Effects of calcium and magnesium on urinary oxalate excretion after oxalate loads. J Urol. 2000;163:1565-1569. [PMID: 10751889 DOI: 10.1016/s0022-5347(05)67680-x]

Influence of Dietary Heritage in a Restricted Geographic Area and Role of Food Additives on Risk of Recurrent Kidney Stone

Dietary factors may be implicated in the formation of kidney stones and should be closely monitored. To achieve this aim, patients are routinely assessed by means of generic dietary recall, a tool widely used by authors in a range of extensive patient populations to record food intake; the findings obtained, however, may be skewed due to dietary variations and underestimation of the effect of food additives. Fifty Frequent Kidney Stone Formers (FKSFs, mean age: 54.3 ± 13.9 years) with normal kidney function, absence of comorbidities, and reliable compliance were selected from a total of 68 patients' resident in Sardinia, an Italian island where genetic admixtures have been relatively rare for generations. The study, conducted from 1 January 2020 to 31 December 2023, was aimed at assessing nutritional values based on the meticulous recording of food quantities, quality, and potential modifications related to food preparation. Patients were selected during an initial clinical check-up and all efforts made to ensure they were capable of reliably recording all food and drinks consumed. A seven-day food diary was provided in which food and drink intake and their impact on 24 h urine output was recorded. The following parameters were measured in both foods and urine output: citrates, oxalates, calcium, phosphorous, uric acid, proteins and nitrogen compounds, magnesium, sulfates, potassium, carbohydrates, free fatty acids. Study outcomes established the presence of hypocitraturia, hyperoxaluria, hypercalciuria, and moderately high levels of nitrogen compounds. Univariate analysis followed by multivariate analysis for further confirmation were performed and the following observations made. Citrate intake correlated with citraturia but did not promote oxaluria; calcium intake promoted onset of sulfaturia, azoturia, and ammoniuria, whilst magnesium correlated with magnesiuria but not with oxaluria, calciuria, phosphaturia, and azoturia; sulfate intake elicited onset of azoturia but not kaliuresis; potassium intake promoted oxaluria and protein intake resulted in onset of ammoniuria and azoturia. (A) The chemical composition of urine based on dietary intake is hard to predict without taking into account the presence of dietary and urinary interferents; (B) the geographic isolation of patients studied underlines the importance of epigenetics in maintaining a traditional dietary heritage. (C) Moreover, the widespread use of food additives should consistently be taken into account to ensure a correct diagnosis of FKSF and set up a valid treatment plan.

Magnesium for disease treatment and prevention: emerging mechanisms and opportunities

Magnesium (Mg2+) is a commonly used dietary supplement for the prevention and treatment of diseases. However, the efficacy and mechanisms of action of Mg2+ in most diseases have been controversial because of conflicting findings in earlier studies. Recent clinical and preclinical studies provide novel insights into the use of Mg2+ for the treatment and prevention of diseases affecting different organ systems. In this review, we provide an overview of recent clinical evidence for, and controversies over, the medical benefits of Mg2+. In addition, we critically discuss recent advances in understanding the mechanisms of action of Mg2+, which could enable the development of novel targeted therapies.

Magnesium Decreases Urine Supersaturation but Not Calcium Oxalate Stone Formation in Genetic Hypercalciuric Stone-Forming Rats

BACKGROUND/AIMS

Hypercalciuria is the most common identifiable risk factor predisposing to CaOx stone formation. Increased oral magnesium intake may lead to decreased CaOx stone formation by binding intestinal Ox leading to decreased absorption and/or binding urinary Ox to decrease urinary supersaturation. This study assessed the effect of oral magnesium on 24-h urine ion excretion, supersaturation, and kidney stone formation in a genetic hypercalciuric stone-forming (GHS) rat model of human idiopathic hypercalciuria.

METHODS

When fed the oxalate precursor, hydroxyproline, every GHS rat develops CaOx stones. The GHS rats, fed a normal calcium and phosphorus diet supplemented with hydroxyproline to induce CaOx, were divided into three groups of ten rats per group: control diet with 4.0 g/kg MgO, low MgO diet (0.5 g/kg), and high MgO diet (8 g/kg). At 6 weeks, 24-h urines were collected, and urine chemistry and supersaturation were determined. Stone formation was quantified.

RESULTS

The GHS rats fed the low and high Mg diets had a significant reduction and increase, respectively, in urinary Mg compared to those fed the control diet. Dietary Mg did not alter urine Ca excretion while the low Mg diet led to a significant fall in urinary Ox. Urine supersaturation with respect to CaOx was significantly increased with low Mg, whereas urine supersaturation was significantly decreased with high Mg. There was no effect of dietary Mg on stone formation within 6 weeks of treatment.

CONCLUSION

Dietary magnesium decreases urine supersaturation but not CaOx stone formation in GHS rats.

Zn2+ regulates human oxalate metabolism by manipulating oxalate decarboxylase to treat calcium oxalate stones

A high concentration of oxalate is associated with an increased risk of kidney calcium oxalate (CaOx) stones, and the degradation of exogenous oxalate mostly depends on oxalate-degrading enzymes from the intestinal microbiome. We found that zinc gluconate supplement to patients with CaOx kidney stones could significantly improve the abundance of oxalate metabolizing bacteria in humans through clinical experiments on patients also subjected to antibiotic treatment. The analysis of clinical samples revealed that an imbalance of Lactobacillus and oxalate decarboxylase (OxDC) was involved in the formation of CaOx kidney stones. Then, we identified that Zn²⁺ could be used as an external factor to improve the activity of OxDC and promote Lactobacillus in the intestinal flora, and this treatment achieved a therapeutic effect on rats with stones aggravated by antibiotics. Finally, by analyzing the three-dimensional structure of OxDC and completing in vitro experiments, we propose a model of the Zn²⁺-induced reduction of CaOx kidney stone symptoms in rats by increasing the metabolism of oxalate through the positive effects of Zn²⁺ on Lactobacillus and OxDC.

