The hydration rule that's worked for 400 million years
Despite popular belief, there's no widely agreed upon benchmark for daily water consumption. That "drink eight 8-ounce glasses a day" thing likely derives from a 1945 recommendation by the U.S. Food and Nutrition Board. The recommendation stated people should ingest 1 milliliter of water for every calorie they consume. The recommendation was apparently not based on any known research, and the sentence immediately following it, which was that "most of this quantity is contained in prepared foods," was soon forgotten. The U.S. Food and Drug Administration began using 2,000 calories a day as an "average" reference point, which translates to 2,000 milliliters of water a day (or about 67 fluid ounces) per the 1945 recommendation. "The origins of that weren't based on any specific series (of studies) or one specific scientific study," says Tamara Hew-Butler, associate professor of Exercise and Sports Science at Wayne State University whose expertise is in exercise-associated hyponatremia. "And that number actually included other beverages, not just water, and it also included the moisture content in food. But from those origins, it caught on." Hew-Butler refers to our level of thirst as a natural "app" hooked up to our brain designed to protect us from drinking too much or too little water. "Everyone's looking for a perfect guideline that works for every individual in every situation. But thirst is governed in your brain. And it's not just the amount of water that's important to your body—your body doesn't really care about the absolute amount of water. The body cares about the balance between the water and sodium," Hew-Butler says. "There are osmoreceptors in our brain that actually, in realtime, every second, sample the combination between water and salt going through the circulation of our brain. So if the water content goes down and it increases the concentration of sodium, then that triggers thirst, so you can actually replace the water back to the amount that you need."
December 9, 2019