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The effects of variable calcium content on daily and fasting urinary calcium and other lithogenic solutes excretion, on the bone turnover index (fasting hydroxyproline urinary excretion) and on the calciotropic hormones were studied in 312 stone former patients with an outpatient protocol and 15 stone former patients in an inpatient study. Furthermore in 60 of these patients, 30 while on a low calcium diet (LCD) and 30 on a free calcium home diet (FCD), the effects of an oral calcium load (OCL) on bone turnover index, calciotropic hormones and calcium excretion were evaluated. The results demonstrate that an LCD is effective in reducing daily calcium excretion. Fasting calcium excretion is apparently not affected by changes in dietary calcium content. On the other hand, LCD induces a marked increase in bone resorption, without apparent signs of increased parathyroid activity. This may explain the failure to reduce fasting urinary calcium excretion by the LCD. The OCL greatly reduced bone resorption rate, without any change in calciotropic hormones, especially in patients on LCD. In conclusion, the LCD induces a reduction in the lithogenic factors in the urine of stone formers, but induces a marked increase in bone resorption. The lack of any change in fasting urinary calcium excretion in conditions of different dietary calcium intake may be due to an opposite change in the intestinal and osseous components that affect this parameter, and is therefore of little value.

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