The efficacy of saliva osmolality as an index of hydration state: is it worth the spit?

Water represents ~60% of the body mass, ranging from 40-80% across individuals. In sedentary people, about 5-10% of this water is turned over daily, with urine flows averaging ~1.5 L.day-1 for normally hydrated individuals. However, when working in stressful environments for extended durations, particularly when wearing personal protective equipment, fluid losses via sweat secretion are dramatically elevated, and can approach 8-16 L.day-1. Indeed, during a moderate exercise-heat stress, whole-body sweat rates typically range between 1-1.5 L.h-1, and sustained and undefended fluid loss at this rate significantly impacts upon body water content, resulting in a 1% body-water loss for every 30-40 min of steady-state sweating without fluid replacement.