Alkali Citrate Content of Common Over-the-Counter and Medical Food Supplements

Objective: Potassium citrate effectively decreases kidney stone recurrence, but it is costly and associated with side effects. While several over-the-counter supplements and medical foods purport to provide sufficient citrate to prevent recurrent stones, corroborating data on their actual citrate content is limited. Materials and Methods: Nine common nonprescription products were purchased online. Reported citrate content was obtained from packaging, promotional materials, or ingredient labels. Using a single serving of each product, actual citrate, sodium, potassium, calcium, magnesium, and oxalate content was measured using spectrophotometry and chromatography. Total alkali citrate, cost, and amounts of each component per 10 mEq of alkali citrate were also calculated. Results: Nearly all products contained more citrate than advertised, except for Litholyte^® powder, Litholyte^® Coffee, and Horbäach^® potassium citrate. Per serving, Moonstone^® powder, LithoBalance™, and KSP tabs™ contained the most citrate (means of 63.9, 33.5, and 26.9 mEq, respectively). Moonstone and LithoBalance had the greatest discrepancy between total citrate and alkali citrate (15.7 and 11.8 mEq per serving, respectively). NOW^® potassium citrate was least expensive ($0.04/10 mEq alkali citrate). KSP tabs delivered the most daily sodium (mean 158 mg/10 mEq alkali citrate, Litholyte Coffee provided the most potassium (mean of 13 mEq/10 mEq alkali citrate), and Kidney COP^® provided the most calcium (mean 147 mg/10 mEq alkali citrate). Conclusion: Some common over-the-counter products contain sufficient alkali to potentially promote a citraturic response; Moonstone provides the most alkali citrate, but at a higher cost than other products. Sodium, potassium, and calcium from these products must also be considered in daily consumption.

Unravelling the Complex Relationship between Diet and Nephrolithiasis: The Role of Nutrigenomics and Nutrigenetics

Nephrolithiasis is an increasingly prevalent condition, especially in high income countries, and is associated with high morbidity. Extraordinary progress in genetics made the identification of genetic forms of nephrolithiasis possible. These genetic diseases are usually rare and do not account for the most common forms of nephrolithiasis that are the result of several factors such as environment, dietary habits, and predisposing genes. This knowledge has shaped what we classify as nephrolithiasis, a condition that is now recognized as systemic. How and to what extent all these factors interact with one another and end in kidney stone formation, growth, and recurrence is not completely understood. Two new research fields have recently been trying to give some answers: nutrigenomics and nutrigenetics. These fields have the aim of understanding the intricate diet/genome interface that influences gene expression regulation mainly through epigenetic mechanisms and results in specific medical conditions such as cancer, metabolic syndrome, and cardiovascular diseases. Epigenetics seems to play a crucial role and could represent the link between environmental factors, that we are constantly exposed to, and risk factors for nephrolithiasis. In this systematic review, we summarize all the available evidence of proven or hypothesized epigenetic mechanisms related to nephrolithiasis.

The effects of drinking bicarbonate-rich mineral water in calcium oxalate stone formers: an open label prospective randomized controlled study in an Asian cohort

PURPOSE

To examine the effects of drinking bicarbonate-rich mineral water in patients with calcium oxalate stones.

MATERIALS AND METHODS

This was an open label prospective randomized controlled study comparing the effects of a bicarbonate-rich mineral water versus plain water on urine biochemistry in patients with calcium oxalate stones. The mineral water group were instructed to consume 1.25 L of mineral water per day at meal times, and supplemented by plain water. Their total intake was up to 3 L/day. Control group consumed only plain water up to 3 L/day. 24 h urine analyses were performed at baseline, 1, 4, 8 and 12 weeks after starting protocol.

RESULTS

58 patients were recruited for the study. 51 patients were included in the final analysis. Baseline data were comparable between the two groups. Over the course of 12 weeks, compared to patients drinking plain water, those drinking mineral water had higher overall urinary volume (difference = 644.0 ml/24 h, 95% CI = (206.7, 1081.3)), higher overall urinary magnesium (difference = 1.894 mmol/24 h, 95% CI = (1.006, 2.782)), and pH (difference = 0.477, 95% CI = (0.149, 0.804)). However, there was no difference in urinary oxalate and Tiselius index. Mineral water group had net increase of urinary citrate (at each study point compared to baseline) which was sustained until week 12, whereas plain water group showed no significant change.

CONCLUSIONS

Drinking bicarbonate-rich mineral water in calcium oxalate stone formers increased stone inhibitors such as magnesium, citrate and moderate degree of urinary alkalinization compared to patients drinking plain water, but it did not alter Tiselius index or urinary oxalate after 12 weeks.

Could the region you live in prevent or precipitate kidney stone formation due to mineral intake through tap water? An analysis of nine distribution regions in Flanders

OBJECTIVES

To analyse the mineral content of tap water in Flanders and assess if the region one lives in could prevent or precipitate stone formation due to a difference in mineral content.

METHODS

Data from six water companies providing tap water to nine regions in Flanders regarding calcium, magnesium, potassium, sodium and sulphate content in tap water was retrieved. Minimum and maximum values were collected and compared between the different geographical regions.

RESULTS

The highest calcium level was found in region 9 with a value of 157.0 mg/L, which is almost 10 times the value found in region 8 (16.1 mg/L). Region 6 had the highest magnesium (31.8 mg/L), potassium (30.5 mg/L), sodium (126.6 mg/L) and sulphate (218.5 mg/L) levels. The lowest level of magnesium (2.7 mg/L) was found in region 1, which was almost 12 times lower as in Region 6. Region 9 had the lowest level of potassium (1.5 mg/L), which is a factor 20 lower than Region 6. The lowest sodium and sulphate levels were found in region 8 (6.8 mg/L and 3.0 mg/L), respectively. The difference between the highest and lowest level of sulphate was a factor 70.

CONCLUSION

There is a broad range in the minerals found in tap water between the different production sites in the nine distribution regions in Flanders. However, due to the high standards tap water has to meet in Flanders, the region one lives in will not lead to a higher or lower risk of kidney stone formation if the advised 2 to 3 L per day are consumed.

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.

Association between Dietary Inflammatory Index and kidney stones in US adults: data from the National Health and Nutrition Examination Survey (NHANES) 2007-2016

OBJECTIVE

We evaluate the association between the Dietary Inflammatory Index (DII) and kidney stones.

DESIGN

We performed a cross-sectional analysis using data from National Health and Nutrition Examination Survey (NHANES). Dietary intake information was assessed using first 24-h dietary recall interviews, and the Kidney Conditions were presented by a questionnaire. The primary outcome was to investigate the association between DII and incidence of kidney stones, and the secondary outcome was to assess the association between DII and nephrolithiasis recurrence.

SETTING

The NHANES, 2007-2016.

PARTICIPANTS

The study included 25 984 NHANES participants, whose data on DII and kidney stones were available, of whom 2439 reported a history of kidney stones.

