Calcium, uric acid excretion reduced by desert exposure

San Diego-Exposure to a desert environment increases lithogenic risk, but the effect is the result of unanticipated changes in excretion of lithogenic material, according to a recent study.

To investigate how living in a desert climate influences stone risk, researchers from the Naval Medical Center San Diego department of urology studied a cohort of 50 young adult infantrymen (ages 19 to 25 years) in the U.S. Marines who were being deployed from a temperate environment at their southern California base to the Mojave Desert for a 1-month, physically intense exercise. A 24-hour urine collection was obtained prior to deployment, 4 weeks into the deployment, and 2 weeks after the men returned to their base.

Analyses of the urine collections showed exposure to the desert environment was associated with an expected extreme reduction in urine volume from 1.29 L prior to deployment to 0.46 L at 4 weeks into the exercise. However, there was also a counterintuitive decrease in urinary excretion of calcium, uric acid, sodium, phosphorous, potassium, and magnesium, while urinary citrate, oxalate, and pH were unchanged. Nevertheless, urinary relative supersaturation for sodium urate, calcium urate, and uric acid increased, which was attributed to increased concentration of the lithogenic minerals. Uric acid crystals were only noted in two specimens: one collected before deployment and one collected after the return.

“Living in a desert environment is a known risk factor for stone disease, but the effect of transient relocation to the desert from a more temperate environment has not been well studied. We were surprised by the decrease in urinary excretion of minerals and electrolytes. However, the data on relative supersaturation is consistent with the link between desert exposure and stone disease,” said first author LCDR James H. Masterson, MD, of the findings, which were presented at the 2012 AUA annual meeting in Atlanta and recently published online in the Journal of Urology (Nov. 19, 2012).

“We believe it may have been because of the intense physical training in combat gear causing extreme diaphoresis. It appears the urine was super concentrated, and this fact makes it difficult to treat stone disease because the Marines were already drinking 17 L of water daily. It would be difficult to ask them to drink more to increase urinary output,” said Dr. Masterson, a fourth-year urology resident at the Naval Medical Center San Diego, who worked on the study with CDR Brian K. Auge, MD, and colleagues.

The authors speculated that the decrease in urinary calcium excretion, which is contrary to some previous desert urinary studies, might partially be the result of reductions in vitamin D levels as the men were wearing full combat gear and were protected from the sun.

“In a previous study involving Israeli lifeguards and British troops in Bahrain, sunlight exposure was shown to increase urinary calcium excretion. However, our study is consistent with a study in Iran that found that the women, whose skin is fully covered by their clothing, had low serum calcium levels,” Dr. Masterson noted.

Decrease in uric acid excretion surprising

The decrease in uric acid excretion was also particularly surprising. A possible explanation for this finding might relate to information suggesting that uric acid has activity as a free-radical scavenger.

“Although generally considered a waste product, uric acid may also serve a physiologically necessary role. Perhaps it was conserved by the body in the extreme desert environment for that purpose,” Dr. Masterson said.

Extrapolations from the study must be made carefully, considering the population was a group of young, healthy men, of whom two had a family history of stones, but none had a personal history. The fact that urinary citrate was unchanged suggests that citrate supplementation is not a viable strategy for minimizing stone risk among persons exposed to a desert environment. As mentioned, neither does increasing fluid consumption seem to be a tenable solution for this group considering that the men were already drinking 17 L per day on average along with electrolyte-enforced drinks with meals.

Dr. Masterson noted that he and his co-authors are investigating strategies to incorporate this data into stone prevention methods for service members deployed to desert environments.

The views expressed in this study are those of the authors and do not reflect the official policy or position of the Department of the Navy, Department of Defense, or the U.S. government.