No one doubts that proper hydration is necessary for optimal functioning. But a debate has raged in recent years about how to determine when you need to drink, and there are at least 13 hydration-assessment methods. The result is more confusion than clarity.
Two camps have received much publicity in running circles. On one side, precision-minded scientists advise you to weigh yourself before and after workouts to determine your sweat loss, and hence your hydration need. On the other side are those who simply say “let thirst be your guide.”
A new paper from a team of heat-and-hydration experts at the University of Connecticut concludes that morning thirst is an accurate indicator of 2% dehydration, which is considered the point at which endurance performance begins to deteriorate. “This is the first study to show that thirst is reliable dehydration guide, at least in the morning, upon waking,” head researcher Lawrence Armstrong, Ph.D., told Runner’s World.
It’s important to note that the study was not an exercise study. The 29 subjects were all young, college-age men who worked out several times a week, but were not training for any specific purpose. They were examined twice: one morning in their normal hydrated state, and another morning after a 19-hour period in which they were instructed to eat their normal foods but to drink nothing. Both mornings they were asked to complete a “dehydration scale” from 1 to 9, with higher numbers indicating more thirst.
When the subjects didn’t drink, they lost about 2% of their normal weight. It also caused subjects to rate their thirst as a “7,” equivalent to “very thirsty.” On mornings when they were normally hydrated, they chose “2” as their thirst rating.
Armstrong, a lifelong runner, says that runners can use morning thirst along with urine colour and daily body weight fluctuations to monitor their hydration status.
“Urine colour should be checked every time a runner voids,” advises Armstrong. “You can do it multiple times a day. Sensing your morning thirst when you wake up gives you another tool.” (Click here for a urine colour chart to gauge your dehydration.)
The research team also measured urine volume of the hydrated or dehydrated runners after they drank varying amounts of water. The dehydrated runners always produced less volume.