RESULTS

For the primary outcome, after fully multivariate adjustment, DII score is positively associated with the risk of kidney stones (OR = 1·07; 95 % CI 1·04, 1·10). Then, compared Q4 with Q1, a significant 38 % increased likelihood of nephrolithiasis was observed. (OR = 1·38; 95 % CI 1·19, 1·60). For the secondary outcome, the multivariate regression analysis showed that DII score is positively correlated with nephrolithiasis recurrence (OR = 1·07; 95 % CI 1·00, 1·15). The results noted that higher DII scores (Q3 and Q4) are positively associated with a significant 48 % and 61 % increased risk of nephrolithiasis recurrence compared with the reference after fully multivariate adjustment (OR = 1·48; 95 % CI 1·07, 2·05; OR = 1·61; 95 % CI 1·12, 2·31).

CONCLUSIONS

Our findings revealed that increased intake of pro-inflammatory diet, as a higher DII score, is correlated with increased odds of kidney stones incidence and recurrence.

The Relationship between Modern Fad Diets and Kidney Stone Disease: A Systematic Review of Literature

OBJECTIVES

Kidney stone disease (KSD) has a strong association with diet metabolic syndrome. This review aims at exploring the lithogenic risk posed by the current most popular diets. Our approach was to search for the effect of each diet type on the major urinary risk factors, to try to draw conclusions regarding the association of a specific diet type and KSD.

METHODS

This systematic review searched for the available literature exploring the association between the existing popular fad diets and KSD. Articles in English, French and Spanish were included, without restriction of the search period with the final search done in August 2021.

RESULTS

Total number of studies and studies for each diet type was as follows: 22 articles for the low carbohydrate diet, 20 articles for high protein diets, 26 articles for vegetarian and vegan diets. There exists a substantial variability in different low carbohydrate and high protein diets, and considerable overlap between modern popular fad diets. High carbohydrate intake might increase urine uric acid, calcium and oxalate levels. High protein diets increase urine calcium and uric acid and lower urine pH and citrate. Consumption of fruits and vegetables increases the urinary volume and urinary citrate. In vegan diets, sufficient daily calcium intake is important to avoid possible secondary hyperoxaluria.

CONCLUSIONS

Few studies evaluated the direct relationship between modern fad diets and KSD. In general, the reduction of carbohydrate in the diet, and counterbalancing protein rich diets with sufficient intake of fruits and vegetables, seem to play a protective role against KSD formation. Maintaining sufficient calcium intake in vegan and vegetarian diets is important. Additional research is needed to directly evaluate the link between KSD and each diet type.

Diet-related urine collections: assistance in categorization of hyperoxaluria

Hyperoxaluria, one of the major risk factors for calcium oxalate urolithiasis and nephrocalcinosis, causes significant morbidity and mortality and should therefore be detected and treated as soon as possible. An early, consequent and adequate evaluation, but also a distinction between primary (PH) and secondary hyperoxaluria (SH) is therefore essential. We evaluated the usefulness of three consecutive 24-h urine collections under different diets [usual diet, (A), low oxalate diet, (B), high oxalate diet, (C)] to prove SH, or to find evidence of PH by changes in urinary oxalate excretion (Uox). We retrospectively analyzed results from 96 pediatric patients (47 females and 49 males, age 3–18 years) who presented with a history of nephrolithiasis, nephrocalcinosis and/or persistent hematuria in whom hyperoxaluria was found in an initial urine sample. The typical pattern of SH was found in 34 patients (mean Uox (A) 0.85 ± 0.29, (B) 0.54 ± 0.15 and (C) 0.95 ± 0.28 mmol/1.73m²/d). PH was suspected in 13 patients [(A) 1.21 ± 0.75; (B) 1.47 ± 0.51 and (C) 1.60 ± 0.82 mmol/1.73m²/d], but genetically proven only in 1/5 patients examined. No hyperoxaluria was found in 16 patients. Data were inconclusive in 33 patients. Urine collection under different diets is helpful to diagnose secondary hyperoxaluria and may provide evidence, that urinary oxalate excretion is normal. We have now established this procedure as our first diagnostic step before further, more extensive and more expensive evaluations are performed.

Prospective Randomized Evaluation of Idiopathic Hyperoxaluria Treatments

PURPOSE

Calcium oxalate(CaOx) stone formation is influenced by urinary oxalate excretion. Stone formers with elevated urinary oxalate are commonly prescribed a low-oxalate diet or oral supplementation with Vitamin B6 and magnesium to reduce urinary oxalate. This study aims to compare the effects of dietary modification versus supplementation versus a combination of both on urinary oxalate.

MATERIALS AND METHODS

We enrolled patients with a documented history of CaOx stones and newly diagnosed idiopathic hyperoxaluria. Patients were randomized into three treatment groups: low oxalate diet(D), supplementation with 25mg vitamin B6 and 400mg magnesium oxide(S), or both low oxalate diet and B6/magnesium supplementation(DS). Baseline and 3-month post-intervention 24-hour urine tests were obtained. The primary endpoint was change in 24-hour urinary oxalate(Ox24) at 12-weeks. Secondary endpoints included changes in other 24-hour urine parameters, compliance rates, and adverse effect rates.

RESULTS

In total, 164 patients were recruited and 62, 47, and 55 were enrolled into the D, S, and DS groups, respectively. Of these, 99 patients completed the study (56.5% of the D, 72.3% of the S, and 54.6% of the DS groups, respectively). Significant differences were noted in median percent reduction in Ox24 values(-31.1% vs.-16.0% vs.-23.9%,p=0.007) in the D, S, and DS groups, respectively. Furthermore, the percentages of patients within each treatment arm who realized a decrease in Ox24 were also significantly different: D=91.4% vs. S=67.6% vs. DS=86.7%, p=0.027. No significant adverse events were observed in any of the study arms.

CONCLUSION

Low oxalate diet is more effective than B6/magnesium supplementation at lowering urinary oxalate in idiopathic hyperoxaluric stone formers. Combination therapy did not produce greater reductions in urinary oxalate than either of the monotherapy arms suggesting it is of little clinical utility. Further study with long-term longitudinal follow-up is required to determine if these treatment strategies reduce recurrent stone events in this population.

