Water balance is one of the most important functions of the body. The cells in the body require a stable environment to survive and work like they should. As originally put by the 19th century French physiologist Claude BernardMD: “La fixité du milieu intérieur est la condition de la vie libre, independante” (The fixedness of the inner environment is the condition of free, independent life). American physiologist Walter B. Cannon, MD, called the process of maintaining this environment in the body “homeostasis,” which is the basis for the study of physiology.
All creatures large and small must protect themselves from dehydration. Lower animals such as worms and water-living species such as frogs and lungfish hibernate in a special way to survive the dry season. This is called “estivation”—or summer torpor—from the Latin for “summer” (aestas).
Physiologically, estivation includes the production of organic compounds, most importantly urea, that increase urine concentration to conserve water. Animals in estivation also experience reduced blood flow to the skin that reduces water loss through the skin. Studies have recently shown that even mammals show similar responses.
The increase of urea production is an important part of how the body responds to estivation. However, increasing urea requires the breakdown of protein, which leads to muscle loss, also called muscle wasting. Muscle wasting is a condition often seen with aging or in disease, particularly critical illness where people require intensive care. Loss of muscle can lead to impaired metabolism, mobility problems and loss of independence after intensive care.
My research group recently measured the increase of amino acids in the blood of people with more severe dehydration. This was directly correlated to the amount of urea in the blood in critically ill patients with COVID-19. These results showed not only that dehydration is associated with increased urea synthesis, but that it is actually associated with muscle wasting and symptoms such as fatigue and weakness.
Some of the underlying causes of muscle wasting are still not well understood. However, with studies such as ours pointing to dehydration as a possible factor, scientists continue to learn more about how to prevent this potentially dangerous condition.
Michael Hultström, MD, PhD, is a critical care anesthesiologist and associate professor of physiology at Uppsala University in Sweden. His research group works with circulatory function and the development of acute and chronic kidney damage both in experimental models and in patients. During the pandemic, Hultström’s research group has worked extensively with critically ill COVID-19 patients. He was featured in the January 2023 issue of The Physiologist Magazine for this work.