ASPECTS OF DIETARY PREVENTION AND TREATME NT OF CALCIUM-OXALATE NEPHROLITHIASIS

Urinary oxalate, which is derived from endogenous production and from dietary oxalate absorption, is a much more important determinant of ca lcium oxalate supersaturation and calcium oxalate nephrolithiasis than is calcium. Dietary supplements of ascorbate, which is reported to be the main endogenous precursor of oxalate, accounting for 35-50 % of u rinary oxalate, did not increase the daily rate of urinary oxalate, wh en added orally or intravenously in healthy volunteers or recurrent st one formers for one or five days with a maximum of 5 g t.i.d. Plenty o f data in the literature, which indicated that hi-her dietary vitamin C intakes increase the urinary oxalate excretion are a result of an in -vitro oxalogenesis, which can be excluded by appropriate methods. Oxa lic acid is mainly absorbed in small and large bowel by a mechanism of simple passive. nonenergy-dependent, nonsaturable diffusion, which me ans, that oxalate uptake may increase linearly with raising oxalate co ncentrations. An oxalate-loading test with 880 mg sodium oxalate in 40 0 ml distilled water after overnight fasting with no food intake for t he next two hours discriminates between apparently healthy volunteers and patients with hyperoxaluria, the threshold measured at greater-tha n-or-equal-to 125 mg urinary oxalic acid/24 h/1.73m2. When the loading test was not performed with fasting probands, but was coupled with a standardized breakfast, intestinal absorption of oxalic acid was reduc ed to more than 50 %. This finding is of special importance for dietar y advice in idiopathic oxalate stone formers. who should abstain from intermediate isolated oxalate-rich food items like chocolate. Thus, th e ingestion of 100-gram chocolate, which contained 158 mg oxalic acid, eaten as an intermediate meal in the afternoon, led to a significant mean increase of about 10 mg or 34.3 % in urinary oxalic acid excretio n, when compared with the corresponding values of the nonsupplemented diet. Therapeutic indications for a high dietary fiber intake in lithi asic patients with absorptive hypercalciuria may increase intestinal o xalic acid absorption in response to their contents in phytic and uron ic acids. Contradictory to dietary supplements of wheat bran, suppleme nts of oat bran and citruspectin did not cause a significant increase of urinary oxalate excretion.