Association Between Serum Magnesium and the Prevalence of Kidney Stones: a Cross-sectional Study

Kidney stones, a painful and costly disease, have become a public health problem worldwide. The aim of this study was to evaluate the association between serum magnesium levels and the prevalence of kidney stones in a large population context. This study was conducted in Xiangya Hospital, Central South University in Changsha, Hunan, China, between October 2013 and December 2015. A total of 6228 subjects aged ≥ 18 years old were included. Kidney stones were diagnosed by (1) direct visualisation of stone(s) on the abdominal ultrasound examination, and (2) presence of stone(s) with a diameter ≥ 4 mm. The chemiluminescence method was used to measure the serum magnesium concentration. The association between serum magnesium and the prevalence of kidney stones was evaluated using logistic and spline regression in a cross-sectional study. The prevalence of kidney stones was 6.1% (7.5% in men and 4.2% in women). Compared with the lowest quartile, the crude odds ratio for kidney stones was 0.62 (95% CI 0.46-0.85) for the highest quartile of serum magnesium, and there was an inverse dose-response relationship (P for trend = 0.038). Similar results were observed for men and women separately. The findings were not materially altered by adjustment for potential confounders. In conclusion, subjects with relatively lower levels of serum magnesium, even though within the normal range, were subject to a higher prevalence of kidney stones in a dose-response relationship manner, indicating that magnesium may play a certain role in the prevention or treatment of kidney stones.

Future treatments for hyperoxaluria

PURPOSE OF REVIEW

The review of potential therapies in the treatment of hyperoxaluria is timely, given the current excitement with clinical trials and the mounting evidence of the importance of oxalate in both kidney stone and chronic kidney disease.

RECENT FINDINGS

Given the significant contribution of both endogenous and dietary oxalate to urinary oxalate excretions, it is not surprising therapeutic targets are being studied in both pathways. This article covers the existing data on endogenous and dietary oxalate and the current targets in these pathways.

SUMMARY

In the near future, there will likely be therapies targeting both endogenous and dietary oxalate, especially in subsets of kidney stone formers.

Supplementation with Bovine Milk or Soy Beverages Recovers Bone Mineralization in Young Growing Rats Fed an Insufficient Diet, in Contrast to an Almond Beverage

BACKGROUND

Nondairy beverages, produced from soy, rice, oat, almond, or coconut, are increasingly being used as alternatives to dairy milk, with the perception that they are healthier and/or more sustainable products than dairy products.

OBJECTIVE

The aim of this study was to compare the effects of supplementing either bovine milk, soy, or almond-based beverages to young, growing rats fed an intact-protein diet or a diet that had protein substituted with amino acids (AA-diet).

METHODS

Three-week-old male Sprague-Dawley rats were randomly assigned to 5 groups (n = 10/group) and fed ad libitum for 4 wk. Two control groups were fed either standard AIN-93G food [20% casein (CN) protein] or AIN-93G with amino acids (AAs) equivalent to CN protein, and water to drink. Three treatment groups were fed AIN-93G AA and supplemented with either bovine ultra-heat treatment (UHT) milk or soy or almond UHT beverages. Rat weight gain and food intakes were recorded. During week 4, body composition was assessed using DEXA to determine lean soft tissue, fat, and bone mass. At trial end, bone biomechanical properties and blood plasma mineral concentrations were measured.

RESULTS

At the end of the trial, animals supplemented with almond beverage were lightest (P > 0.05), with higher plasma calcium concentrations (P > 0.05) and lower bone mineral content (BMC) and bone density (P > 0.05) than animals supplemented with milk or soy beverage. Soy-supplemented animals had similar BMC and bone density compared with milk-supplemented animals, although the soy group gained most weight (P > 0.05) and had the highest fat:lean ratio (P > 0.05) compared with other groups.

CONCLUSIONS

In the model tested, supplementing rats with bovine UHT milk and soy UHT beverage provided favorable bone health outcomes. Conversely, almond UHT beverage was not an effective supplement and could be detrimental to bone mineralization and strength outcomes.

Dietary oxalate and kidney stone formation

Dietary oxalate is plant-derived and may be a component of vegetables, nuts, fruits, and grains. In normal individuals, approximately half of urinary oxalate is derived from the diet and half from endogenous synthesis. The amount of oxalate excreted in urine plays an important role in calcium oxalate stone formation. Large epidemiological cohort studies have demonstrated that urinary oxalate excretion is a continuous variable when indexed to stone risk. Thus, individuals with oxalate excretions >25 mg/day may benefit from a reduction of urinary oxalate output. The 24-h urine assessment may miss periods of transient surges in urinary oxalate excretion, which may promote stone growth and is a limitation of this analysis. In this review we describe the impact of dietary oxalate and its contribution to stone growth. To limit calcium oxalate stone growth, we advocate that patients maintain appropriate hydration, avoid oxalate-rich foods, and consume an adequate amount of calcium.

Influence of protein concentration and quality in a canned diet on urine composition, apparent nutrient digestibility and energy supply in adult cats

BACKGROUND

Protein concentration and quality in cat food can vary considerably, and the impact on feline urine composition and nutrient supply is of high practical relevance. In the present study, 6 canned diets with varying protein concentrations and qualities were fed to 10 healthy adult cats. Protein quality in the diet differed depending on the amount of collagen-rich ingredients. Hydroxyproline concentrations were 2.56-4.45 g/kg dry matter in the high quality and 3.76-9.44 g/kg dry matter in the low quality diets. Protein levels were 36.2, 43.3 and 54.9% in the high quality and 36.7, 45.0 and 56.1% in the low quality groups. Each diet was fed for 6 weeks, using a randomized cross-over design. In the last 2 weeks of each feeding period, urine and faeces of the cats were collected.

RESULTS

Renal calcium (Ca), oxalate (Ox) and citrate excretion were unaffected by the dietary protein concentration, possibly mediated by a high urine volume (24.2-34.2 ml/kg bodyweight (BW)/day) in all groups. However, renal Ox excretion was lower when the high quality diets were fed (P = 0.013). Urinary relative supersaturation (RSS) with calcium oxalate (CaOx) was low in general, but reduced in the high quality groups (P = 0.031). Urinary RSS values for magnesium ammonium phosphate (MAP) were high (2.64-5.00) among all groups. Apparent digestibility of crude protein and most minerals was unaffected by the different diets. Feed intake was higher in the low quality groups (P = 0.026), but BW of the cats did not differ depending on dietary protein quality. BW of the cats increased with increasing dietary protein concentrations (P = 0.003).

CONCLUSION

In conclusion, a high protein canned diet might not be a specific risk factor for CaOx urolith formation in cats. In contrast, all diets resulted in high RSS MAP values, which might be critical concerning MAP crystallization. Protein quality had a minor, but significant impact on urine composition, necessitating further research on this subject. A lower energy supply when feeding a low protein quality can be assumed. Changes in BW were only small and require a careful interpretation.

