In a recent review article published in the journal Nature Reviews Nephrology, researchers synthesized what is known about the long-term consequences of being chronically underhydrated.
Most people know hydration is vital to staying healthy, but surveys suggest that underhydration is a common problem affecting nearly half of surveyed adults in the United States and Europe. Experts believe it to be exacerbated by the lack of inclusion in health guidelines and reinforcement by doctors during preventive check-ups.
While the effects of acute dehydration in the short term are widely known, less clinical focus has been given to the long-term effects of chronic underhydration.
Epidemiological studies now suggest that even mild but chronic underhydration can lead to the development of coronary heart disease, heart failure, diabetes, obesity, kidney function deterioration, premature mortality, and faster aging. These findings have been strengthened by observing mice subjected to chronic water restriction.
Review: Long-term health outcomes associated with hydration status. Image Credit: Love the wind / Shutterstock
Adaptive responses to underhydration
Homeostasis is the state of balance among all the body systems needed for the body to survive and function correctly, and water balance is a key mechanism through which this happens. Osmoregulation, or the control of water and salt balance, is known to provide an osmotically stable environment for many cells.
Underhydration leads to adaptive changes across the body. At all times, water losses through respiration, the gastrointestinal tract, sweat, and urine must be matched by water gains through ingestion of food and liquids.
The body needs to maintain a stable composition and volume of intracellular fluids (ICF) and extracellular fluids (ECF); changes in the body’s water content, either because of a surplus or a deficit, affect all organs, tissues, and cells.
One response to a water balance deficit is the release of the antidiuretic hormone arginine vasopressin (AVP), which decreases water loss through excretion. Vasoconstriction, blood pressure stabilization, and increased heart rate occur while water reabsorption is facilitated.
The sensations of thirst and appetite for salt are also stimulated by AVP and angiotensin II (ANGII). However, thirst regulation or anticipatory inhibition operates to prevent overdrinking; thirst responses may also decline with age. Sweating is also attenuated, which can lead to compromised thermoregulatory responses in sweltering conditions and cause chronic kidney disease.
Over time, chronic underhydration may establish a new water balance steady state, matching water losses with water intake. This response may be triggered under conditions where access to water is limited. Over time, this could lead to the deterioration of physiological systems, but these mechanisms are poorly understood.
Diseases associated with underhydration
Researchers consider underhydration or hypohydration to be a moderate decrease in the body’s water levels that leads to water conservation mechanisms being activated so that normal levels of plasma osmolality and sodium can be maintained. This contrasts with dehydration, a state in which the water conservation mechanisms cannot maintain water balance.
Acute water loss is most visible in military personnel and athletes, increasing heart rate and reducing anaerobic exercise performance. Acute dehydration can also adversely affect cognitive functioning, including alertness, concentration, short-term memory, and visual perception.
Certain diseases and conditions are thought to predispose people to underhydration. For example, uncontrolled diabetes mellitus can lead to water loss because it reduces renal water reabsorption. Increased urine output can result from nephrogenic diabetes insipidus and some forms of polycystic kidney disease. Conversely, certain cancers and drugs can also stimulate AVP secretion without low water intake, leading to hyponatremia.
Interventions to increase hydration
In response to emerging evidence regarding the adverse effects of underhydration, researchers have begun to explore whether optimal hydration can prevent or slow metabolic and cardiovascular diseases.
Interventions that focused on regimens designed to increase water intake found that participants faced barriers such as forgetting to drink, lack of access, lack of thirst, dislike for the taste of water, and not appreciating the benefits of proper hydration; another issue was work-related disruptions resulting from an increased frequency of urination.
These challenges were demonstrated by an experiment that found lower adherence to water intake regimens compared to a control liquid of inactive syrup.
One trial that successfully increased hydration in the intervention group found that higher water intake was significantly associated with reduced copeptin levels among people with chronic kidney disease. Another found that the greatest changes were seen in people who were habitually low drinkers.
The literature identifies connections between chronic underhydration markers and a heightened risk of several chronic diseases, but the mechanisms underlying these phenomena are not well understood. There are indications that hydration could be a preventive tool, but causal inference has been limited by the lack of adherence to hydration regimens.
However, even with these limitations, there is evidence that optimal hydration can be beneficial for people who are habitually low drinkers. Future studies on the efficacy of interventions to increase water intake should monitor compliance more closely and continue to focus on chronically underhydrated populations who stand to gain the most.