can in turn cause other heat-related injuries such as heat cramps, heat syncope, heat exhaustion, and heat stroke. Therefore, the outdoor adventurer should be familiar with using oral rehydration solutions (ORS) to counteract dehydration. 

The World Health Organization (WHO) recommends ORS that have an osmolality of approximately 245 mOsm/kg, which is lower than typical blood osmolality of 275–295 mOsm/kg. Commercially available ready-made low-osmolality ORS packets are recommended over homemade solutions because of the potential for mixing errors, and having ORS packets with abundant clean water is recommended for prolonged and  strenuous  outdoor adventures. However, when these are not available, carefully made homemade solutions can be used (Table 1).

Table 1. Two different protocols for making oral rehydration solutions to address dehydration.

Familiarity with ORS is essential for those who are active in the wilderness, since intravenous fluids and other advanced care are unavailable out in the field. Individuals with mild heat-related dehydration, exhaustion, cramps, and syncope can benefit from ORS. These solutions are also useful for treating dehydration caused by other conditions that might be experienced in the field, including severe diarrhea, acute mountain sickness, and exposure to cold. Some important contraindications to ORS include seizure, exercise-associated hyponatremia (EAH, see below), and altered mental status that may occur during heat stroke. Evacuation and higher level of care should be immediately sought for those with altered mental status, and individuals who are unable to take sufficient oral rehydration for more than 24 hours should be swiftly evacuated.

Avoiding Unhelpful Hydration Practices and Preventing Exercise-Associated Hyponatremia

For endurance and wilderness athletes, two critical hydration principles must be balanced: (1) adequate fluid intake to prevent dehydration, and (2) avoidance of excessive fluid intake that can lead to EAH, a serious and potentially lethal state of low serum sodium concentration. The development of EAH in athletes is complex, but the most common cause is overconsumption of hypotonic fluids. Exertion at high temperatures can speed the dehydration process and increase the risk of EAH. Extended exercise duration >4 hours, high sweat rate, high sweat sodium concentration, and small body size are additional factors that can predispose athletes to this complication. EAH is extremely rare among athletes who consume fluids at a rate below 700 mL/hr, so athletes can reduce the risk of EAH through sound hydration practices.

What is the optimal strategy for fluid replacement in such a setting?  Let’s consider several sub-optimal hydration strategies that may best be avoided. First, restoring lost sodium with salt tablets or capsules is popular among endurance athletes, but studies have not shown that they increase in plasma sodium concentration or provide a performance benefit. Also, pre-exercise glycerol with energy gels and gummies may delay dehydration during strenuous activity (a benefit for endurance athletes), but glycerol also has a dilutional effect on both intracellular and extracellular fluids and could therefore predispose athletes to EAH when combined with aggressive fluid intake. One possibly effective but difficult-to-implement hydration strategy is the “drinking plan” approach, in which the athlete determines their personal sweat rate and drinks according to a finely tuned plan. However, this method requires meticulous advance planning, regular body weight measurements, and a precise sweat rate calculation,  and is not relevant for less structured and more unpredictable endurance activities such as through-hiking or mountain climbing. A final hydration strategy that some athletes use is drinking only when they “feel” thirsty (aka, “drinking to thirst”). While drinking to thirst is appealing in its simplicity, studies have shown that the thirst mechanism is typically inadequate to fully compensate for fluid loss. Even so, this loss is typically so minimal that it does not negatively impact performance or cause clinically significant symptoms of dehydration.  Because of this, drinking to thirst, as opposed to bolus drinking, represents the best overall balance of feasibility and hydration for most activities.

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