Effect of high and low calcium diets on stone forming risk during liberal oxalate intake

PURPOSE

Recent studies suggest that a high calcium diet protects against calcium oxalate stone formation. We compared the effect of high and low calcium diets on urinary saturation of calcium oxalate during liberal oxalate intake.

MATERIALS AND METHODS

A total of 10 healthy subjects (5 male, 5 female) participated in a 2-phase, randomized, crossover study comparing high (1,000 mg daily) and low (400 mg daily) calcium intake on a liberal oxalate diet (200 mg daily). During each phase subjects adhered to an instructed diet for 3 days followed by a controlled, metabolic diet for 4 days. Blood and 24-hour urine specimens collected on the last 2 days of each phase were analyzed for serum biochemistry studies and stone risk factors, respectively.

RESULTS

Urinary calcium was higher (mean +/- SD 171 +/- 64 vs 124 +/- 49 mg daily, p = 0.002) and oxalate was lower (25 +/- 4.8 vs 27 +/- 4 mg daily, p = 0.02) on the high vs low calcium diet. Overall, the urinary relative saturation ratio of calcium oxalate was higher on the high compared with the low calcium diet (3.3 vs 2.5, p <0.0001) even after adjusting for confounding variables.

CONCLUSIONS

In normal subjects urinary saturation of calcium oxalate was higher on a high calcium diet than a low calcium diet during liberal oxalate intake because the decrease in urinary oxalate did not overcome the effect of increased calcium. A high calcium diet during liberal oxalate intake may pose an increased risk of calcium oxalate stone formation.

[The effect of urinary tract calculosis to levels of low molecular inhibitors of crystallization in the urine]

The incidence of urinary tract calculosis continuously progresses. The triggering event in the process of stone formation is decreased urinary level of crystallizing inhibitors. The aim of our study was to investigate whether the existing stone or applied therapeutic procedure - extracorporeal shock waves lithotripsy (ESWL) - has effect to urinary levels of Mg, citrate and pyrophosphate. Study included 128 patients with the upper urinary tract stones. ESWL using the Lithostar (Siemens) device was used as a mode of treatment. Out of all patients, 76 (59%) were free of stone particles before 1 month, while 52 (41%) had residual stone fragments even 3 months after ESWL. Mg, citrate and pyrophosphate were measured in 24h-urine specimens: before, between days 2 and 3, as well as 1 and 3 months after ESWL. The analysis of the results revealed that stone itself had no effect on urinary crystallizing inhibitors. Detected increased urinary levels of Mg, citrate and pyrophosphate after ESWL, compared with pre-treatment values, could be attributed to applied therapeutic procedure.

Aetiological factors in paediatric urolithiasis

The aetiology of stones in children differs from that in adults. Young children, especially boys, are prone to infective stones, although this type of calculi is decreasing in frequency over time in prosperous countries. Two monogenic causes, cystinuria and hyperoxaluria, each account for 5-15% of paediatric stones. Increased factors for stone formation in children include prematurity, neurological problems, ketogenic diet and reconstructed or augmented bladders. Hypercalciuria is commonly found in paediatric stone formers, is usually idiopathic and is only rarely associated with hypercalcaemia. All children with stones should undergo a metabolic evaluation.

Problems in the metabolic evaluation of renal stone disease: audit of intra-individual variation in urine metabolites

Preliminary metabolic assessment of patients with renal stones includes measurement of urine metabolites. This paper reports on the degree of intra-individual variation in some key urine metabolites. Over 80 medically untreated patients under initial metabolic investigation were audited from whom 24-h urine results were available as three separate urine pairs collected at intervals not less than 1 month apart. Ranking patients by intra-individual variation, above the 75th centile, the highest calcium was at least 216% of the lowest calcium, the respective figures for phosphate, urate, oxalate, citrate, creatinine and sodium were 207, 190, 271, 412, 175 and 233%. In order to estimate pre-treatment excretion within 30% of a true mean at the 95% confidence limit, for calcium and oxalate, the number of 24-h samples required were 3 and 4 respectively with 6 and 9 required to be within 20%. These observations illustrate significant practical clinical problems in assessing patients with renal stones when assessing these basic parameters. Regimens based on small numbers of urine collections are flawed, hence evidence based protocols should be devised. A minimum of three pairs of 24-h urine samples based upon predicting metabolite output within 20-30% or less of the true mean is recommended.

