The urinary response to an oral oxalate load in recurrent calcium stone formers

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

Compliance in patients with dietary hyperoxaluria: A cohort study and systematic review

Objective

Hyperoxaluria leads to calcium oxalate crystal formation and subsequent urolithiasis. This study aims to analyse the effect of treatment compliance in hyperoxaluria, firstly by analysis of patients with non-primary hyperoxaluria and secondly via systematic review in patients with any hyperoxaluria.

Methods

In a retrospective cohort study, adults with non-primary hyperoxaluria managed with dietary counselling in 2013 were enrolled. Twenty-four-hour (24 h) urine collections initially and at 6 months were obtained. Compliance was assessed by self-reported dietary compliance and 24 h urinary volume >2 L. Patients were followed for 24 months. Primary outcomes were urinary oxalate and calcium 24 h load at 6 months, and urolithiasis-related procedural rates at 24 months. A Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-compatible systematic review of compliance among hyperoxaluric patients was performed.

Results

In the cohort study, of 19 eligible patients (4 female) with median age 52 years, 10 (53%) were considered compliant. Compared with the non-compliant group, these patients had significantly increased subsequent 24 h urinary volume (2250 mL vs. 1600 mL; p = 0.008) and lower procedural rates (10% vs. 56%; p = 0.033). Subsequent 24 h urinary oxalate load was non-significantly lower in compliant patients. Systematic review regarding compliance in hyperoxaluric patients revealed five studies. Only one utilised dietary counselling or analysed compliant vs. non-compliant patients, finding no difference. None examined the effect of compliance on procedural rates.

Conclusion

Hyperoxaluria is an important cause of recurrent urolithiasis. Increasing fluid intake and reducing dietary oxalate reduce the risk of operative intervention and remain fundamental to the treatment of hyperoxaluria.

A Double-Blind, Placebo Controlled, Randomized Phase 1 Cross-Over Study with ALLN-177, an Orally Administered Oxalate Degrading Enzyme

BACKGROUND

Hyperoxaluria may result from increased endogenous production or overabsorption of dietary oxalate in the gastrointestinal tract leading to nephrolithiasis and, in some, to oxalate nephropathy and chronic kidney disease. ALLN-177 is an oral formulation of a recombinant, oxalate specific, microbial enzyme oxalate decarboxylase intended to treat secondary hyperoxaluria by degrading dietary oxalate in the gastrointestinal tract, thereby reducing its absorption and subsequent excretion in the urine.

METHODS

This double-blind, placebo controlled, randomized, cross-over, phase 1 study of ALLN-177 evaluated the tolerability of ALLN-177 and its effect on urinary oxalate excretion in 30 healthy volunteers with hyperoxaluria induced by ingestion of a high oxalate, low calcium (HOLC) diet. The primary end point was the difference in the mean 24-hour urinary oxalate excretion during the ALLN-177 treatment period compared with the placebo treatment period.

RESULTS

The daily urinary oxalate excretion increased in the study population from 27.2 ± 9.5 mg/day during screening to 80.8 ± 24.1 mg/day (mean ± SD) on the HOLC diet before introducing ALLN-177 or placebo therapy for 7 days. Compared to placebo, ALLN-177 treatment reduced urinary oxalate by 11.6 ± 2.7 mg/day, p = 0.0002 (least squares mean ± SD).

CONCLUSIONS

In healthy volunteers, with diet-induced hyperoxaluria treatment with ALLN-177, when compared to placebo, significantly reduced urinary oxalate excretion by degrading dietary oxalate in the gastrointestinal tract and thereby reducing its absorption. ALLN-177 may represent a new approach for managing secondary hyperoxaluria and its complications.

