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.

体内水平衡由维持稳定的体积、渗透压以及细胞外液和细胞内液的组成的基本稳态机制决定。水平衡是通过多种机制维持的，这些机制不断地将通过尿液、皮肤、胃肠道和呼吸损失的水与通过饮水、饮食和代谢水产生获得的水相匹配。水合状态由水平衡状态决定。当体内水分由于高损失或低增益而减少时，就会发生水合不足，这会刺激水平衡网络内旨在减少损失和增加增益的适应性反应。这种刺激还伴随着心血管的调整。流行病学和实验研究已将液体摄入量低和水分不足的标志物（例如加压素和钠的血浆浓度增加，以及尿液渗透压升高）与新发慢性疾病、加速衰老和过早死亡的风险增加联系起来，这表明适应性反应的持续激活可能不利于长期健康结果。这些关联的因果性质目前正在干预试验中进行测试。了解对水合不足的生理反应可能有助于确定水合不足潜在不利、长期影响的可能机制，并为未来的研究提供信息，以开发优化水合状态的预防和治疗方法。