By Roberta H. Anding, M.S., Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley, The Great Courses Daily

Conventional wisdom says we must drink 64 ounces of water each day to be healthy. Is that really true, though? Professor Anding challenges this myth as she lays out the science of hydration.

Calculating Water Needs

How much water do we need? Back in school, we all memorized that we need to drink eight glasses of eight-ounces of water per day. 

If you think about it, though, we are not all identical twins, nor do we all have the same amount of muscle and fat. Although eight glasses a day might be a general public health recommendation, we are unique when it comes to our fluid needs. For some of us, 64 ounces of water a day isn’t going to touch our fluid needs, and for others, it’s going to overestimate our needs.

“I was working with a professional football player, and through our series of discussions, he was really struggling with some performance issues on the field,” Professor Anding said. “It wasn’t his talent, and it wasn’t his coaching. I live in Houston, Texas. It was the heat and humidity.”

His actual, calculated fluid needs after a game, or after a practice, to get him rehydrated were up to 240 ounces of fluid. If he only drank eight glasses, or 64 ounces, of water per day, he would be significantly dehydrated. 

“In fact, I would argue he would end up in the emergency room for heat-related illness,” Professor Anding said.

Fluid and Body Composition

Our bodies are 65 percent to 70 percent water, plus or minus. It is less if you have more body fat. Body fat is what is known as anhydrous, meaning it has very little stored water. Maybe less than 10 percent of a pound of body fat is going to be water. 

This is not the case with muscle. Muscle can be 70 percent water, according to estimates. 

As the typical American ages, he or she gains body fat and loses muscle, and that changes the amount of body water. You may think that’s not a significant issue, but it can be if you become a hospitalized patient or if someone is trying to calculate intravenous fluid requirements for you if you need an IV for fluid restoration. 

Dehydration can cause issues such as constipation, heat stroke, kidney stones, seizures, sudden drops in blood pressure, and urinary tract infections. Therefore, it is important for older individuals, in particular, to monitor their fluid intake and stay hydrated.

This changing body composition does have implications, then, when it comes to fluid intake. Keep in mind, though, that this shift in body composition is not an inevitable consequence of aging. It can be prevented by resistance or strength training, which will build muscle mass while reducing body fat.

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.