Fad diets and their effect on urinary stone formation

The influence of unhealthy dietary habits on urinary stone formation has been widely recognized in literature. Dietary advice is indeed the cornerstone prescription for prevention of nephrolithiasis as well. However, only a small amount of medical literature has addressed the influence of popular or fad diets, often self-prescribed for the management of obesity and overweight or for cultural beliefs, on the risk of kidney stones. Thereby in this paper we analyze the current knowledge on the effects of some popular diets on overall lithogenic risk. High-protein diets, like Dukan diet, raise some concerns, since animal proteins are able to increase urinary calcium and to decrease urinary citrate excretion, thus leading to a high overall lithogenic risk. Low-carbohydrate diets, like Atkins diet or zone diet, may have a protective role against kidney stone formation, but there are also evidences stating that this dietary approach may rise calciuria and decrease citraturia, since it is generally associated to a relatively high intake of animal proteins. Vegan diet can be harmful for urinary stone disease, especially for the risk of hyperuricemia and micronutrient deficiencies, even if only few studies have addressed this specific matter. On the other side, the benefits of a lacto-ovo-vegetarian diet on kidney stone prevention have been largely emphasized, provided that the intake of calcium and oxalate is balanced. Traditional Mediterranean diet should exert a protective effect on nephrolithiasis as well, even if specific studies have not been carried out yet. High phytate and antioxidant content of this diet have however demonstrated to be beneficial in preventing the formation of new or recurrent calculi. Anyway, at the current state of knowledge, the most effective dietary approach to prevent kidney stone disease is a mild animal protein restriction, a balanced intake of carbohydrates and fats and a high intake of fruit and vegetables. Other fundamental aspects, which are often neglected in fad diets, are a normal intake of milk and dairy products and salt restriction. All these nutritional aspects should be greatly taken into account when patients who are willing to undergo fad or commercial diets ask for dietary advice.

Dietary management of idiopathic hyperoxaluria and the influence of patient characteristics and compliance

OBJECTIVE

To assess the efficacy of dietary management for the treatment of idiopathic hyperoxaluria in a large tertiary care center and examine the influence of patient factors, compliance, and follow-up on oxalate reduction, which has not been previously investigated.

METHODS

Retrospectively, 149 patients with kidney stones with idiopathic hyperoxaluria who received dietary management at our stone clinic were evaluated. Changes in urinary parameters on 24-hour urine collections were calculated for all patients and those with abnormal values in the overall short-term (30-240 days) and long-term (>240 days) time periods. Changes in urinary oxalate were evaluated with respect to patient characteristics and compliance measures.

RESULTS

Urine oxalate and supersaturation of calcium oxalate were significantly (P < .001) reduced by 8.9 ± 19.2 mg/d and 1.7 ± 4.3, respectively. A total of 48.3% of the patients reduced their urinary oxalate to normal. Urine oxalate reductions were similar in the short-term and long-term periods. Women lowered urine oxalate nearly twice as much as men (12.7 ± 2.0 mg/d vs 6.7 ± 2.2 mg/d, P = .022) and body mass index (BMI) negatively correlated with oxalate reduction (Pearson's r = -0.213). Reported noncompliance and keeping follow-up appointments did not affect oxalate, however, there was a significant correlation between increasing urine volume and reducing oxalate (Pearson's r = -0.21).

CONCLUSION

This study confirms that meaningful reductions of urine oxalate and supersaturation of calcium oxalate can be achieved with dietary management of hyperoxaluria on a larger clinical scale. Furthermore, we identified that women and patients with low BMIs had greater urine oxalate reductions and urine volume may also be used by clinicians as a measure of dietary compliance.

Response to dietary oxalate after bariatric surgery

BACKGROUND AND OBJECTIVES

Bariatric surgery (BS) may be associated with increased oxalate excretion and a higher risk of nephrolithiasis. This study aimed to investigate urinary abnormalities and responses to an acute oxalate load as an indirect assessment of the intestinal absorption of oxalate in this population.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Twenty-four-hour urine specimens were collected from 61 patients a median of 48 months after BS (post-BS) as well as from 30 morbidly obese (MO) participants; dietary information was obtained through 24-hour food recalls. An oral oxalate load test (OLT), consisting of 2-hour urine samples after overnight fasting and 2, 4, and 6 hours after consuming 375 mg of oxalate (spinach juice), was performed on 21 MO and 22 post-BS patients 12 months after BS. Ten post-BS patients also underwent OLT before surgery (pre-BS).