Urinary macromolecules and renal tubular cell protection from oxalate injury: Comparison of normal subjects and recurrent stone formers

AIM

To determine whether urinary macromolecules (UMM), which are the high molecular weight substances in urine, can provide protection against the oxalate-associated injury to the renal tubular cells.

METHODS

UMM were extracted from 24-h urine of 12 healthy adult male volunteers and 13 recurrent-stone-former male patients. Urine parameters in relation to urolithiasis were measured, including the level of glycosaminoglycans (GAG) in the UMM. Madin-Darby canine kidney (MDCK) cells were used to evaluate the protective activity of UMM from oxalate-induced cytotoxicity by LDH release measurement and methyl-thiazolyl tertrazolium (MTT) assay.

RESULTS

Considering urinary parameters, citrate was significantly higher in urine from normal subjects than stone-former subjects; the other parameters show no differences between the groups. Total UMM and the level of GAG in the UMM were also significantly higher in the normal subject group. Compared with normal subject and stone-former subject UMM, after cells were treated with the UMM and then exposed to oxalate solution, LDH release was significantly higher in stone-former group. In the MTT assay, we found that more viable cells were observed after treatment with UMM compared to control in both groups. Moreover, UMM from the normal subjects showed higher protective activity against oxalate-related cytotoxicity than UMM from the stone-former subjects.

CONCLUSION

UMM protected renal epithelial cells from oxalate-related injury. This protective activity was found to be higher in normal subject UMM than stone-former UMM. Among other factors, a higher concentration of GAG and citrate in normal subject UMM might affect some parts in this finding.

Calcium oxalate urolithiasis in mice lacking anion transporter Slc26a6

Urolithiasis is one of the most common urologic diseases in industrialized societies. Calcium oxalate is the predominant component in 70-80% of kidney stones, and small changes in urinary oxalate concentration affect the risk of stone formation. SLC26A6 is an anion exchanger expressed on the apical membrane in many epithelial tissues, including kidney and intestine. Among its transport activities, SLC26A6 mediates Cl(-)-oxalate exchange. Here we show that mutant mice lacking Slc26a6 develop a high incidence of calcium oxalate urolithiasis. Slc26a6-null mice have significant hyperoxaluria and elevation in plasma oxalate concentration that is greatly attenuated by dietary oxalate restriction. In vitro flux studies indicated that mice lacking Slc26a6 have a defect in intestinal oxalate secretion resulting in enhanced net absorption of oxalate. We conclude that the anion exchanger SLC26A6 has a major constitutive role in limiting net intestinal absorption of oxalate, thereby preventing hyperoxaluria and calcium oxalate urolithiasis.

Effect of oxalate test dose size on absolute and percent oxalate absorption

The purpose of this pilot study was to establish the dependence or independence of oxalate absorption on the quantity of the test dose of sodium oxalate over a range of test doses corresponding to physiological dietary oxalate intake values. Gastrointestinal oxalate absorption was measured with the [13C2]oxalate absorption test. Six healthy volunteers were always tested under standardized dietary conditions with 63 mg dietary oxalate and 800 mg dietary calcium per day. The volunteers were tested thrice each with sodium oxalate test doses of 25, 50, 200, and 600 mg. Additionally, 1000 mg sodium oxalate was applied once to three of these volunteers. The oxalate absorption of the six volunteers tested under the standardized conditions with 50 mg sodium [13C2]oxalate was 7.2 +/- 2.62 % (mean +/- SD), similar to the 120 volunteers tested previously: 8.0 +/- 4.4 % (mean +/- SD). The tests with sodium [13C2]oxalate doses in the range 25-1000 mg revealed similar percent oxalate absorption values. In conclusion, in healthy volunteers, the amount of oxalate absorbed in the gastrointestinal tract increased proportionally with the higher test doses of oxalate. However, percent oxalate absorption remained unchanged with test doses in the dose range of physiological dietary oxalate intakes.

