Body water AND electrolytes - what is ideal hydration?

Zachary J. Schlader, PhD, FACSM

Approximately sixty percent of the human body is composed of water. Ensuring adequate levels of body water is essential for health and necessary for optimal functioning of body systems, such as the cardiovascular, thermoregulatory, and central nervous systems, with the impacts of inadequate body water levels being at the cellular and system levels. Body water is maintained at optimal levels via a process called body fluid regulation, which is mediated by activities that modify fluid intake (i.e., drinking) and fluid output, such as urine and sweat loss and fluid loss due to respiration and feces. When fluid output exceeds fluid intake, such as might occur during exercise in the heat without drinking, body water is reduced. In this state, drinking then acts to raise body water to normal levels. Moreover, when fluid intake exceeds fluid output, such as might occur with overdrinking, the kidneys produce more urine to reduce body water.

Ideal hydration is not only a function of body water but is also dependent on electrolytes, or minerals in the body that have an electric charge. Electrolytes play an important role in body fluid regulation via a process called osmosis, which dictates the movement of fluid between cells and tissues. Notably, the kidneys use osmosis to modify fluid output by controlling the volume and concentration of urine. However, electrolytes also help to ensure optimal functioning of nerves, muscles, the brain and other organs throughout the body. The body can lose electrolytes in sweat, urine, and feces, while electrolytes can be gained through the diet, including both food and drinks.

The body is never at a completely steady level of body water and electrolytes. Rather it is constantly balancing fluid and electrolyte intake with fluid and electrolyte output to maintain body water and electrolytes within normal ranges. Thus, ideal hydration is the maintenance of adequate levels of body water and electrolytes.

Optimal fluid and electrolyte intake is vital to ensuring that body water and electrolytes are maintained within normal levels. Under standard day-to-day activities normal levels of body water and electrolytes can usually be achieved via regular water intake while eating a normal diet. In this instance, fluid intake is accomplished from both food and drink consumption, while adequate electrolyte intake can be accomplished largely through the food. However, during instances in which fluid and electrolyte output may acutely exceed input there is often a need to simultaneously replace fluid and electrolytes. This can typically be accomplished by drinking an electrolyte containing beverage. Importantly, such situations are relatively common. For example, during prolonged intense manual labor in a hot environment, sweat (body water) and electrolyte loss can be profound enough to displace body water and electrolytes levels out of normal ranges. Similarly, severe diarrheal illness can cause a quick and extreme loss of body water and electrolytes. In these instances, amongst many others, it is likely that a drinking an electrolyte containing beverage will promote adequate levels of body water and electrolytes - that is, ideal hydration.