RESULTS

There was a higher percentage of low urinary volume (<1.5 L/d) in post-BS versus MO (P<0.001). Hypocitraturia and hyperoxaluria (P=0.13 and P=0.36, respectively) were more frequent in BS versus MO patients. The OLT showed intragroup (P<0.001 for all periods versus baseline) and intergroup differences (P<0.001 for post-BS versus MO; P=0.03 for post-BS versus pre-BS). The total mean increment in oxaluria after 6 hours of load, expressed as area under the curve, was higher in both post-BS versus MO and in post-BS versus pre-BS participants (P<0.001 for both).

CONCLUSIONS

The mean oxaluric response to an oxalate load is markedly elevated in post-bariatric surgery patients, suggesting that increased intestinal absorption of dietary oxalate is a predisposing mechanism for enteric hyperoxaluria.

Curative treatment with extracts of Bombax ceiba fruit reduces risk of calcium oxalate urolithiasis in rats

CONTEXT

Drawbacks of presently available treatments for urolithiasis necessitate finding the treatment of hyperoxaluria specifically aimed at reduction in oxalate excretion. Interestingly, many Indian tribes use Bombax ceiba L. (Bombacaceae) fruits as a traditional medicine for the treatment of urinary stones.

OBJECTIVE

The present study investigated the efficacy of B. ceiba fruit extracts as curative agents in experimentally induced calcium oxalate urolithiatic rats.

MATERIALS AND METHODS

Calcium oxalate lithiasis was induced in rats by oral administration of 0.75% ethylene glycol for 14 consecutive days. Treatments with aqueous and ethanol extract of B. ceiba fruit (400 mg/kg body weight) was performed in the same manner for further 14 consecutive days. Cystone (750 mg/kg body weight) was used as reference antiurolithiatic drug. The urinary excretion and kidney deposition of offending salt components, and serum biochemical parameters were investigated.

RESULTS

Oral administration of ethylene glycol resulted in hyperoxaluria and increased renal excretion of calcium and phosphate. However, supplementation with aqueous and ethanol extracts of B. ceiba fruit significantly (p < 0.05) reduced the elevated urinary oxalate, showing a regulatory action on endogenous oxalate synthesis. The increased deposition of stone forming constituents in kidneys of calculogenic rats was also significantly lowered with curative treatment of aqueous and ethanol extract.

DISCUSSION AND CONCLUSION

The results indicate that the fruit of B. ceiba is endowed with lithontriptic activity warranting further development for curative treatment of urolithiasis.

Hyperoxaluria: a gut-kidney axis?

Hyperoxaluria leads to urinary calcium oxalate (CaOx) supersaturation, resulting in the formation and retention of CaOx crystals in renal tissue. CaOx crystals may contribute to the formation of diffuse renal calcifications (nephrocalcinosis) or stones (nephrolithiasis). When the innate renal defense mechanisms are suppressed, injury and progressive inflammation caused by these CaOx crystals, together with secondary complications such as tubular obstruction, may lead to decreased renal function and in severe cases to end-stage renal failure. For decades, research on nephrocalcinosis and nephrolithiasis mainly focused on both the physicochemistry of crystal formation and the cell biology of crystal retention. Although both have been characterized quite well, the mechanisms involved in establishing urinary supersaturation in vivo are insufficiently understood, particularly with respect to oxalate. Therefore, current therapeutic strategies often fail in their compliance or effectiveness, and CaOx stone recurrence is still common. As the etiology of hyperoxaluria is diverse, a good understanding of how oxalate is absorbed and transported throughout the body, together with a better insight in the regulatory mechanisms, is crucial in the setting of future treatment strategies of this disorder. In this review, the currently known mechanisms of oxalate handling in relevant organs will be discussed in relation to the different etiologies of hyperoxaluria. Furthermore, future directions in the treatment of hyperoxaluria will be covered.