Why does the Bonn Risk Index discriminate between calcium oxalate stone formers and healthy controls?

PURPOSE

The BRI has been shown to discriminate between calcium oxalate stone formers and controls. BRI is the ratio of the concentration of ionized calcium and the amount of oxalate that must be added to 200 ml urine to initiate crystallization. Higher BRI values are predictive of being a stone former and a value of 1.0 has been found to be the cutoff value to distinguish stone formers and controls. It is not easy to present a consistent argument based on the thermodynamics of calcium oxalate crystallization to account for the success of this index. For instance, why should 2 samples sharing the same BRI but with different ionized calcium and oxalate values have the same likelihood of being obtained from a stone former?

MATERIALS AND METHODS

Using data on 195 samples the distribution and interrelationships of measured variables were examined. They were used to calculate illustrative data with which it was possible to examine the effects of varying the parameters and their relationships.

RESULTS

Data simulations identified 3 necessary and sufficient conditions that must be met for BRI to be an effective discriminator between stone former and nonstone former urine samples.

CONCLUSIONS

The success of BRI can be explained as the natural outcome of there being significantly different distributions (stone formers vs nonstone formers) of the concentration of ionized calcium and the formation product minus activity product difference as well as the correlation between these 2 variables.

Effect of potential renal acid load of foods on urinary citrate excretion in calcium renal stone formers

The aim of this study was to investigate the influence of the potential renal acid load (PRAL) of the diet on the urinary risk factors for renal stone formation. The present series comprises 187 consecutive renal calcium stone patients (114 males, 73 females) who were studied in our stone clinic. Each patient was subjected to an investigation including a 24-h dietary record and 24-h urine sample taken over the same period. Nutrients and calories were calculated by means of food composition tables using a computerized procedure. Daily PRAL was calculated considering the mineral and protein composition of foods, the mean intestinal absorption rate for each nutrient and the metabolism of sulfur-containing amino acids. Sodium, potassium, calcium, magnesium, phosphate, oxalate, urate, citrate, and creatinine levels were measured in the urine. The mean daily PRAL was higher in male than in female patients (24.1+/-24.0 vs 16.1+/-20.1 mEq/day, P=0.000). A significantly (P=0.01) negative correlation (R=-0.18) was found between daily PRAL and daily urinary citrate, but no correlation between PRAL and urinary calcium, oxalate, and urate was shown. Daily urinary calcium (R=0.186, P=0.011) and uric acid (R=0.157, P=0.033) were significantly related to the dietary intake of protein. Daily urinary citrate was significantly related to the intakes of copper (R=0.178, P=0.015), riboflavin (R=0.20, P=0.006), piridoxine (R=0.169, P=0.021) and biotin (R=0.196, P=0.007). The regression analysis by stepwise selection confirmed the significant negative correlation between PRAL and urinary citrate (P=0.002) and the significant positive correlation between riboflavin and urinary citrate (P=0.000). Urinary citrate excretion of renal stone formers (RSFs) is highly dependent from dietary acid load. The computation of the renal acid load is advisable to investigate the role of diet in the pathogenesis of calcium stone disease and it is also a useful tool to evaluate the lithogenic potential of the diet of the individual patient.

Effect of dietary modification on urinary stone risk factors

BACKGROUND

This study was undertaken to ascertain the effect of dietary modification on urinary stone risks, and to determine whether the response depends on the prevailing urinary calcium.

METHODS

A retrospective data analysis was conducted from our stone registry involving 951 patients with calcareous stones undergoing ambulatory evaluation, whereby 24-hour urine samples were collected during random diet and after dietary modification composed of restriction of calcium, oxalate, sodium, and meat products. Samples were analyzed for stone risk factors. Urinary calcium was also obtained after overnight fast and following a 1 g-calcium load. Changes produced by dietary modification from the random diet were evaluated in 356 patients with moderate-severe hypercalciuria (>6.88 mmol/day, group I), 243 patients with mild hypercalciuria (5.00-6.88 mmol/day, group II), and 352 with normocalciuria (<5.00 mmol/day, group III).

