Increased Incidence of Nephrolithiasis (N) in Lifeguards (LG) in Israel

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Eleven of 45 (24%) LG had proven N. This is approximately twenty times the incidence of N in the general population. The present study was undertaken to determine the factors that contribute to this high incidence of N in LG.

On the job LG are exposed to heat and intense sunlight over almost their entire body surface, for at least 8 hr/d, 6 mo/yr. In an attempt to study the influence of these conditions on vitamin D and calcium metabolism the following were measured: mean serum 25-hydroxycholecalciferol (25-HCC) of the LG of 59±42 (SD) ng/ml (n=34) was greater than the mean of 26±10 in season and age matched controls (n=25, p< .01). Mean serum iPTH of 421±234 pg/ml in LG (n=33) was lower than mean of 566±175 in matched controls (n=50, p< .01). Mean urinary calcium of the LG of 308±45 mg/24 hr was greater than mean control of 168±87 mg/24 hr (n=20, p< 0.001). Mean urinary Mg/Ca ratio (mEq/24 hr) of those LG who had N was lower than control values (72.3±33.6 vs. 125±63, p<0.001). This was due to increase in calciuria rather than to reduced magnesuria in LG. Daily urine volumes of the 11 LG who worked in the warm and arid Gulf of Eilat had mean daily urinary volumes of 856±270 ml/24 hr, which was lower than in controls and lower than in LG and in controls from Northern Israel (p<0.01). Levels of total serum calcium, magnesium, and phosphate in LG were within normal limits. There was a slight but statistically significant hyperuricemia in the LG.

The data show that LG have: a) enhanced incidence of N; b) increase in serum 25-HCC, and decrease in serum iPTH, and c) hypercalciuria. It is postulated that excessive exposure to sunlight induced a state of hypervitaminosis D in LG. This led to hyperabsorption of calcium from the gut, which partially suppressed the parathyroid glands. Each of these factors contributed to the hypercalciuria and to the high incidence of N in LG. Relative oliguria in certain instances, and the low urinary Mg/Ca ratio, as well as the hyperuricemia were additional independent, potentially lithogenic factors in LG.