Importance of hydration in dry periods and cooler months

Zachary J. Schlader, PhD, FACSM

Most of the human body is water. Due to the metabolism of glycogen, the human body can produce only very small amounts of water and has a limited capacity to store water. Therefore, the body is constantly balancing fluid intake (drinking) and fluid output (for example, urine, sweat, fecal and respiratory water losses) to maintain adequate levels of body water. Being in a state of overhydration is relatively rare. That said, being underhydrated (also termed hypohydrated or dehydrated) is comparatively common, as this situation occurs when fluid intake is less than fluid output. The most extreme example of hypohydration is likely that of exercise and/or heat exposure, where sweat losses are not fully offset by drinking. However, hypohydration can occur in more subtle situations, that are independent of exercise or heat exposure. An understanding of the vital role of hydration in these ‘subtle situations’ is important because the evidence supports that preventing hypohydration is essential for health and is necessary for ideal functioning of bodily systems. Thus, ensuring optimal physical and mental functioning requires healthy hydration practices. 

Body water can be lost without it being sensed. That is, we can lose body water in ways that do not involve the production of sweat or copious volumes of urine. For example, our body adds water to the air we breathe-in so that the relative humidity of the air is approximately 100% by the time that air is ready for gas exchange in the alveoli of the lungs. In this instance, the amount of water moved from the body to the air in the lungs is dependent on the humidity of the air breathed in. Thus, breathing ‘dry air’ (or air with a low humidity) requires the loss of a greater amount of body water compared to breathing air with a higher humidity. Furthermore, low ambient humidity can also increase the loss of water from tissues in contact with the low humidity air, including water loss from the skin and other tissues.

Low humidity air is commonly encountered in day-to-day life. Because cooler air has a lower capacity to hold water, the volume of water dissolved in air is lower in the cooler months. The use of heating in homes and buildings further reduces the relative humidity of the air. The importance of maintaining some water in the air during the winter months is highlighted by the use of humidifiers in many homes and buildings. Another classic example is air travel, as the air circulated in aircraft is known to have a low humidity. This low humidity air is often cited as a reason for the development of many symptoms (e.g., dry skin, headache, etc.), which are often considered hallmarks of hypohydration, during and following long-haul flights. 

The challenge of subtle loses in body water are that they occur gradually and are not directly sensed, as opposed to body water that is lost due to sweating and/or urination. That said, the response of the body to the resulting hypohydration is the same no matter how it is manifested. When body water is low, several processes are triggered that act to increase the sensation of thirst (to stimulate drinking) and cause the kidneys to reduce urine production and increase urine concentration. These responses, amongst others, can have short- and long- term health and performance consequences, as outlined in previous blog posts. Specific to the cooler months, however, is that dry air occurs at the same time as when respiratory viruses are prevalent in the population. Notably, some evidence supports that breathing dry air may increase the susceptibility of the lungs to viral infection. Whether preventing hypohydration modifies this relation remains poorly understood. Nevertheless, it is clear that hypohydration should be avoided and that the challenge of maintaining body water is increased during exposure to dry air (e.g., air travel, cooler months, etc.). Thus, to ensure adequate hydration consider following these helpful tips.

   - Ensure easy access to drinkable fluids.
      o  Water is generally fine, but if exposed to low humidity air for a prolonged period of time without eating consider fluids with electrolytes.
      o  Consider drinking something with flavoring if you do not like the taste of water but be sure that it does not contain too much sugar (e.g., avoid soft drinks).
      o  Some drinks marketed as hydration beverages may improve fluid absorption and better help maintain body water.
   - Remember to drink early and often – you should drink before you get too thirsty
   - Monitor urine volume and color to estimate hydration status – if the need to urinate is infrequent and the urine is dark, drink more.