Effect of dietary modification on urinary stone risk factors

Beyond the Urine Anion Gap: In Support of the Direct Measurement of Urinary Ammonium

Ammonium is a major urinary buffer that is necessary for the normal excretion of the daily acid load. Its urinary rate of excretion (UNH₄) may be increased several fold in the presence of extrarenal metabolic acidosis. Therefore, measurement of UNH₄ can provide important clues about causes of metabolic acidosis. Because UNH₄ is not commonly measured in clinical laboratories, the urinary anion gap (UAG) was proposed as its surrogate about 4 decades ago, and it is still frequently used for that purpose. Several published studies strongly suggest that UAG is not a good index of UNH₄ and support the concept that direct measurement of UNH₄ is an important parameter to define in clinical nephrology. Low UNH₄ levels have recently been found to be associated with a higher risk of metabolic acidosis, loss of kidney function, and death in persons with chronic kidney disease, while surrogates like the UAG do not recapitulate this risk. In order to advance the field it is necessary for the medical community to become more familiar with UNH₄ levels in a variety of clinical settings. Herein, we review the literature, searching for available data on UNH₄ under normal and various pathological conditions, in an attempt to establish reference values to interpret UNH₄ results if and when UNH₄ measurements become available as a routine clinical test. In addition, we present original data in 2 large populations that provide further evidence that the UAG is not a good predictor of UNH₄. Measurement of urine NH₄ holds promise to aid clinicians in the care of patients, and we encourage further research to determine its best diagnostic usage.

Initial Results from the M-STONE Group: A Multi-Center Collaboration to Study Treatment Outcomes in Nephrolithiasis Evaluation

Introduction: Despite proven effectiveness of medications in preventing stone recurrence, compliance with pharmacotherapy (PT) is often poor because of cost, side effects, and impact on lifestyle. We sought to compare the risk of stone recurrence between patients managed with conservative therapy (CT) vs PT controlling for aggressiveness of stone disease. Materials and Methods: The Multi-center collaboration to Study Treatment Outcomes in Nephrolithiasis Evaluation (MSTONE) database contains patient data and outcomes from July 2001 to April 2015 across four centers. The database was queried for patients whose stone disease was managed with CT alone (fluid and dietary recommendations) vs PT. Patients were risk stratified according to number of previous passed stones. Within each risk group, we compared CT vs PT with respect to 2-year stone event rate and stone event-free survival (SEFS) using the Kaplan-Meier method. Results: A total of 245 patients, with a median follow-up of 29 months (interquartile range = 16-44), were identified, including 93 on CT and 152 on PT. The overall 2-year stone event rate was 38% for all patients. Stone events at 2 years occurred less frequently in the PT group compared with the CT group (31% vs 44%, p = 0.043), with the difference most pronounced in the high-risk group (71% vs 32% for CT and PT, respectively, p = 0.058). The 30-month SEFS was significantly higher for PT (58%) than CT (46%) overall. When stratified by risk group, 30-month SEFS was statistically significantly higher for PT than CT in the intermediate risk group (65% vs 45% for PT and CT, respectively). Conclusion: Controlling for aggressiveness of stone disease, PT was more effective than CT in reducing and delaying stone-related events. However, CT appeared to be as effective as PT in low-risk patients. PT is best reserved for recurrent stone formers, regardless of metabolic background.

The Serum Metabolome Identifies Biomarkers of Dietary Acid Load in 2 Studies of Adults with Chronic Kidney Disease

BACKGROUND

Dietary acid load is a clinically important aspect of the diet that reflects the balance between acid-producing foods, for example, meat and cheese, and base-producing foods, for example, fruits and vegetables.

METHODS

We used metabolomics to identify blood biomarkers of dietary acid load in 2 independent studies of chronic kidney disease patients: the African American Study of Kidney Disease and Hypertension (AASK, n = 689) and the Modification of Diet in Renal Disease (MDRD, n = 356) study. Multivariable linear regression was used to assess the cross-sectional association between serum metabolites whose identity was known (outcome) and dietary acid load (exposure), estimated with net endogenous acid production (NEAP) based on 24-h urine urea nitrogen and potassium, and adjusted for age, sex, race, randomization group, measured glomerular filtration rate, log-transformed urine protein-to-creatinine ratio, history of cardiovascular disease, BMI, and smoking status.