Addition of calcium compounds to reduce soluble oxalate in a high oxalate food system

Spinach (Spinacia oleracea L.) is often used as a base vegetable to make green juices that are promoted as healthy dietary alternatives. Spinach is known to contain significant amounts of oxalates, which are toxic and, if consumed regularly, can lead to the development of kidney stones. This research investigates adding 50-500mg increments of calcium carbonate, calcium chloride, calcium citrate and calcium sulphate to 100g of raw homogenates of spinach to determine whether calcium would combine with the soluble oxalate present in the spinach. Calcium chloride was the most effective additive while calcium carbonate was the least effective. The formation of insoluble oxalate after incubation at 25°C for 30min is a simple practical step that can be incorporated into the juicing process. This would make the juice considerably safer to consume on a regular basis.

Oxalate, inflammasome, and progression of kidney disease

PURPOSE OF REVIEW

Oxalate is an end product of metabolism excreted via the kidney. Excess urinary oxalate, whether from primary or enteric hyperoxaluria, can lead to oxalate deposition in the kidney. Oxalate crystals are associated with renal inflammation, fibrosis, and progressive renal failure. It has long been known that as the glomerular filtration rate becomes reduced in chronic kidney disease (CKD), there is striking elevation of plasma oxalate. Taken together, these findings raise the possibility that elevation of plasma oxalate in CKD may promote renal inflammation and more rapid progression of CKD independent of primary cause.

RECENT FINDINGS

The inflammasome has recently been identified to play a critical role in oxalate-induced renal inflammation. Oxalate crystals have been shown to activate the NOD-like receptor family, pyrin domain containing 3 inflammasome (also known as NALP3, NLRP3, or cryopyrin), resulting in release of IL-1β and macrophage infiltration. Deletion of inflammasome proteins in mice protects from oxalate-induced renal inflammation and progressive renal failure.

SUMMARY

The findings reviewed in this article expand our understanding of the relevance of elevated plasma oxalate levels leading to inflammasome activation. We propose that inhibiting oxalate-induced inflammasome activation, or lowering plasma oxalate, may prevent or mitigate progressive renal damage in CKD, and warrants clinical trials.

Kinetics of calcium oxalate crystal formation in urine

It is routinely observed that persons with increased urinary stone risk factors do not necessarily form uroliths. Furthermore, stone formers can present with urinalyses that do not reflect the clinical picture. We explain this discrepancy by differences in crystallization kinetics. In 1162 urines, crystallization of Ca-oxalate was induced according to the BONN-Risk-Index (BRI) method. The urine's relative light transmissivity (RLT) was recorded from 100 % at start of titration to 95 % due to nuclei formation and crystal growth. From the RLT changes, a measure of the thermodynamic inhibition threshold of crystal formation (BRI) and of crystal growth kinetics is derived ("turbidity slope" after crystallization onset). On average, subjects presenting with a low inhibition threshold, i.e., high BRI, also present significantly higher crystal growth rates compared with subjects in lower BRI classes. Only subjects in the highest BRI class show a lower growth rate than expected, probably due to a depletion of supersaturation by massive initial nucleation. With increasing thermodynamic risk of crystal formation (i.e., increasing BRI) due to an imbalance between inhibitors and promoters of crystal formation, an increase in the imbalance between inhibitors and promoters of crystal growth (i.e., increasing growth rate) is observed. Both lead to an increased urolith formation risk. Healthy subjects with increased BRI are an exception to this trend: their urine is thermodynamically prone to form stones, but they show a kinetic inhibition preventing nuclei from significant growth.

The management of patients with enteric hyperoxaluria

Enteric hyperoxaluria is a common occurrence in the setting of fat malabsorption, usually due to intestinal resection or intestinal bypass surgery. Enhanced intestinal absorption of dietary oxalate leads to elevated renal oxalate excretion, frequently in excess of 100 mg/d (1.14 mmol/d). Patients are at increased risk of urolithiasis and loss of kidney function from oxalate nephropathy. Fat malabsorption causes increased binding of diet calcium by free fatty acids, reducing the calcium available to precipitate diet oxalate. Delivery of unabsorbed bile salts and fatty acids to the colon increases colonic permeability, the site of oxalate hyper-absorption in enteric hyperoxaluria. The combination of soluble oxalate in the intestinal lumen and increased permeability of the colonic mucosa leads to hyperoxaluria. Dietary therapy consists of limiting oxalate and fat intake. The primary medical intervention is the use of oral oxalate binding agents such as calcium salts to reduce free intestinal oxalate levels. Bile acid sequestrants can be useful in patients with ileal resection and bile acid malabsorption. Oxalate degrading bacteria provided as probiotics are being investigated but as of yet, no definite benefit has been shown with currently available preparations. The current state of medical therapy and potential future directions will be summarized in this article.

Lowering urinary oxalate excretion to decrease calcium oxalate stone disease

Dietary modifications should be considered as a first line approach in the treatment of idiopathic calcium oxalate nephrolithiasis. The amounts of oxalate and calcium consumed in the diet are significant factors in the development of the disease due to their impact on urinary oxalate excretion. There are a number of strategies that can be employed to reduce oxalate excretion. The consumption of oxalate-rich foods should be avoided and calcium intake adjusted to 1000-1200 mg/day. To encourage compliance it should be emphasized to patients that they be vigilant with this diet as a deviation in any meal or snack could potentially result in significant stone growth. The evidence underlying these two modifications is outlined and other strategies to reduce urinary oxalate excretion are reviewed.

Medical and alternative therapies in urinary tract stone disease

Nephrolithiasis is a serious problem for both patients and the health system. Recurrence stands out as a significant problem in urinary system stone disease, the prevalence of which is increasing gradually. If recurrence is not prevented, patients may go through recurrent operations due to nephrolithiasis. While classical therapeutic options are available for all stone types, the number of randomized controlled studies and extensive meta-analyses focusing on their efficiency are inadequate. Various alternative therapeutic options to these medical therapies also stand out in recent years. The etiology of urolithiasis is multifactorial and not always related to nutritional factors. Nutrition therapy seems to be useful, either along with pharmacological therapy or as a monotherapy. General nutrition guidelines are useful in promoting public health and developing nutrition plans that reduce the risk or attenuate the effects of diseases affected by nutrition. Nutrition therapy involves the evaluation of a patient’s nutritional state and intake, the diagnosis of nutrition risk factors, and the organization and application of a nutrition program. The main target is the reduction or prevention of calculus formation and growth via decreasing lithogenic risk factors and increasing lithogenic inhibitors in urine. This review focuses briefly on classical medical therapy, along with alternative options, related diets, and medical expulsive therapy.

Antinephrolithiatic and antioxidative efficacy of Dolichos biflorus seeds in a lithiasic rat model

CONTEXT

Dolichos biflorus sensu auct non L. (Fabaceae) is widely used for the treatment of kidney stones, leucorrhoea, urinary disorders, and menstrual troubles, and is known for its antioxidant activity.