Impact of dietary counseling on urinary stone risk parameters in recurrent stone formers

OBJECTIVES

The aim of this study was to investigate the current impact of dietary counseling on the risk for urolithiasis.

METHODS

A retrospective cohort study of the patients treated in our stone clinics from July 2007 to February 2009 was carried out. Patients' urinary risk factors for stone disease were evaluated with pre- and postintervention 24-hour urine collections. All patients received dietary recommendations from a registered dietician at each visit.

RESULTS

One hundred thirty-seven subjects were identified and managed initially with only dietary interventions to address their urinary stone risk parameters. Average follow-up for this group was 15.19 ± 13.7 months. Subjects showed significant changes in urine volume (71.1%, 1.68 ± 0.68 to 2.59 ± 0.80 L/day, p < 0.0001), urine sodium (58.1%, 229.68 ± 72.51 to 144.65 ± 52.70 mmol/day, p < 0.0001), urine calcium (43.8%, 314.33 ± 95.75 to 216.81 ± 80.90 mg/day, p < 0.0001), urinary uric acid (50%, 0.821 ± 0.210 to 0.622 ± 0.128 g/day, p < 0.0001), urinary citrate (50.7%, 583.19 ± 330.86 to 797.36 ± 412.31, p < 0.0001), and urine oxalate (55.5%, 46.28 ± 10.31 to 32.56 ± 9.02 mg/day, p < 0.0001). The supersaturation for calcium oxalate also decreased significantly from baseline (9.34-5.03, p < 0.0001).

CONCLUSION

Urolithiasis is a multifactorial disease requiring a multidisciplinary approach. Our results support the use of dietary counseling by a registered dietician in the management of urolithiasis.

Pyridoxine and dietary counseling for the management of idiopathic hyperoxaluria in stone-forming patients

OBJECTIVES

To examine the effects of dietary manipulation and pyridoxine medical management for idiopathic hyperoxaluria in patients with nephrolithiasis.

METHODS

A retrospective longitudinal study of the patients treated in our stone clinics from July 2007 to February 2009 was performed. All patients were evaluated with pre- and postintervention 24-hour urine collection and met a registered dietician. Recommendations to keep urine volume above 2 L per day, sodium restriction, protein moderation, increased calcium intake with meals and low oxalate diet combined with oral pyridoxine were given. Initial dosage ranged from 50 to 100 mg per day depending on the baseline oxalate level, and was titrated to a maximum of 200 mg daily. Subjects with at least two 24-hour urine collections were included in the study.

RESULTS

Of 314 patients with complete metabolic and urinary profile evaluation, 95 subjects were identified with idiopathic hyperoxaluria. Mean follow-up was 18.4 ± 14.8 months and mean age was 50.3 ± 12.8 years. In patients treated with the combination of dietary counseling and pyridoxine, there was a significant change in urinary parameters in 75% of patients with a significant decrease in urinary oxalate excretion (58.26 ± 27.05 to 40.61 ± 15.04, P < .0001). In all, 39% of the patients had a decrease from a high urine oxalate levels (>40 mg/d) to a normal range urine oxalate (55.30 ± 22.04 to 33.45 ± 3.93, P = .0004). No peripheral neuropathy was reported.

CONCLUSIONS

Dietary management and medical treatment using pyridoxine may be an effective first-line therapy to decrease hyperoxaluria in patients who form stones.