RESULTS

Urinary calcium postcalcium load and the percentage of patients with absorptive hypercalciuria type I were highest in group I, intermediate in group II, and lowest in group III. During dietary modification, urinary calcium declined by 29% in group I, 19% in group II, and 10% in group III. Urinary oxalate did not change. Urinary saturation of calcium oxalate declined by only 12% in group I, 6% in group II, and nonsignificantly in group III, owing to various physicochemical changes in urinary biochemistry, which attenuated the effect of the decline in urinary calcium. Urinary saturation of brushite declined in all 3 groups due to the fall in urinary calcium, phosphorus, and pH. This reduction was more marked in the hypercalciuric groups than in the normocalciuric group. Urinary saturation of monosodium urate also decreased from a decline in urinary sodium and uric acid.

CONCLUSION

Secondary rise in urinary oxalate occurring from calcium restriction can be avoided by concurrent dietary oxalate restriction. Dietary modification (restriction of dietary calcium, oxalate, sodium, and meat products) is more useful in reducing urinary saturation of calcium oxalate among patients with hypercalciuria than among those with normocalciuria.

Renal tubular impairment in children with idiopathic hypercalciuria

Idiopathic hypercalciuria (IH) is defined as hypercalciuria that persists after correction of dietary inbalances and has no detectable cause. The excretion of urinary N-acetyl-beta-D-glucosaminidase (U-NAG), a marker of proximal tubular damage, has been previously reported as either increased or normal in children with IH. We evaluated U-NAG in 20 children (13 boys and 7 girls, mean age 10.3 years +/- 5.7 SD) with IH (urinary calcium excretion above 0.1 mmol/kg/24 hours, with no detectable cause) and with otherwise normal renal function tests. Ultrasound examination revealed urolithiasis (n=4) and nephrocalcinosis (n=1). The U-NAG values were evaluated in the spot urine collected from the second morning void and calculated as the urinary NAG/creatinine ratio (U-NAG/Cr) and expressed in nkat/mmol. The 24-hour urinary calcium excretion (U-Ca/24h) was assessed in a urinary sample from 24-hour collected urine and calculated in mmol/kg. The obtained results of U-Ca/24h and U-NAG/Cr were expressed as Z-scores. When compared to the reference data, the U-Ca/24h and U-NAG/Cr were significantly higher (p = 0.0004 and p = 0.006, respectively). There was no correlation between the U-NAG/Cr and U-Ca/24h (r = 0.18, p = 0.20). The U-NAG/Cr values were significantly higher in the 5 patients with urolithiasis/nephrocalcinosis, whether compared to the rest of the group (p = 0.02), or to the reference data (p = 0.01). The U-NAG/Cr activity was higher in 15 children without urolithiasis/nephrocalcinosis when compared to reference data (p < 0.01). There was no difference in U-Ca/24h between the children with and without urolithiasis/nephrocalcinosis (p = 0.58). These findings suggest that tubular impairment, as reflected by U-NAG/Cr, might occur in children with IH, especially in patients with urolithiasis/nephrocalcinosis. There doesn't seem to be a direct relationship between the U-NAG/Cr activity and the degree of calcium leakage.

Influence of estrus status on urinary chemical parameters related to urolithiasis

The present study examines the urinary chemical parameters related to urolithiasis in healthy female volunteers during premenopause and menopause, and discusses the role of menopause in stone formation. We investigated 24-h urine parameters associated with urinary stones and focused upon estrus status. Participants comprised 30 healthy women, 15 childless, premenopausal women and 15 menopausal women without a history of urolithiasis. Our results showed that menopausal women have lower citrate and higher calcium excretion, which might enhance calcium stone crystallization. We propose that the estrus status of female patients should be considered when evaluating metabolic abnormalities.

