Hydration as an important modifier of disease risk
Zachary J. Schlader, PhD, FACSM
Representing 40-60% of body mass, most of the human body is water. The body is constantly balancing fluid intake and fluid output to maintain adequate levels of body water. This is important because the body can produce only very small amounts of water (due to metabolism) and has a limited capacity to store water. Given this latter point, a state of being overhydrated is relatively rare. Conversely, being underhydrated (i.e., hypohydrated) is comparatively common, as this situation occurs when fluid intake, due to drinking, is less than fluid output, due to mostly to urine, sweating, fecal and respiratory water losses. In the short term, the evidence clearly supports that preventing hypohydration is essential for health and is necessary for optimal functioning of bodily systems. This is often manifested as impairments in both physical and mental functioning. Strikingly, recent evidence indicates that maintaining adequate hydration habits may even have more long-term health effects.
Due to robust regulatory mechanisms body water is generally well maintained across a wide array of daily fluid intakes. This is possible because of the kidneys tremendous ability to conserve body water. There is no water sensor in the human body. Rather, our kidneys use the concentration of particles dissolved in the blood (termed osmolality) and alterations in blood pressure to get an idea of the volume of water in the body. When osmolality is high and/or blood pressure is low, several processes are triggered that act to increase the sensation of thirst (to stimulate drinking) and stimulate the kidneys to reduce urine production and increase urine concentration. When these processes are active over a long period of time, this can produce negative health consequences particularly as it relates to the kidneys and the urinary tract. For example, increasing fluid intake may help prevent the development of kidney stones and urinary tract infection. This is likely because increasing fluid intake results in lower urine concentration and increases urine production. Reducing urine concentration may be important is preventing crystal formation in the kidneys, while increasing urine production (and urine flow rate) encourages the flushing of the urinary tract that is likely helpful for both kidneys stones and urinary tract infection.
The physiological responses to low body water (i.e., elevations in blood osmolality) are partially due to the release of the hormone arginine vasopressin (or simply vasopressin), which acts on receptors located in the collecting ducts in the kidneys. When blood osmolality is high, blood levels of vasopressin are elevated, whereas when blood osmolality is normal (or low) blood levels of vasopressin are reduced. Importantly, sustained elevations in circulating vasopressin have been associated with an increased risk of some metabolic and kidney diseases. The increased risk of metabolic disease, such as type 2 diabetes, is likely due to vasopressin’s role in modulating glucose regulating hormones, whereas the links between vasopressin and kidney disease risk is likely tied to the higher likelihood of long-term kidney function declines when water intake is low and vasopressin concentrations are high.
Collectively, ideal levels of body water, which requires optimal fluid intake, positively impacts both short- and long-term health and wellbeing. Thus, drink up for your health!