OBJECTIVES

To evaluate the preventive effect of hydro-alcoholic extract of Dolichos biflorus seeds (DBE) in ethylene glycol induced nephrolithiasis.

MATERIALS AND METHODS

In vitro antioxidative capacity of DBE was estimated in terms of reducing power, superoxide radical, 2,2- diphenyl-1-picrylhydrazyl radical, and nitric oxide scavenging activity. A validated HPLC method was used for standardization using quercetin as a marker. Adult female Wistar rats were administered with DBE (150 and 300 mg/kg body weight/day) along with ethylene glycol (0.75%, v/v) for 28 d. The various biochemical parameters were measured in urine, serum, and kidney followed by histochemistry.

RESULTS

Ethylene glycol caused a significant increase in calcium, oxalate, phosphate, and total protein in urine as well as in kidney whereas decrease in calcium, sodium, and magnesium in serum was observed (p < 0.001). Ethylene glycol also caused a significant increase in lipid peroxidation and concurrent decrease in activities of antioxidant enzymes in kidney (p < 0.001). However, the seed extract of D. biflorus caused significant restoration of all these parameters (p < 0.001). Histopathological and histochemical studies also showed the reduced calcifications in kidney of seed extract treated rats.

DISCUSSION AND CONCLUSION

These results indicated that seeds of D. biflorus have significant prophylactic effect in preventing the nephrolithiasis, which might be due to the antioxidant activity of the active compounds of the plant.

Secondary hyperoxaluria: a risk factor for kidney stone formation and renal failure in native kidneys and renal grafts

Secondary hyperoxaluria is a multifactorial disease affecting several organs and tissues, among which stand native and transplanted kidneys. Nephrocalcinosis and nephrolithiasis may lead to renal insufficiency. Patients suffering from secondary hyperoxaluria, should be promptly identified and appropriately treated, so that less renal damage occurs. The aim of this review is to underline the causes of hyperoxaluria and the related pathophysiologic mechanisms, which are involved, along with the description of seven cases of irreversible renal graft injury due to secondary hyperoxaluria.

Diet and risk of kidney stones in the Oxford cohort of the European Prospective Investigation into Cancer and Nutrition (EPIC)

The lifetime prevalence of kidney stones is around 10 % and incidence rates are increasing. Diet may be an important determinant of kidney stone development. Our objective was to investigate the association between diet and kidney stone risk in a population with a wide range of diets. This association was examined among 51,336 participants in the Oxford arm of the European Prospective Investigation into Cancer and Nutrition using data from Hospital Episode Statistics in England and Scottish Morbidity Records. In the cohort, 303 participants attended hospital with a new kidney stone episode. Cox proportional hazards regression was performed to calculate hazard ratios (HR) and their 95 % confidence intervals (95 % CI). Compared to those with high intake of meat (>100 g/day), the HR estimates for moderate meat-eaters (50-99 g/day), low meat-eaters (<50 g/day), fish-eaters and vegetarians were 0.80 (95 % CI 0.57-1.11), 0.52 (95 % CI 0.35-0.8), 0.73 (95 % CI 0.48-1.11) and 0.69 (95 % CI 0.48-0.98), respectively. High intakes of fresh fruit, fibre from wholegrain cereals and magnesium were also associated with a lower risk of kidney stone formation. A high intake of zinc was associated with a higher risk. In conclusion, vegetarians have a lower risk of developing kidney stones compared with those who eat a high meat diet. This information may be important to advise the public about prevention of kidney stone formation.

A functional analysis of the formyl-coenzyme A (frc) gene from Lactobacillus reuteri 100-23C

AIM

To examine the role of the Lactobacillus reuteri 100-23C frc gene product in oxalate metabolism, host colonization and the acid stress response.

METHODS AND RESULTS

Genes encoding putative formyl-CoA transferase (frc) and oxalyl-CoA decarboxylase (oxc) enzymes are present in the genome sequences of Lact. reuteri strains. Two strains isolated from humans harboured an IS200 insertion sequence in the frc ORF and a group 2 intron-associated transposase downstream of the frc gene, both of which were lacking in two strains of animal origin, which contained intact frc and oxc genes. An frc(-) insertional mutant of Lact. reuteri 100-23C was compared with the parent strain with respect to oxalate degradation, colonization of an RLF-mouse host model and growth in the presence of acids. Neither parent nor mutant degraded oxalate in vitro or in vivo. However, the parent outcompeted the frc(-) mutant in the mouse intestine during co-colonization and the frc(-) mutant showed a reduced growth rate in the presence of hydrochloric acid.

CONCLUSIONS

Intact oxc and frc genes do not ensure oxalate degradation under the conditions tested. The frc gene product is important during host colonization and survival of acid stress by Lact. reuteri 100-23C.

SIGNIFICANCE AND IMPACT OF THE STUDY

Oxalate metabolism by oxalate-degrading intestinal bacterial strains may be important in preventing urolithiasis and might lead to the derivation of probiotic products. To produce safe and efficacious probiotics, however, an understanding of the genetic characteristics of potential oxalate degraders must be obtained, together with knowledge of their functional ramifications.

Effects of potassium chloride and potassium bicarbonate in the diet on urinary pH and mineral excretion of adult cats

Low dietary K levels have been associated with increasing renal Ca excretion in humans, indicating a higher risk of calcium oxalate (CaOx) urolith formation. Therefore, the present study aimed to investigate whether dietary K also affects the urine composition of cats. A total of eight adult cats were fed diets containing 0·31 % native K and 0·50, 0·75 and 1·00 % K from KCl or KHCO₃ and were evaluated for the effects of dietary K. High dietary K levels were found to elevate urinary K concentrations (P<0·001). Renal Ca excretion was higher in cats fed the KCl diets than in those fed the KHCO₃ diets (P=0·026), while urinary oxalate concentrations were generally lower in cats fed the KCl diets and only dependent on dietary K levels in cats fed the KHCO₃ diets (P<0·05). Fasting urine pH increased with higher dietary K levels (P=0·022), reaching values of 6·38 (1·00 % KCl) and 7·65 (1·00 % KHCO₃). K retention was markedly negative after feeding the cats with the basal diet (-197 mg/d) and the 0·50 % KCl diet (-131 mg/d), while the cats tended to maintain their balance on being fed the highest-KCl diet (-23·3 mg/d). In contrast, K from KHCO₃ was more efficiently retained (P=0·018), with K retention being between -82·5 and 52·5 mg/d. In conclusion, the dietary inclusion of KHCO₃ instead of KCl as K source could be beneficial for the prevention of CaOx urolith formation in cats, since there is an association between a lower renal Ca excretion and a generally higher urine pH. The utilisation of K is distinctly influenced by the K salt, which may be especially practically relevant when using diets with low K levels.

