The Influence of Fluid Intake on Daily Biological Rhythm and Mental Performance in Healthy Young Adults

Chronic low water intake may raise the risk of morbidity and mortality by influencing key water regulating hormones (e.g., AVP), which are known to modulate glucoregulation and renal function. For example, AVP stimulates the HPA axis to release the glucocorticoid stress hormone cortisol with potentially far-reaching effects on metabolism, immunity and inflammation. One study observed elevated blood cortisol in a group of low water drinkers, albeit cortisol was measured at one time of day only. However, in the field of psychobiology, researchers have traditionally related more dynamic assessments of cortisol with health outcomes; by evoking cortisol responses to acute standardised laboratory stressors, such as The Trier social stress test. More recently, researchers have appreciated the importance of circadian variability in cortisol levels, by examining influences on, and consequences of individual differences in the diurnal variation of cortisol. The major measurable parameters of the diurnal variation are; the cortisol awakening response (CAR), which is the rise in cortisol during the first 30-45 minutes following awakening, and the diurnal cortisol slope, which is the rate of decline in cortisol levels across the day, from morning to evening. These parameters are considered to reflect different aspects of HPA axis function; with the CAR best reflecting the adrenal capacity to respond to stress and awakening and diurnal slope more indicative of daily cortisol exposure. Although distinct, both blunted CAR and a flattened diurnal cortisol slope appear to be consistent markers of HPA axis dysfunction and related to a variety of poor health outcomes. Therefore, it has been recommended that contemporary research should simultaneously estimate an individual's awakening cortisol responsiveness, and diurnal slope, thereby capturing distinct and important components of HPA axis function. The shared pathways that regulate body water, diurnal variation in cortisol and our re