Oxalate and its handling in a low stone risk vs a stone-prone population group

Despite hyperoxalurogenic eating habits relative to white subjects, South African blacks have urinary oxalate excretions, Tiselius risk indices (AP(CaOx)) and calcium oxalate saturations, which do not differ significantly from those of their white counterparts. The present study was undertaken to establish whether the BONN-Risk-Index (BRI) might discriminate between the urines of the two population groups and whether differences might exist in their respective gastrointestinal absorption rates of oxalate. Participants (n = 15 in each group) provided 24 h urines on their free diets for BRI determination. Gastrointestinal oxalate absorption was measured using the [13C2]oxalate absorption test. Results showed that BRI values were significantly lower in black subjects (2.04 vs 4.90, P = 0.034), but that there was no difference in the oxalate absorption between the groups (10.30 vs 9.95%, P = 0.87). These results suggest that South African black subjects handle dietary oxalate more efficaciously than white subjects and that this occurs via some endogenous mechanism, which has not yet been identified or characterized.

Urine sediment from a Chihuahua

A 6-year-old, intact male Chihuahua was presented with stranguria and painful urination of 5 days duration. Cystine crystals were observed in low numbers in unstained urine sediment preparations, and a diagnosis of cystinuria was made. Uroliths were removed surgically from the urethra and the bladder, and mineral analysis indicated the stones were composed of 100% cystine. Cystinuria results from an inherited defect in renal tubular transport of cystine that affects many breeds and has been found as an autosomal recessive trait in Newfoundlands. Accurate identification of cystine crystals in urine is an important means of diagnosing cystinuria.

Prevalence and risk factors for urolithiasis in primary gout: is a reappraisal needed?

OBJECTIVE

To assess the prevalence and risk factors for urolithiasis in primary gout.

METHODS

One hundred forty patients with primary gout were studied. Urolithiasis was defined as a history of urolithiasis, or nephrolithiasis detected via ultrasonography in patients with no previous record of urolithiasis. Patient age, duration of gout, presence of tophi, obesity, alcoholism, high blood pressure, diabetes, hyperlipidemia, family history of urolithiasis, daily urine output, uricemia, urine pH, FeNa, FeUrate, urine pH/FeUrate index, and daily urine excretion of urate, sodium, calcium and potassium were compared between lithiasic and non-lithiasic subjects.

RESULTS

Fifty-five (39%; 95% CI 31-47) patients had urolithiasis, of which 37 (26%) were diagnosed by clinical history and 18 (13%) by ultrasonography. Patients with a silent kidney stone diagnosed by ultrasound tended to have shorter evolution of gout. Aside from urinary H+ ion concentration (lithiasic subjects 5.17 +/- 3.9 microM/l; non-lithiasic subjects 3.80 +/- 3.01 microM/l; p = 0.02), no difference was found between lithiasic and non-lithiasic subjects for the other variables studied.

CONCLUSION

Ultrasonography increased the probability of diagnosing urolithiasis by 50%, meaning the prevalence of urolithiasis in gout is likely higher than previously reported. A higher urinary H+ ion concentration was the only variable associated with urolithiasis. Due to advances in diagnosis of gout and urolithiasis, as well as biochemical assays, the prevalence and risk factors for urolithiasis in gout require reassessment.

[Calcium oxalate stones and hyperoxaluria. What is certain? What is new?]

Approximately 4 million Germans suffer from stone disease. In the majority of cases (70-75%) it is calcium oxalate. Its pathophysiology is complex and comprises disorders such as hypercalciuria, hyperoxaluria, hypocitraturia, hyperuricosuria, and hypomagnesuria. These biochemical changes in urine are well known as "classic" risk factors of calcium oxalate stone formation. However, studies in the last decade showed that calcium oxalate stones are strongly related with other diseases or disorders such as overweight, hypertension, or a lack of oxalate-degrading bacteria in the gut. The evidence for these "new" risk factors in the literature is very strong. It is particularly important in regard to effective treatment and aftercare of patients with calcium oxalate stones to be familiar with both the "classic" and the new risk factors.

