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

Edited by Kate Findley, The Great Courses Daily

You probably know that if you go for a run on a hot summer day, you will need to drink more water. We can become dehydrated without even being aware of it, though. Professor Anding explains.

Dehydration and Humidity

Maintaining adequate hydration levels is essential for our health, as water not only satisfies our thirst but is responsible for the proper functioning of our internal systems. Staying hydrated is not as simple as drinking eight glasses of water a day, though. Many factors influence dehydration, including the environment you live in.

“I live in Houston, Texas, where in the summertime it’s 95 degrees and 95 percent humidity,” Professor Anding said. “In this case, my body’s going to produce more sweat to cool me off, but the really bad news is, when the humidity is that high, the sweat doesn’t evaporate, and in order to cool you, sweat has to dry off of your skin.”

If you live in an area that has low relative humidity, this may not be an issue for you. In fact, when you sweat, you might actually see salt crystals on your skin, and your skin might feel a little gritty. That’s not the case in an area with high relative humidity. 

When you sweat more, you’re not cooling yourself off because you’re not able to evaporate that sweat loss. Your body has only one response, and that is to produce more sweat. It’s not sensing that the sweat that’s being produced is not effective in dissipating the heat, so it’s going to produce more and more sweat.

If you live in a cold environment, oftentimes the humidity in your home is relatively low. You can sense because your skin and lips get very dry, and your mouth dries out quickly. You might not think you need to drink as much because you don’t have that same sensation of dehydration that you get in hot weather. 

However, the cold can still dehydrate you, even though you don’t have the environmental trigger. You can’t feel sweat on your skin, but it doesn’t mean that you’re not losing moisture through your skin in a different way.

Dehydrating Beverages and Medications

Contrary to popular belief, caffeinated beverages such as coffee or tea do not cause significant dehydration as long as you drink them in moderation. If you have acclimated to that level of caffeine, you don’t lose a significant amount of fluid. 

However, energy drinks often have three to four times the amount of caffeine that’s found in coffee and tea, which can have a significant dehydrating effect. Additionally, energy drinks have synthetically added vitamins such as taurine and B vitamins. 

Coffee and tea, by contrast, have actual nutrients in them such as phytochemicals and antioxidants. Because the caffeine in these beverages is wrapped in whole food, it behaves differently than the caffeine in an energy drink.

Drinking excessive amounts of alcohol can also cause dehydration, as alcohol stimulates the body to produce more urine. Finally, medications and disease states can cause intentional or accidental dehydration.

For example, laxatives promote body water loss through frequent bowel movements, while many blood pressure medications cause dehydration as they’re intended to reduce the amount of fluid in your blood vessels. 

Travel Leads to Thirst

Airplane travel can really contribute to excess dehydration. Your breath is humidified at 100 percent, and when you get on a plane, it’s essentially zero percent humidity in a pressurized cabin. 

You’re not sweating or exercising, but just that act of breathing can increase the amount of fluid losses that you have. If you do international travel, flight attendants will offer you water more often. 

You’re not exercising and you don’t feel any sweat on your skin, but the fluid loss that’s coming in your breath is significant. Some reports suggest that when the plane lands, the relative humidity in the cabin is about 40 percent, and that’s all been put into the air by passengers’ breaths.

Therefore, although you should definitely drink more water after a strenuous workout or a salty meal, it’s important not to overlook other causes of dehydration. Stock up on extra water on a cold day or a long flight—even if you don’t feel like you need it. 

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.