Evidence for net renal tubule oxalate secretion in patients with calcium kidney stones

Little is known about the renal handling of oxalate in patients with idiopathic hypercalciuria (IH). To explore the role of tubular oxalate handling in IH and to evaluate whether differences exist between IH and normal controls, we studied 19 IH subjects, 8 normal subjects, and 2 bariatric stone formers (BSF) during a 1-day General Clinical Research Center protocol utilizing a low-oxalate diet. Urine and blood samples were collected at 30- to 60-min intervals while subjects were fasting and after they ate three meals providing known amounts of calcium, phosphorus, sodium, protein, oxalate, and calories. Plasma oxalate concentrations and oxalate-filtered loads were similar between patients (includes IH and BSF) and controls in both the fasting and fed states. Urinary oxalate excretion was significantly higher in patients vs. controls regardless of feeding state. Fractional excretion of oxalate (FEOx) was >1, suggesting tubular secretion of oxalate, in 6 of 19 IH and both BSF, compared with none of the controls (P < 0.00001). Adjusted for water extraction along the nephron, urine oxalate rose more rapidly among patients than normal subjects with increases in plasma oxalate. Our findings identify tubular secretion of oxalate as a key mediator of hyperoxaluria in calcium stone formers, potentially as a means of maintaining plasma oxalate in a tight range.

Liquid chromatography-tandem mass spectrometry determination of oxalate in spot urine

BACKGROUND

For assessment of total oxalic acid (OX) status, reliable quantification of OX in both urine and plasma is important. For urine, but not plasma, a commercial kit is available. We have recently described a LC-MSMS method for OX in plasma. The aim of the present study was to evaluate the usefulness of this assay for urine. We also wanted to evaluate if 24 h urine collection could be substituted by OX/creatinine-ratio (U-OX/crea) in spot-urine, and establish precursory reference intervals for U-OX/crea in children and adults.

METHODS

Acidified urines were analysed and relevant validation parameters assessed. Diurnal excretion patterns were investigated in nine healthy volunteers on self-chosen diets. For method comparison, 29 urine samples were analysed with both the present method and a commercial urine-oxalate kit. Precursory reference values for U-OX/crea in children and adults (N=103, 1 month-76 years) were calculated.

RESULTS

The within-batch coefficient of variation (CV) was 2.5% and a relative recovery of 97% in urine spiked with 5-200 micromol/L OX was found. The LC-MSMS method gave 7.9% higher OX values compared to the kit. No significant diurnal pattern of U-OX/crea was observed. U-OX/crea in children decreases with age, with no gender dependency. In adults no age variation was found, but females had somewhat higher U-OX/Crea compared to males.

CONCLUSION

The LC-MSMS method has proven useful for urinary OX quantification. Random spot-urine samples can be used. Age-dependent reference limits for U-OX/crea must be applied in children, in contrast to adults.

Diet, but not oral probiotics, effectively reduces urinary oxalate excretion and calcium oxalate supersaturation

We examined the effect of a controlled diet and two probiotic preparations on urinary oxalate excretion, a risk factor for calcium oxalate kidney stone formation, in patients with mild hyperoxaluria. Patients were randomized to a placebo, a probiotic, or a synbiotic preparation. This tested whether these probiotic preparations can increase oxalate metabolism in the intestine and/or decrease oxalate absorption from the gut. Patients were maintained on a controlled diet to remove the confounding variable of differing oxalate intake from food. Urinary oxalate excretion and calcium oxalate supersaturation on the controlled diet were significantly lower compared with baseline on a free-choice diet. Neither study preparation reduced urinary oxalate excretion nor calcium oxalate supersaturation. Fecal lactobacilli colony counts increased on both preparations, whereas enterococcal and yeast colony counts were increased on the synbiotic. Total urine volume and the excretion of oxalate and calcium were all strong independent determinants of urinary calcium oxalate supersaturation. Hence, dietary oxalate restriction reduced urinary oxalate excretion, but the tested probiotics did not influence urinary oxalate levels in patients on a restricted oxalate diet. However, this study suggests that dietary oxalate restriction is useful for kidney stone prevention.