RESULTS OF A PROSPECTIVE TRIAL TO COMPARE NORMAL URINE SUPERSATURATION IN CHILDREN AND ADULTS

PURPOSE

Urinary stone disease is relatively rare in children but is often due to metabolic abnormalities. Urinary supersaturation (SS) indices have been suggested as more precise predictors of stone recurrence. We previously reported urinary metabolic evaluations in stone-forming children using adult references. We now assess normal SS values in children.

MATERIAL AND METHODS

A prospective trial was performed to assess urinary metabolic profiles in children with no personal or family history of urinary calculi. The 24-hour urine collections were performed and evaluated at an outside central laboratory. Urine chemistries were adjusted for urine creatinine. Urine pH and SS for calcium oxalate, calcium phosphate and uric acid were also assessed. The data were compared to a cohort of normal adults.

RESULTS

A total of 168 samples from 168 adults (58% males) and 51 samples from 30 children (59% males) were evaluated. The mean age of the children was 8.8 years (range 5 to 18). Calcium oxalate SS was similar between children and adults, and urine pH was the major cause of differences seen in the calcium phosphate and uric acid SS indices. Children had a much higher excretion of calcium, oxalate and citrate when adjusted for creatinine.

CONCLUSIONS

There are significant differences in the normal ranges of urine chemistries in children and adults. An understanding of normal supersaturation values may allow metabolic abnormalities in stone-forming children to be more precisely diagnosed and treated.

Urinary macromolecular inhibition of crystal adhesion to renal epithelial cells is impaired in male stone formers

BACKGROUND

Retention of microcrystals that form in tubular fluid could be a critical event in kidney stone formation. This study was performed to determine if urinary macromolecules from stone-forming (SF) individuals have reduced ability to inhibit crystal adhesion to renal cells.

METHODS

A first morning whole urine (WU) sample was obtained from 24 SF subjects (17 males and 7 females) and 24 age-, race-, and sex-matched controls (C). An aliquot of urine was centrifuged and an ultrafiltrate (UF) free of macromolecules >10 kD and 10x concentrate (U(conc)) were prepared.

RESULTS

Supplementing UF with increasing amounts of U(conc) to return the macromolecule concentration to 0.25x, 0.5x, or 1x of baseline progressively decreased crystal binding to cells. This effect was blunted in the male SF group compared to controls (P < 0.05, SF vs. C, for UF plus 0.25x macromolecules). No difference was apparent in the female groups. In order to identify responsible macromolecule(s), calcium oxalate monohydrate (COM) crystals were coated with U(conc) and adherent proteins then released and probed by Western blot. Coated COM crystals from male controls contained 3.5-fold more Tamm-Horsfall protein (THP) than SF subjects (P < 0.01). COM crystal coating with other proteins did not consistently differ between the groups. COM crystal coating by urinary prothrombin fragment 1 (UPTF1, P < 0.05) and crystal adhesion inhibitor (CAI) (P= 0.09) correlated with decreased crystal binding to cells, whereas coating with osteopontin (OPN) correlated with increased adhesion tendency (P < 0.05).

CONCLUSION

Urinary macromolecules >10 kD coat COM crystals and block their adhesion to renal cells. This capacity appears to be blunted in male but not female SF individuals. Multiple urinary proteins may play a role in renal cell-urinary crystal interactions, and THP appears to be one of the more important ones.

Intracrystalline proteins and urolithiasis: a comparison of the protein content and ultrastructure of urinary calcium oxalate monohydrate and dihydrate crystals

OBJECTIVE

To compare the ultrastructure and protein content, particularly prothrombin fragment 1 and osteopontin, of calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) crystals precipitated from human urine, and their susceptibility to proteolysis, to try to clarify the role of intracrystalline proteins in urolithiasis, as differences between these types of crystal may determine whether calcium oxalate crystals nucleated in urine progress to stone formation.