Lanthanum carbonate inhibits intestinal oxalate absorption and prevents nephrocalcinosis after oxalate loading in rats

PURPOSE

Increased intestinal oxalate absorption leads to increased urinary oxalate excretion (secondary hyperoxaluria) and calcium oxalate crystal formation, contributing to nephrocalcinosis/lithiasis. Lanthanum carbonate is an intestinal phosphate binder that is orally administered to patients on dialysis to treat hyperphosphatemia. It is hypothesized that lanthanum can also bind oxalate, in addition to phosphate. We evaluated this in vitro and in vivo.

MATERIALS AND METHODS

In vitro oxalate binding was evaluated by oxalate precipitation from a solution by lanthanum. In vivo oxalate absorption kinetics and the effect of lanthanum carbonate on nephrocalcinosis development were assessed in male Sprague-Dawley® rats that received 1) 1,000 mg lanthanum carbonate and oxalate, 2) carboxymethylcellulose and oxalate or 3) carboxymethylcellulose by gavage for up to 12 hours (kinetics) or 7 days (nephrocalcinosis). Plasma and urinary oxalate concentrations were measured at several time points after gavage. The degree of nephrocalcinosis was assessed histomorphometrically on von Kossa stained sections and by measuring total calcium content in renal tissue.

RESULTS

In vitro lanthanum bound oxalate in a pH range comparable to the range of the intestine. In vivo oxalate administration in untreated animals resulted in a biphasic pattern of increased plasma oxalate levels, which was almost abolished in lanthanum treated rats. In the urine of treated rats oxaluria and calcium oxalate crystalluria were blunted. Moreover, significantly decreased nephrocalcinosis was observed compared with that in untreated rats.

CONCLUSIONS

Lanthanum carbonate is a promising agent for the future prevention/treatment of secondary hyperoxaluria.

Dietary and animal-related factors associated with the rate of urinary oxalate and calcium excretion in dogs and cats

This paper reports the results of a cohort study and randomised clinical trial (RCT) in cross-over design. In the cohort study, the range of urinary oxalate (Uox) and calcium (Uca) excretion was determined within a sample of the Dutch population of dogs and cats, and dietary and animal-related factors associated with these urine parameters were identified. Spot urine samples were collected from privately owned dogs (n=141) and cats (n=50). The RCT determined the effect of a commercial raw meat diet versus a dry diet on Uox and Uca excretion rate in 23 dogs. In the cohort study, Uox excretion ranged from 21.1 to 170.6 mmol oxalate/mol creatinine in dogs and 27.5 to 161.6 in cats. Urinary calcium excretion ranged from 3.4 to 462.8 mmol calcium/mol creatinine in dogs and 10.1 to 128.0 in cats. In dogs, increased Uox and Uca excretion was associated with (1) the intake of a dry diet as the primary source of energy, (2) receiving no snacks and (3) breed. Increased Uox excretion was associated with males as well. In cats, urine collection in anaesthetised subjects was identified as a confounder. In the RCT, feeding the dry diet resulted in higher Uox (P<0.001) and Uca (P=0.021) excretion rates in dogs.

Changes in dietary macronutrient profile do not appear to affect endogenous urinary oxalate excretion in healthy adult cats

The progressive increase in calcium oxalate uroliths reported in cats diagnosed with urolithiasis may partly be due to changes in nutrition. Since cats have a predominant mitochondrial alanine:glyoxylate aminotransferase 1 (AGT1) location, high carbohydrate intake may induce endogenous oxalate synthesis. This hypothesis was tested by feeding 12 adult, female cats three diets differing in macronutrients, namely, high protein (HP), high carbohydrate (HC) and high fat (HF), using a randomised Latin square design in a 36-day study. In addition to plasma, urine was collected quantitatively using modified litter boxes. A pilot study with four cats, conducted to determine the adaptation time of urinary oxalate (Uox) excretion to a dietary change, indicated a mean (± SEM) adaptation time of 5.9 ± 0.7 days, with the urinary oxalate:creatinine (Ox:Cr) ratio increasing from 36.1 ± 3.7 to 81.6 ± 2.3 mmol/mol. In the main study, plasma oxalate concentration was significantly lower when feeding the HP compared to the HF (P=0.003) diet, whereas Uox excretion (μmol/kg BW(0.75)/day) and the urinary Ox:Cr ratio were unaffected by diet. The Uox concentration (mmol/L) was significantly lower when feeding the HP compared to the HC (P=0.004) and HF (P=0.001) diets. The results indicate that changes in macronutrient profile may not influence endogenous Uox excretion in cats but high dietary protein did reduce Uox concentration and may therefore help to lower the risk of calcium oxalate formation.

Net intestinal transport of oxalate reflects passive absorption and SLC26A6-mediated secretion

Mice lacking the oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium-oxalate stones as a result of a defect in intestinal oxalate secretion, but what accounts for the absorptive oxalate flux remains unknown. We measured transepithelial absorption of [(14)C]oxalate simultaneously with the flux of [(3)H]mannitol, a marker of the paracellular pathway, across intestine from wild-type and Slc26a6-null mice. We used the anion transport inhibitor DIDS to investigate other members of the SLC26 family that may mediate transcellular oxalate absorption. Absorptive flux of oxalate in duodenum was similar to mannitol, insensitive to DIDS, and nonsaturable, indicating that it is predominantly passive and paracellular. In contrast, in wild-type mice, secretory flux of oxalate in duodenum exceeded that of mannitol, was sensitive to DIDS, and saturable, indicating transcellular secretion of oxalate. In Slc26a6-null mice, secretory flux of oxalate was similar to mannitol, and no net flux of oxalate occurred. Absorptive fluxes of both oxalate and mannitol varied in parallel in different segments of small and large intestine. In epithelial cell lines, modulation of the charge selectivity of the claudin-based pore pathway did not affect oxalate permeability, but knockdown of the tight-junction protein ZO-1 enhanced permeability to oxalate and mannitol in parallel. Moreover, formation of soluble complexes with cations did not affect oxalate absorption. In conclusion, absorptive oxalate flux occurs through the paracellular "leak" pathway, and net absorption of dietary oxalate depends on the relative balance between absorption and SLC26A6-dependent transcellular secretion.