Probiotic-induced reduction of gastrointestinal oxalate absorption in healthy subjects

Both a high dietary oxalate intake and increased intestinal absorption appear to be major causes of elevated urine oxalate, a risk factor for kidney stone formation. By favorably altering the gastrointestinal bacterial population, probiotics have the potential to lower oxalate absorption/urinary excretion. This study assessed whether a 4-wk daily consumption of a commercially available probiotic by 11 healthy volunteers (8 females, 3 males), aged 21-36 y, would decrease oxalate absorption. The study involved the ingestion of a probiotic (VSL#3) for a 4 wk period followed by a 4 wk washout period. Oxalate load tests, providing a total of 80 mg oxalate, were conducted at baseline (pre-probiotic), and after the probiotic and washout periods. In the total subject population, mean total 22 h oxalate absorption at baseline (30.8 %) was significantly higher than after the probiotic (11.6 %) and washout (11.5 %) periods. However, four subjects identified as high oxalate absorbers at baseline had a particularly marked probiotic-induced reduction in oxalate absorption, which largely accounted for the reduction observed in the total subject population. The overall data suggested that in individuals characterized by high oxalate absorption levels, VSL#3 ingestion has the potential to reduce gastrointestinal oxalate absorption, which could decrease risk of kidney stones and other disorders related to hyperoxaluria.

Genetic basis of renal cellular dysfunction and the formation of kidney stones

Nephrolithiasis is a result of formation and retention of crystals within the kidneys. The driving force behind crystal formation is urinary supersaturation with respect to the stone-forming salts, which means that crystals form when the concentrations of participating ions are higher than the thermodynamic solubility for that salt. Levels of supersaturation are kept low and under control by proper functioning of a variety of cells including those that line the renal tubules. It is our hypothesis that crystal deposition, i.e., formation and retention in the kidneys, is a result of impaired cellular function, which may be intrinsic and inherent or triggered by external stimuli and challenges. Cellular impairment or dysfunction affects the supersaturation, by influencing the excretion of participating ions such as calcium, oxalate and citrate and causing hypercalciuria, hyperoxaluria or hypocitraturia. The production and excretion of macromolecular promoters and inhibitors of crystallization is also dependent upon proper functioning of the renal epithelial cells. Insufficient or ineffective crystallization modulators such as osteopontin, Tamm-Horsfall protein, bikunin, etc. are most likely produced by the impaired cells.

Renal physiology of SLC26 anion exchangers

PURPOSE OF REVIEW

The multifunctional anion exchanger family (Slc26) encompasses 11 identified genes, but only 10 encode real proteins (Slc26a10 is a pseudogene). Most of the Slc26 proteins function primarily as anion exchangers, exchanging sulfate, iodide, formate, oxalate, hydroxyl ion, and bicarbonate anions, whereas other Slc26 proteins function as chloride ion channels or anion-gated molecular motors. The aim of this review is to present recent studies on the molecular function of the Slc26 family and its role in renal physiology and pathophysiology.

RECENT FINDINGS

In proximal tubules, Slc26a1 (Sat-1) mediates sulfate and oxalate transport across the basolateral membrane, while Slc26a6 (CFEX, Pat-1) mediates a variety of anion exchange at the apical membrane to facilitate transcellular sodium chloride absorption. Targeted deletion of murine Slc26a6 leads to intestinal hyperabsorption of oxalate, hyperoxaluria, and kidney stones. Slc26a4 (pendrin) and Slc26a7 are expressed in intercalated cells, and are involved in acid-base homeostasis and blood pressure regulation. Messenger RNA for Slc26a2, Slc26a9, and Slc26a11 is also present in the kidney, yet the roles of these family members in renal physiology or pathophysiology are not clear.

SUMMARY

Members of this multifunctional anion transporter family play evolving roles in the etiology of nephrolithiasis (Slc26a6) and hypertension (Slc26a4 and Slc26a6). Other Slc26 family members (Slc26a2, Slc26a9, Slc26a11) express mRNA in the kidney but their roles in renal physiology are not yet known.

