Table 1.

The relative effects of thirst, drinking, and physiological responses on fluid-electrolyte balance during ordinary daily activities and endurance exercise.

Activity	Thirst & Drinking Behavior	Sweat Gland Secretion of Hypotonic Fluid	Kidney Regulation of Water & Electrolytes	Neuroendocrine Homeostatic Responses a	Effects on Water & Electrolyte Balance
Sedentary daily activities (16 h)	Basal b	Negligible	Basal b	Basal b	CNS responses are sufficient to maintain water and electrolyte homeostasis
Brief exercise (5–30 min) at moderate-to-high intensity	Minor	Minor-to-moderate	Minor	Minor, brief	Water and electrolyte losses are minor
Endurance exercise (0.5–5 h) at low-to-high intensity	Minor-to-large	Moderate-to-large	Minor-to-moderate	Minor-to-large, prolonged	Moderate-to-large turnover c due to sweating and drinking
Ultraendurance exercise (5–24 h) at low-to-moderate intensity	Moderate-to-large	Large	Moderate-to-large	Large, prolonged	Water and electrolyte losses in sweat and urine exceed 24 h dietary intake

CNS, central nervous system (i.e., brain and spinal cord); TBW, total body water. ^a, CNS effects involving nerves and hormones that regulate whole-body water volume and concentration, blood volume/pressure/osmolality, and thirst (see [9] for a review of this topic). ^b, a standard low level maintenance of whole-body fluid-electrolyte balance with small turnover (intake versus loss) and minor perturbations. ^c, turnover refers to the sum of gains and losses of water and electrolytes.