MATERIALS AND METHODS

Sodium dodecyl sulphate gel electrophoresis and Western blotting were used to analyse demineralized extracts of COM and/or COD crystals deposited from the same centrifuged and filtered urine (which contains abundant urinary proteins) by adjusting the calcium concentration to 2 and 7 mmol/L, respectively. Similar analyses were performed on COM and COD crystals deposited from ultrafiltered urine (which contains only proteins of < 10 kDa) and then incubated in centrifuged and filtered urine, as well as crystals generated in the presence of increasing concentrations of proteins derived from the organic matrix of urinary calcium oxalate crystals. Field-emission scanning electron microscopy was used to assess effects of proteinase K and cathepsin D on internal and superficial crystal structure.

RESULTS

Osteopontin was undetectable in COM extracts, but clearly visible in COD. Prothrombin fragment 1 was abundant in COM, but present in COD in lesser amounts than osteopontin. The selectivity was also the same with crystals from ultrafiltered urine that were incubated in centrifuged and filtered urine: prothrombin fragment 1 binding was favoured by low calcium concentration, while osteopontin bound at higher levels. Scanning electron microscopy of COM and COD digested with proteinase K and cathepsin D revealed superficial and internal texture, as wells as surface erosion, in crystals from centrifuged and filtered urine, thus confirming the presence of intracrystalline proteins. Such features were absent from crystals precipitated from ultrafiltered urine.

CONCLUSION

Binding of osteopontin and prothrombin fragment 1 to calcium oxalate is dictated primarily by ambient calcium concentration. Each protein may inhibit urolithiasis by inhibiting crystallization of its preferred crystal habit, and by facilitating the intracellular disintegration and dissolution of crystals attached to and internalized by renal epithelial cells.

[Influence of urinary citrate levels on spontaneous calcium oxalate dihydrate crystalluria]

INTRODUCTION

Calcium oxalate is the leading cause of renal stones and is mainly due to hypercalciuria, hyperoxaluria and/or hypocitraturia. Citrate is considered to be an effective inhibitor of calcium oxalate crystallization and is therefore increasingly prescribed as maintenance therapy for patients with calcium stones, but no study has investigated the effect of urinary citrate levels on spontaneous calcium oxalate crystalluria in human urine. In this study, the authors examined the relationships between the calcium oxalate molar product, the urinary citrate concentration and weddellite (oxalate calcium dihydrate) crystalluria, the most frequent crystalline form of calcium oxalate in human urine.

MATERIAL AND METHODS

Crystalluria analysis and calcium, oxalate and citrate assays were performed on a series of 10,222 first morning urine samples from 4,809 stone patients and 453 first morning urine samples from 317 control subjects. The frequency and characteristics of weddellite crystalluria were determined as a function of the calcium oxalate molar product (pCaOx) and urinary citrate concentration.

RESULTS

1,940 urine samples (18.2%) presented weddellite crystalluria, which was pure in 1,378 urine samples from stone patients (13.5%) and 43 urine samples (9.5%) from controls (p < 0.05). The crystalluria rate in stone patients ranged from 4% for pCaOx < 1 (mmol/l)2 to 81.3% for pCaOx > or = 3 (mmol/l)2 (p < 0.0001). Over the same interval of pCaOx, weddellite crystalluria ranged from 1.5% to 72.2% in control subjects. An increase of urinary citrate excretion from 0.5 to 5 mmol/l significantly lowered the frequency of crystalluria from 32.4% to 10.1% for a pCaOx between 1 and 2 (mmol/l)2 (p < 0.0001) and from 63% to 27.9% for a pCaOx between 2 and 3 (mmol/l)2 (p < 0.001). For pCaOx values > or = 3 (mmol/l)2, urinary citrate excretion no longer significantly influenced the frequency of crystalluria. The number of crystals and aggregates and the maximum dimensions of aggregates were only influenced by the urinary citrate concentration when the pCaOx product was < 2 (mmol/l)2.