Food oxalate: factors affecting measurement, biological variation, and bioavailability

Food and nutrition professionals provide medical nutrition therapy for patients with kidney stones. If the stones contain oxalate or the patient has been diagnosed with hyperoxaluria, reduction of dietary oxalate may be appropriate. Differences in oxalate values for a single food may be due to analytical methods, and/or biological variation from several sources, including cultivar, time of harvest, and growing conditions. Bioavailability of food oxalate and, thus, urine oxalate, will also be affected by salt forms of oxalate, food processing and cooking methods, meal composition, and the presence of Oxalabacter formigenes in the patient's gut. Dietary advice for reducing urinary oxalate should include both reduction of dietary oxalate and simultaneous consumption of calcium-rich food or supplement to reduce oxalate absorption.

Pathophysiology and Management of Calcium Stones

Nephrolithiasis is a common disorder that accounts for significant cost, morbidity, and loss of work. There is a one in eight lifetime chance of being diagnosed with urinary stones. Calcium is the most common component of renal stones in individuals in industrialized nations. Calcium stones form as a result of a variety of environmental and metabolic abnormalities that change the urinary environment and increase supersaturation of stone-forming salts. Understanding the pathophysiology of stone disease can help direct treatment toward correction of the underlying abnormalities. Current medical and dietary therapeutic regimens have been shown to significantly reduce the risk of recurrent stone formation.

Intestinal and renal handling of oxalate loads in normal individuals and stone formers

The renal handling and intestinal absorption of dietary oxalate are believed to be risk factors for calcium oxalate stone formation. In this study, we have examined the time and dose effects of soluble oxalate loads on the intestinal absorption and renal handling of oxalate in six stone formers (SF) and six normal individuals (N) who consumed diets controlled in oxalate and other nutrients. Urinary and plasma oxalate changes were monitored over 24 h after ingestion of 0, 2, 4, and 8 mmole oxalate loads, containing a mixture of (12)C- and (13)C(2)-oxalate. There were significant time and dose dependent changes in urinary oxalate excretion and secretion after these loads. However, there were no significant differences between SF and N in both the intestinal absorption and the renal handling of oxalate loads, as measured by the urinary excretion of oxalate (P = 0.96) and the ratio of oxalate to creatinine clearance (P = 0.34). (13)C(2)-oxalate absorption studies showed three of the subjects, two SF and one N, had enhanced absorption with the 8 mmole load. A clear difference in absorption was demonstrated in these individuals during the 8-24 h interval, suggesting that in these individuals there was greater oxalate absorption in the large intestine as compared to the other subjects. This enhanced absorption of oxalate warrants further characterization.

Use of a probiotic to decrease enteric hyperoxaluria

BACKGROUND

Patients with inflammatory bowel disease have a 10- to 100-fold increased risk of nephrolithiasis, with enteric hyperoxaluria being the major risk factor for these and other patients with fat malabsorptive states. Endogenous components of the intestinal microflora can potentially limit dietary oxalate absorption.

METHODS

Ten patients were studied with chronic fat malabsorption, calcium oxalate stones, and hyperoxaluria thought to be caused by jejunoileal bypass (1) and Roux-en-Y gastric bypass surgery for obesity (4), dumping syndrome secondary to gastrectomy (2), celiac sprue (1), chronic pancreatitis (1), and ulcerative colitis in remission (1). For 3 months, patients received increasing doses of a lactic acid bacteria mixture (Oxadrop), VSL Pharmaceuticals), followed by a washout month. Twenty-four-hour urine collections were performed at baseline and after each month.

