Article – Science Magazine – What is Considered Normal Hydration May not Be Enough

A recent article in Science showed that there was a wide variation in drinking habits in the world, and made the erroneous conclusion that this means that we can vary our hydration and do just fine. However, in response, a number of experts on hydration (including yours truly) pointed out that what is normal intake does not mean it is healthy intake (see link). Indeed, as many people may have heard, last week an article from the National Institute of Health reported that people whose hydration status is “in the normal range” as noted by measuring serum sodium (a marker of hydration status and salt intake) but on the higher end of normal are at marked risk for developing obesity, diabetes, accelerated aging, and increased mortality. Everyone, keep well hydrated!

Link: https://www.science.org/doi/10.1126/science.abm8668

Letter to the Editor

Water Intake: What is Normal may not be What is Healthy

Yamada et al report daily water turnover (measured by double-labeled water) averaged about 4.3 L in young adult men and 3.4 L in young adult women, but with substantial variations, being higher in people living in hot climates, those with high lean body mass, and those who are pregnant, while lower in people with obesity and the elderly (1). While these data aid our understanding of normal water intake and turnover, the authors suggest this information means that drinking water guidelines should vary, and that the “common suggestion that we should drink eight 8-ounce glasses of water per day (~2 liters) is not backed up by objective evidence.”

Certainly water needs will vary among individuals, especially related to climate and exercise, but while these data may show what is normal water intake, this does not necessarily equate with what is healthy water intake. The majority of the population are “underhydrated” in which they maintain normal serum osmolality by raising vasopressin levels as opposed to drinking enough water (2, 3). Most obese people show signs of dehydration (by bioimpedance or urine osmolality) and elevated vasopressin levels (4). In turn, vasopressin has been shown to drive obesity and metabolic syndrome via its stimulation of the V1b receptor, and blocking vasopressin stimulation in animals with increased hydration can reverse hyperinsulinemia and stabilize or lower body fat (5). Underhydration is associated with increased risk for mortality (3), and even individuals with high normal serum sodium levels are at increased risk for cardiovascular disease (6). Indeed, Yamada, observed an association of obesity with lower water turnover (1).

While the data by Yamada et al are important, we recommend clinical trials to determine the optimal hydration to prevent and treat obesity and metabolic diseases.

References

Y. Yamada et al., Variation in human water turnover associated with environmental and lifestyle factors. Science 378, 909-915 (2022).
R. J. Johnson et al., Current Hydration Habits: The Disregarded Factor for the Development of Renal and Cardiometabolic Diseases. Nutrients 14, (2022).
J. D. Stookey, S. Kavouras, H. Suh, F. Lang, Underhydration Is Associated with Obesity, Chronic Diseases, and Death Within 3 to 6 Years in the U.S. Population Aged 51-70 Years. Nutrients 12, (2020).
J. D. Stookey, D. Barclay, A. Arieff, B. M. Popkin, The altered fluid distribution in obesity may reflect plasma hypertonicity. Eur J Clin Nutr 61, 190-199 (2007).
A. Andres-Hernando et al., Vasopressin mediates fructose-induced metabolic syndrome by activating the V1b receptor. JCI Insight 6, (2021).
S. Gao, X. Cui, X. Wang, M. B. Burg, N. I. Dmitrieva, Cross-Sectional Positive Association of Serum Lipids and Blood Pressure With Serum Sodium Within the Normal Reference Range of 135-145 mmol/L. Arterioscler Thromb Vasc Biol 37, 598-606 (2017).