CONCLUSION

The main determinant of the frequency and characteristics of weddellite crystalluria is the pCaOx molar product. The beneficial effect of the urinary citrate concentration on the frequency of crystalluria is observed for pCaOx values < 3 (mmol/l)2, but only for pCaOx values < 2 (mmol/l)2 for the characteristics of crystalluria such as the number and dimensions of crystals and aggregates. This means that therapeutic measures designed to increase urinary citrate concentrations can only be effective when pCaOx has been previously lowered by increased diuresis or specific reduction of urinary calcium and/or urinary oxalate levels.

Comparison of 24 hours urinary citrate levels in urolithiasis patients and healthy controls

OBJECTIVE

To identify the difference in urinary citrate excretion between Stone Formers (SF) and Healthy Volunteers (HV) as a metabolic risk factor, that predisposes to urinary stone formation and to compare levels of urinary citrate in (HV) with reference values.

METHODS

The 24 hours urinary citrate was evaluated in 40 patients treated for renal citrate and declared stone free, and 40 age matched healthy adults taken as controls. Both the groups had a similar living environment, extrinsic factors, diet and similar genetic descent.

RESULTS

There was no significant difference in urinary citrate excretion level among stone formers (mean 262 SD 197) and normal volunteer subjects (mean 269 SD 140). Using the previously defined normal values (200) of urinary citrate in the local population, 55% of stone patients in our study group were hypocitric. While using the same value, 45% of our normal volunteers were also hypocitric. If 320 was taken as normal limit, 70% of the patient's population and 72% of controls were hypocitric. The prevalence of hypocitraturia was similar in the age matched adult groups.

CONCLUSION

Certain intrinsic factors in our local subjects may account for the high prevalence of urolithiasis than in western population. Although the urinary citrate excretion of stone patients is similar to normal volunteers, uniformly low urinary citrate excretion may be a feature as a nation and not a predisposing factor for the lithogenesis. This supports the view that there may be more often prominent influences in stone formers possibly of genetic origin.

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.

[Study of crystalluria in diabetic patients]

INTRODUCTION

The incidence of diabetes, particularly non-insulin-dependent diabetes, is on the increase in industrialized and developing countries. The prevalence of renal stones in the diabetic population was recently estimated to be 21%, i.e. more than twice the prevalence of stones in the general population. Other studies have emphasized the high frequency of uric acid stones in this particular population. The present study was designed to verify whether diabetic patients present a particular type of crystalluria predisposing them to a high frequency of uric acid stones than other types of stones, which could allow detection of this risk and the proposal of therapeutic measures to prevent these stones.

MATERIAL AND METHOD

The first morning urine of 208 diabetic patients was examined by polarized light microscopy to detect and identify crystalluria. Patients were distributed into 3 age-groups: less than 40 years, 40 to 59.9 years and 60 years or more. The results are expressed as the predominant crystalline species.

RESULTS

The overall frequency of crystalluria was 29.8%, i.e. about one half that observed in calcium stones. However, the crystallogenic profile was very unusual, as 61.3% of cases of crystalluria consisted of purine. Uric acid crystalluria was twice as frequent in women than in men (66% vs 33.3%, p < 0.05). The mean pH of the urine of diabetic subjects was 5.5, i.e. significantly more acidic than that of normal subjects or patients with calcium stones. pH was negatively correlated with the patient's age, decreasing from 5.54 in patients younger than 40 to 5.3 in patients over the age of 60 (p < 0.05). The mean pH of crystalluric urine was significantly more acidic than that of crystal-free urine (pH 5.2 +/- 0.46 vs 5.5 +/- 0.67, p < 0.01).

CONCLUSION

Diabetic patients have an acidic urinary pH which tends to decrease with age, predisposing to uric acid crystalluria, which is particularly frequent in women. The high prevalence of uric acid crystalluria and the high proportion of uric acid stones reported in diabetic women suggest that women are at greater risk than men of developing uric acid stones in the context of diabetes. The study of crystalluria could be useful to detect this risk and to propose preventive measures. Complementary studies are necessary to identify factors accounting for the increased risk of uric acid stones in diabetic women and to verify whether good glycaemic control can reduce the crystallogenic risk.