Intestinal oxalate absorption is higher in idiopathic calcium oxalate stone formers than in healthy controls: measurements with the [(13)C2]oxalate absorption test.

PURPOSE: We assessed the importance of oxalate hyperabsorption for idiopathic calcium oxalate urolithiasis, oxalate absorption in healthy volunteers and recurrent calcium oxalate stone formers was compared. MATERIALS AND METHODS: The [(13)C2]oxalate absorption test, a standardized, radioactivity-free test, was performed. On 2 days 24-hour urine was collected and an identical standard diet containing 800 mg Ca daily was maintained. On the morning of day 2 a capsule containing 0.37 mmol sodium [(13)C2]oxalate was ingested. A total of 120 healthy volunteers (60 women and 60 men) and 120 patients (30 women and 90 men) with idiopathic CaOx urolithiasis (60% or greater CaOx) were tested. RESULTS: Mean intestinal oxalate absorption in the volunteers was 8.0 +/- 4.4%, and in the patients was 10.2 +/- 5.2% (p <0.001). There was no significant difference in mean absorption values between men and women within both groups. A high overlap between the absorption values of volunteers and patients was found. Only in the patient group did absorption values greater than 20% occur. Oxalate absorption correlated with oxalate excretion in the patients, r = 0.529 (p <0.01) and in the volunteers, r = 0.307 (p <0.01). CONCLUSIONS: In high oxalate absorbers dietary oxalate has a significant role in oxalate excretion and, therefore, increases the risk of calcium oxalate stone formation.

Influence of a high-oxalate diet on intestinal oxalate absorption.

Hyperoxaluria is a major risk factor for renal stones. In most cases, it is sustained by increased dietary loads. In healthy individuals with a normal Western diet, the majority of urinary oxalate is usually derived from endogenous metabolism. However, up to 50% may be derived from the diet. We were interested in the effect of a high-oxalate diet on oxalate absorption, not merely on the frequently studied increased oxalate excretion. In study I, 25 healthy volunteers were tested with the [13C2]oxalate absorption test once while following a low-oxalate (63 mg) and once while following a high-oxalate (600 mg) diet for 2 days each. In study II, four volunteers repeated study I, and afterwards continued with a high-oxalate diet (600 mg oxalate/day) for 6 weeks. In the last week, the [13C2]oxalate absorption test was repeated. After 4 weeks of individual normal diet, the oxalate absorption test with a high-oxalate diet was performed again. The results of study I show that the mean [13C2]oxalate absorption under low-oxalate diet was 7.9 +/- 4.0%. In the presence of oxalate-rich food, the percent absorption for the soluble labelled oxalate almost doubled (13.7 +/- 6.3%). The results of study II show that the mean [13C2]oxalate absorption of the four volunteers under low-oxalate diet was 7.3 +/- 1.4%. The absorption increased to 14.7+/-5.2% under 600 mg oxalate. After 6 weeks under a high-oxalate diet, the [13C2]oxalate absorption was significantly decreased (8.2 +/- 1.7%). After the wash-out phase, the absorption was again high (14.1 +/- 2.2%) under the 600 mg oxalate challenge.

Importance of magnesium in absorption and excretion of oxalate.

INTRODUCTION: Magnesium treatment for calcium oxalate urolithiasis is discussed controversially. The aim of this study was to investigate the influence of magnesium supplementation on the oxalate absorption. MATERIALS AND METHODS: The [13C2]oxalate absorption test was always performed three times in 6 healthy volunteers under standardized conditions, with one 10-mmol magnesium supplement together with the labeled oxalate and with two 10-mmol magnesium supplements given in 12-hour intervals. RESULTS: The mean intestinal oxalate absorption under standard conditions was 8.6 +/- 2.83%. The oxalate absorption with one 10-mmol magnesium supplement was 5.2 +/- 1.40% and with two supplements 5.5 +/- 1.62%. Both decreases were statistically significant relative to the standard test, however, not significantly different from each other. CONCLUSIONS: The results show that magnesium administration decreases the oxalate absorption, when magnesium is taken together with oxalate. However, magnesium administration does not decrease the oxalate absorption, when magnesium and oxalate intake differ by 12 h.

The effect of oral administration of calcium and magnesium on intestinal oxalate absorption in humans.

Calcium oxalate (CaOx) urolithiasis is the most common urinary stone disease (70-75 % of all stones consist of CaOx in countries with western diet). Oxalate is the most lithogenic substance in CaOx crystallisation in urine. Oxalate is either synthesized within the body or absorbed from food. As oxalate is not metabolized in the human body, it appears unchanged in urine. Conventional analysis methods cannot distinguish between endogenous and exogenous oxalate. Our [13C2]oxalate absorption test enabled measurement of intestinal oxalate absorption and quantification of the influence of Ca- and Mg-supplementation on it. The effects of the oral administration of these supplements were compared in order to obtain valid data for recommendations for CaOx urolithiasis patients. A 10 mmol supplement of both ions decreased the oxalate absorption significantly, calcium being more than twice as effective.

The influence of freezer storage of urine samples on the BONN-Risk-Index for calcium oxalate crystallization.

This study was performed to quantify the effect of a 1-week freezer storage of urine on its calcium oxalate crystallization risk. Calcium oxalate is the most common urinary stone material observed in urolithiasis patients in western and affluent countries. The BONN-Risk-Index of calcium oxalate crystallization risk in human urine is determined from a crystallization experiment performed on untreated native urine samples. We tested the influence of a 1-week freezing on the BONN-Risk-Index value as well as the effect of the sample freezing on the urinary osmolality. In vitro crystallization experiments in 49 native urine samples from stone-forming and non-stone forming individuals were performed in order to determine their calcium oxalate crystallization risk according to the BONN-Risk-Index approach. Comparison of the results derived from original sample investigations with those obtained from the thawed aliquots by statistical evaluation shows that i) no significant deviation from linearity between both results exists and ii) both results are identical by statistical means. This is valid for both, the BONN-Risk-Index and the osmolality data. The differences in the BONN-Risk-Index results of both procedures of BONN-Risk-Index determination, however, exceed the clinically acceptable difference. Thus, determination of the urinary calcium oxalate crystallization risk from thawed urine samples cannot be recommended.