Recent study findings suggest that drinking water during the night may be particularly beneficial.


SAN FRANCISCO—By drinking more water, especially during the night, calcium stone formers may reduce their stone risk, according to a study presented here at Renal Week 2007. 

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The finding is based on the first study to look at urine supersaturation with calcium oxalate (CaOx) and calcium phosphate (CaP) in calcium stone formers using urine samples collected at various time points over a 24-hour period.


Investigators at the University of Chicago collected urine samples at 14 hourly intervals during the day while subjects were fasting and after they ate meals. They also collected a 10-hour overnight specimen. Most urine studies have been conducted using 24-hour urine collections.


“Supersaturation appears to be the greatest risk factor and the greatest differentiator between stone formers and normal [individuals],” said investigator Kristin Bergsland, PhD, a research associate and an assistant professor of medicine. “We also found that supersaturation seems to peak during the overnight period.  Drinking more water results in reduced saturations, so that could potentially be therapeutic for many stone formers.”                                                               


Calcium stone formers (CaSF) have higher urine supersaturation (SS) with CaOx and CaP than healthy individuals, Dr. Bergsland explained. Stone formation is opposed by inhibitors of crystallization. One measure of inhibition is the distance between urinary SS and the upper limit of metastability (ULM), which is the supersaturation point at which crystallization occurs. She and her colleagues have previously shown in 24-hour urine specimens that ULM varies directly with SS in both SF and normal controls. ULM is lower in relation to SS among SF compared with controls, however.


The study, which was led by Fredric Coe, MD, and Elaine Worcester, MD, involved 16 idiopathic CaSF and 14 normal controls. Urine samples were collected at one-hour intervals while volunteers were fasting and after they ate three meals with known amounts of calcium, phosphorus, sodium, protein, and calories. The investigators grouped their findings according to fasting, postprandial (PP), and overnight periods.


During each period, SS CaOx was significantly higher in SF compared with controls. The same was true for SS CaP during the postprandial and overnight periods; maximum SS for both CaOx and CaP converged overnight. In addition, the ULM for both CaOx and CaP exceeded SS and tracked SS closely. The ULM-SS difference can be interpreted as residual inhibitory capacity. When ULM-SS is near zero, crystallization is imminent, Dr. Bergsland said. ULM-SS CaP was lower during the postprandial period compared with fasting or overnight among both SF and normals, suggesting that the risk of CaP crystallization is likely to increase after eating, Dr. Bergsland commented.


Unexpectedly, the researchers found that throughout the day the mean ULM-SS for both CaOx and CaP tended to be higher for SF than for controls, indicating greater protection against crystallization in SF. This was statistically significant during the postprandial period. The ULM-SS CaOx was 32.1 in SF versus 28.1 for controls; the ULM-SS CaP was 2.7 in SF versus 2.1 for controls. The findings contradict those of previous 24-hour urine studies showing lower ULM in relation to SS among SF versus normals, but the difference may be due to use of urine samples freshly collected and studied promptly. This may more closely approximate conditions in the



ULM-SS distance may not be a useful gauge of stone-forming propensity because by this measure stone formers do not appear to be at higher risk than normal individuals for CaOx or CaP crystallization. This work favors the view that high SS itself (especially CaP SS) most differentiates SF from normal individuals throughout the day and night and is, at present, the more appropriate clinical measurement. Whether nocturnal hydration would be especially effective in stone prevention remains an open question and further studies are needed to confirm these findings.