Long-term health outcomes associated with hydration status

Body water balance is determined by fundamental homeostatic mechanisms that maintain stable volume, osmolality and the composition of extracellular and intracellular fluids. Water balance is maintained by multiple mechanisms that continuously match water losses through urine, the skin, the gastrointestinal tract and respiration with water gains achieved through drinking, eating and metabolic water production. Hydration status is determined by the state of the water balance. Underhydration occurs when a decrease in body water availability, due to high losses or low gains, stimulates adaptive responses within the water balance network that are aimed at decreasing losses and increasing gains. This stimulation is also accompanied by cardiovascular adjustments. Epidemiological and experimental studies have linked markers of low fluid intake and underhydration - such as increased plasma concentration of vasopressin and sodium, as well as elevated urine osmolality - with an increased risk of new-onset chronic diseases, accelerated aging and premature mortality, suggesting that persistent activation of adaptive responses may be detrimental to long-term health outcomes. The causative nature of these associations is currently being tested in interventional trials. Understanding of the physiological responses to underhydration may help to identify possible mechanisms that underlie potential adverse, long-term effects of underhydration and inform future research to develop preventative and treatment approaches to the optimization of hydration status. A number of epidemiological studies have linked markers of underhydration with an increased risk of adverse health outcomes. This Review provides an overview of the mechanisms that regulate body water balance and the adaptive responses to decreased hydration, and describes current understanding of the physiological consequences of underhydration on health outcomes. A growing number of epidemiological studies have linked markers of underhydration, such as elevated plasma vasopressin, sodium at the upper end of the normal range, low urine volume and high urine osmolality, with an increased risk of adverse health outcomes such as the future development of chronic diseases and premature mortality.Worldwide population surveys estimate that more than 50% of people drink less than recommended, indicating that consistent proper hydration may be beneficial for many individuals.Chronic underhydration results in a new steady state of water balance with constantly activated water conservation mechanisms accompanied by an accumulation of organic osmolytes and persistent vasopressin secretion.Adaptive changes to underhydration affect multiple physiological systems and may promote their deterioration.Interventional trials with increased water intake are needed to prove causality and facilitate implementation of optimal fluid intake recommendations into general clinical practice.