Just add water.

“Just add water” is the modern chime turning various prepared foods into family dinner; however, is it good advice for people at the table as well? In this issue of JASN, Berl1 describes how solute intake affects the kidney’s handling of water. What of the converse: How does intake of water affect kidney function and other physiologic variables? A brief search of the Internet will find multiple web sites warning health-conscious readers they must drink eight glasses of 8 oz/d to remove dangerous “poisons.”2,3 Is there evidence behind these recommendations? Furthermore, if targeted amounts of water consumption are therapeutic, then what are the improved outcomes? It is widely known that humans cannot survive for more than a few days without ingesting water in excess of solutes.4 The dangers of severe hypertonicity and volume depletion are not up for debate. It is also obvious that individuals in hot, dry climates have increased need for water, as do people who engage in strenuous physical exertion.5 There are certainly well-recognized disease states, such as nephrolithiasis, for which increased fluid intake is therapeutic,6 but do average, healthy individuals living in a temperate climate need to drink extra fluid— even when not thirsty—to maintain health? The classic recommendation is known as “8 8”: Eight glasses of 8 oz of liquid per day—not including caffeinated and alcoholic beverages. Where did this recommendation come from? In his exceedingly thorough review of this subject, Valtin7 reached the following conclusion: Nobody really knows. There is no single study—and therefore no single outcome—that has led to these recommendations. Different authors make different claims regarding the potential benefits of water drinking, and it is instructive to examine some of these in turn. First is the notion that increased water intake improves kidney function and clearance of toxins. The kidney manifests several mechanisms to rid the body of toxins, including glomerular filtration, tubular secretion, and various degradative metabolic pathways. If excess water intake were to have an impact on toxin removal, then it would be through one of these mechanisms. Water ingestion can acutely affect GFR, although not necessarily in the direction one might expect. Using 12 young, healthy individuals as their own controls, Anastasio et al.8 found increased water intake actually decreases GFR. It might therefore seem that any “toxin” removed purely by glomerular filtration is cleared less efficiently in the setting of increased water intake; however, it is not certain such changes in GFR persist over time. Indeed, GFR was unchanged during a 6-mo randomized trial of increased water intake in older men who had benign prostatic hypertrophy.9 Of course, the populations in the two studies are different, and the main goal of the randomized trial was to evaluate bladder function rather than kidney function; as an aside, the study did show some improvement in bladder function, although the clinical significance of the findings is unclear.10 Of course, most endogenous substances are not cleared purely by glomerular filtration alone. Anastasio et al.8 found the total clearance of osmoles increased as water intake increased, probably as a result of reduced reabsorption. If there are “dangerous” substances among these osmoles, then increased water intake might indeed help in their clearance. Interestingly, one of the osmoles whose clearance was increased was sodium. Given the suspected role of long-term sodium retention in the development of hypertension,11 one could speculate that increased clearance of sodium is beneficial. Urea clearance also increases with high water intake, but urea is not a toxin. It is unclear whether any of these changes persist in the long term. In short, increased water intake does have some impact on renal clearance of various substances, but current data are insufficient to assess the clinical significance of these observations. In fact, given how little is known about the identity of toxic substances cleared by the kidney, it is unlikely this type of data can conclusively demonstrate a benefit from excess water drinking. Another popular idea found on Internet sites is that ingested water is retained in various organs and improves their function. For this hypothesis to be plausible, one must first show that “normal” individuals who are not thirsty will nevertheless retain ingested water in their body rather than excrete it in the urine. One study of 14 individuals in “good health” suggested that water retention is quite variable and depends significantly on the speed with which water is ingested. A water load ingested over 15 min is largely excreted, whereas a water load ingested over 2.5 h is largely retained.12 In addition, water mixed with a poorly absorbed sugar, thereby slowing absorption of water from the gut, is largely retained, whereas water mixed with an easily absorbed sugar is largely excreted. This pattern is similar in both men and Published online ahead of print. Publication date available at www.jasn.org.