RESULTS

Mean urinary oxalate excretion fell by 19% after 1 month (1 dose per day, P < 0.05), and oxalate excretion remained reduced by 24% during the second month (2 doses per day, P < 0.05). During the third month on 3 doses per day oxalate excretion increased slightly, so that the mean was close to the baseline established off treatment. Urinary oxalate again fell 20% from baseline during the washout period. Calcium oxalate supersaturation was reduced while on Oxadrop, largely due to the decrease in oxalate excretion, although mean changes did not reach statistical significance.

CONCLUSION

Manipulation of gastrointestinal (GI) flora can influence urinary oxalate excretion to reduce urinary supersaturation levels. These changes could have a salutary effect on stone formation rates. Further studies will be needed to establish the optimal dosing regimen.

Risk factors associated to kidney stones in primary hyperparathyroidism

Nephrolithiasis is the most important clinical manifestation of primary hyperparathyroidism (PHPT), although nowadays this disorder is often asymptomatic. Clinical or biochemical differences between PHPT patients with and without nephrolithiasis have not been clearly identified in most of the previous studies. The aim of the study was to investigate clinical and biochemical parameters in kidney stone former (SF) and non-stone former (NSF) patients with PHPT in order to identify potential risk factors. Serum and plasma samples from 55 consecutive patients (43 females, 12 males) with PHPT were collected after overnight fasting; 24-h urine collection and a fresh sample of urine for sediment analysis were obtained from all patients. Clinical data were recorded in all. Out of 55 patients, 22 had kidney stones, which were symptomatic in 73%. SFs showed circulating PTH, total and ionized calcium, 1,25 dihydroxyvitamin D3, urinary calcium excretion and 24-h urine oxalate levels significantly higher than NSFs. Hypercalciuria was often concomitant with massive quantities of calcium oxalate crystals in urine sediment. Hypercalciuria and relatively high oxaluria were associated with stone formation with an odds ratio (OR) of 4.0 and 7.0, respectively, which rose to 33.5 when they coexisted. Hypomagnesuria and hypocitraturia were common in at least one third of all PHPT patients, but they were not associated to an increased OR. As expected, they were positively correlated with urine calcium excretion, suggesting that calcium, magnesium and citrate are commonly regulated at renal level. In conclusion, hypercalciuria, higher oxalate excretion and severe PHPT are associated with kidney stones in PHPT.

Intestinal transport of an obdurate anion: oxalate

In this review, we focus on the role of gastrointestinal transport of oxalate primarily from a contemporary physiological standpoint with an emphasis on those aspects that we believe may be most important in efforts to mitigate the untoward effects of oxalate. Included in this review is a general discussion of intestinal solute transport as it relates to oxalate, considering cellular and paracellular avenues, the transport mechanisms, and the molecular identities of oxalate transporters. In addition, we review the role of the intestine in oxalate disease states and various factors affecting oxalate absorption.

The impact of dietary oxalate on kidney stone formation

The role of dietary oxalate in calcium oxalate kidney stone formation remains unclear. However, due to the risk for stone disease that is associated with a low calcium intake, dietary oxalate is believed to be an important contributing factor. In this review, we have examined the available evidence related to the ingestion of dietary oxalate, its intestinal absorption, and its handling by the kidney. The only difference identified to date between normal individuals and those who form stones is in the intestinal absorption of oxalate. Differences in dietary oxalate intake and in renal oxalate excretion are two other parameters that are likely to receive close scrutiny in the near future, because the research tools required for these investigations are now available. Such research, together with more extensive examinations of intestinal oxalate absorption, should help clarify the role of dietary oxalate in stone formation.

Medical management of calcium oxalate urolithiasis

Dietary modifications and medical therapy can reduce the risk of urinary stone formation. Using the described stepwise approach, the medical management of stone disease should become a less daunting task for physicians. Unfortunately, the optimal medical and dietary management of urolithiasis remains uncertain and recurrent stone disease remains a significant cause of morbidity. Approaches to diagnosis and medical management have improved considerably in the last decade and it is hoped that well-designed studies in the near future will help further optimize management.