Challenges for epidemiologic research on the verge of a new era

Although risk factor epidemiology has achieved much, it has its limitations (e.g., a failure to reveal causal mechanisms at multiple levels). To illustrate contemporary challenges for epidemiological research, we present a dialog with examples and argue for incorporating a "systems thinking through a life course" paradigm in epidemiological research. There is an increasing interest in moving part of public health from a discipline concerned primarily with risk factors at the individual level toward one concerned with complex causal patterns which often operate across different levels in time and space (e.g., from the molecular to the population, from the past to the future, and from the distal to the proximal). However, the methodology for discovering these complex and dynamic relationships remains to be improved. We propose strategies for taking up this challenge.

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.

Probiotics and other key determinants of dietary oxalate absorption

Oxalate is a common component of many foods of plant origin, including nuts, fruits, vegetables, grains, and legumes, and is typically present as a salt of oxalic acid. Because virtually all absorbed oxalic acid is excreted in the urine and hyperoxaluria is known to be a considerable risk factor for urolithiasis, it is important to understand the factors that have the potential to alter the efficiency of oxalate absorption. Oxalate bioavailability, a term that has been used to refer to that portion of food-derived oxalate that is absorbed from the gastrointestinal tract (GIT), is estimated to range from 2 to 15% for different foods. Oxalate bioavailability appears to be decreased by concomitant food ingestion due to interactions between oxalate and coingested food components that likely result in less oxalic acid remaining in a soluble form. There is a lack of consensus in the literature as to whether efficiency of oxalate absorption is dependent on the proportion of total dietary oxalate that is in a soluble form. However, studies that directly compared foods of varying soluble oxalate contents have generally supported the proposition that the amount of soluble oxalate in food is an important determinant of oxalate bioavailability. Oxalate degradation by oxalate-degrading bacteria within the GIT is another key factor that could affect oxalate absorption and degree of oxaluria. Studies that have assessed the efficacy of oral ingestion of probiotics that provide bacteria with oxalate-degrading capacity have led to promising but generally mixed results, and this remains a fertile area for future studies.

Influence of nutrition on feline calcium oxalate urolithiasis with emphasis on endogenous oxalate synthesis

The prevalence of calcium oxalate (CaOx) uroliths detected in cats with lower urinary tract disease has shown a sharp increase over the last decades with a concomitant reciprocal decrease in the occurrence of struvite (magnesium ammonium phosphate) uroliths. CaOx stone-preventative diets are available nowadays, but seem to be marginally effective, as CaOx urolith recurrence occurs in patients fed these diets. In order to improve the preventative measures against CaOx urolithiasis, it is important to understand its aetiopathogenesis. The main research focus in CaOx formation in cats has been on the role of Ca, whereas little research effort has been directed towards the role and origin of urinary oxalates. As in man, the exogenous origin of urinary oxalates in cats is thought to be of minor importance, although the precise contribution of dietary oxalates remains unclear. The generally accepted dietary risk factors for CaOx urolithiasis in cats are discussed and a model for the biosynthetic pathways of oxalate in feline liver is provided. Alanine:glyoxylate aminotransferase 1 (AGT1) in endogenous oxalate metabolism is a liver-specific enzyme targeted in the mitochondria in cats, and allows for efficient conversion of glyoxylate to glycine when fed a carnivorous diet. The low peroxisomal activity of AGT1 in cat liver is compatible with the view that felids utilised a low-carbohydrate diet throughout evolution. Future research should focus on understanding de novo biosynthesis of oxalate in cats and their adaptation(s) in oxalate metabolism, and on dietary oxalate intake and absorption by cats.

Bioavailability of magnesium from different pharmaceutical formulations

Magnesium is suggested to reduce intestinal oxalate absorption and to act as an inhibitor of calcium oxalate crystallization in the urine. However, previous studies have shown only minimal increase in urinary magnesium excretion following oral magnesium supplementation, possibly due to its low bioavailability. This study was performed to examine the bioavailability of magnesium from two different pharmaceutical formulations of magnesium oxide (MgO). Thirteen healthy male volunteers (22-31 years) were recruited from university students and staff, and all completed the study. During the baseline phase, subjects collected two 24-h urines while on their usual diet. Throughout the control and test phases, the subjects consumed a standardized diet calculated according to the recommendations. During the test phases, subjects received two magnesium preparations in a cross-over procedure. With each preparation, MgO-capsules and MgO-effervescent tablets, 450 mg magnesium was supplemented. On the control day and the two test days, fractional urine collection was performed and six corresponding blood samples were taken. In the follow-up phase, subjects continued to take the respective preparation while on their usual diet and collected 24-h urines weekly. With standardized conditions, urinary magnesium excretion increased by 40% after ingestion of the effervescent tablets, and by only 20% after intake of the capsules. The results indicate better bioavailability of magnesium from the effervescent tablets than from the capsules. This may be attributed to the fact that the tablets have to be dissolved in water before ingestion so that magnesium becomes ionized, which is an important precondition for absorption.

In-vivo evaluation in rats of colon-specific microspheres containing 5-fluorouracil

The aims of this investigation were to determine the distribution in the gastrointestinal (GI) tract of Eudragit S-100 encapsulated colon-specific sodium alginate microspheres containing 5-fluorouracil (5-FU) in rats, and to perform pharmacokinetic and pharmacodynamic studies. Comparisons were with a control immediate-release (IR) formulation of 5-FU. 5-FU was distributed predominantly in the upper GI tract from the IR formulation but was distributed primarily to the lower part of the GI tract from the microsphere formulation. No drug was released in the stomach and intestinal regions from the colon-specific microspheres. Significantly, a high concentration of the active drug was achieved in colonic tissues from the colon-specific microspheres (P < 0.001), which was higher than the IC50 required to halt the growth of and/or kill colon cancer cells. Colon cancer was induced in rats by subcutaneous injection of 1,2-dimethylhydrazine (40 mg kg (-1)) for 10 weeks. The tumours induced were non-invasive adenocarcinomas and were in Duke's stage A. The 5-FU formulations were administered for 4 weeks after tumour induction. Non-significant reductions in tumour volume and multiplicity were observed in animals given the colon-specific microspheres. Enhanced levels of liver enzymes (SGOT, SGPT and alkaline phosphatase) were found in animals given the IR formulation of 5-FU, and values differed significantly (P < 0.001) from those in animals treated with the colon-specific microspheres. Elevated levels of serum albumin and creatinine, and leucocytopenia and thrombocytopenia were observed in the animals given the IR formulation. In summary, Eudragit S-100 coated alginate microspheres delivered 5-FU to colonic tissues, with reduced systemic side-effects. A long-term dosing study is required to ascertain the therapeutic benefits.