Effects of the submarine environment on renal-stone risk factors and vitamin D metabolism.

The effects of total sunlight deprivation on urinary risk factors for nephrolithiasis and vitamin D metabolism were studied in 20 healthy male subjects. Blood and 24-h urine samples were collected before submarine deployment and 68 days later while still at sea. No subject received sunlight exposure during the test interval. Significant decreases in daily urinary excretion of calcium, uric acid, sodium, sulfate, and phosphorus were found. The relative supersaturation ratio of monosodium urate also fell. There was no change in urinary citrate or urine volume. Mean serum levels of 25-hydroxyvitamin D [25(OH)D] declined from 31 to 19 pg/ml (P < 0.0001), parathyroid hormone increased from 22 to 30 pg/ml (P < 0.0001), and osteocalcin (GLA) increased from 2.7 to 3.3 ng/ml (P = 0.005). Mean serum levels of 1,25 dihydroxy-vitamin D were unchanged. Four subjects had 25(OH)D levels below 10 ng/ml by the end of the submarine patrol. These findings suggest that exposure to the submarine environment produces physiologic changes that decrease the risk for renal stone formation. The data are consistent with the role of vitamin D metabolism in sunlight deprivation and demonstrate that compensatory mechanisms are well established within 68 days.