RESULTS

Out of the 757 known, nondrug metabolites identified in AASK, 26 were significantly associated with NEAP at the Bonferroni threshold for significance (P < 6.6 × 10-5). Twenty-three of the 26 metabolites were also identified in the MDRD study, and 13 of the 23 (57%) were significantly associated with NEAP (P < 2.2 × 10-3), including 5 amino acids (S-methylmethionine, indolepropionylglycine, indolepropionate, N-methylproline, N-δ-acetylornithine), 2 cofactors and vitamins (threonate, oxalate), 1 lipid (chiro-inositol), and 5 xenobiotics (methyl glucopyranoside, stachydrine, catechol sulfate, hippurate, and tartronate). Higher levels of all 13 replicated metabolites were associated with lower NEAP in both AASK and the MDRD study.

CONCLUSION

Metabolomic profiling of serum specimens from kidney disease patients in 2 study populations identified 13 replicated metabolites associated with dietary acid load. Additional studies are needed to validate these compounds in healthy populations. These 13 compounds may potentially be used as objective markers of dietary acid load in future nutrition research studies.

Association of Urinary Oxalate Excretion With the Risk of Chronic Kidney Disease Progression

IMPORTANCE

Oxalate is a potentially toxic terminal metabolite that is eliminated primarily by the kidneys. Oxalate nephropathy is a well-known complication of rare genetic disorders and enteric hyperoxaluria, but oxalate has not been investigated as a potential contributor to more common forms of chronic kidney disease (CKD).

OBJECTIVE

To assess whether urinary oxalate excretion is a risk factor for more rapid progression of CKD toward kidney failure.

DESIGN, SETTING, AND PARTICIPANTS

This prospective cohort study assessed 3123 participants with stages 2 to 4 CKD who enrolled in the Chronic Renal Insufficiency Cohort study from June 1, 2003, to September 30, 2008. Data analysis was performed from October 24, 2017, to June 17, 2018.

EXPOSURES

Twenty-four-hour urinary oxalate excretion.

MAIN OUTCOMES AND MEASURES

A 50% decline in estimated glomerular filtration rate (eGFR) and end-stage renal disease (ESRD).

RESULTS

This study included 3123 participants (mean [SD] age, 59.1 [10.6] years; 1414 [45.3%] female; 1423 [45.6%] white). Mean (SD) eGFR at the time of 24-hour urine collection was 42.9 (16.8) mL/min/1.73 m2. Median urinary excretion of oxalate was 18.6 mg/24 hours (interquartile range [IQR], 12.9-25.7 mg/24 hours) and was correlated inversely with eGFR (r = -0.13, P < .001) and positively with 24-hour proteinuria (r = 0.22, P < .001). During 22 318 person-years of follow-up, 752 individuals reached ESRD, and 940 individuals reached the composite end point of ESRD or 50% decline in eGFR (CKD progression). Higher oxalate excretion was independently associated with greater risks of both CKD progression and ESRD: compared with quintile 1 (oxalate excretion, <11.5 mg/24 hours) those in quintile 5 (oxalate excretion, ≥27.8 mg/24 hours) had a 33% higher risk of CKD progression (hazard ratio [HR], 1.33; 95% CI, 1.04-1.70) and a 45% higher risk of ESRD (HR, 1.45; 95% CI, 1.09-1.93). The association between oxalate excretion and CKD progression and ESRD was nonlinear and exhibited a threshold effect at quintiles 3 to 5 vs quintiles 1 and 2. Higher vs lower oxalate excretion (at the 40th percentile) was associated with a 32% higher risk of CKD progression (HR, 1.32; 95% CI, 1.13-1.53) and 37% higher risk of ESRD (HR, 1.37; 95% CI, 1.15-1.63). Results were similar when treating death as a competing event.

CONCLUSIONS AND RELEVANCE

Higher 24-hour urinary oxalate excretion may be a risk factor for CKD progression and ESRD in individuals with CKD stages 2 to 4.

Idiopathic hypercalciuria and formation of calcium renal stones

The most common presentation of nephrolithiasis is idiopathic calcium stones in patients without systemic disease. Most stones are primarily composed of calcium oxalate and form on a base of interstitial apatite deposits, known as Randall's plaque. By contrast some stones are composed largely of calcium phosphate, as either hydroxyapatite or brushite (calcium monohydrogen phosphate), and are usually accompanied by deposits of calcium phosphate in the Bellini ducts. These deposits result in local tissue damage and might serve as a site of mineral overgrowth. Stone formation is driven by supersaturation of urine with calcium oxalate and brushite. The level of supersaturation is related to fluid intake as well as to the levels of urinary citrate and calcium. Risk of stone formation is increased when urine citrate excretion is <400 mg per day, and treatment with potassium citrate has been used to prevent stones. Urine calcium levels >200 mg per day also increase stone risk and often result in negative calcium balance. Reduced renal calcium reabsorption has a role in idiopathic hypercalciuria. Low sodium diets and thiazide-type diuretics lower urine calcium levels and potentially reduce the risk of stone recurrence and bone disease.

The Vacuolar H+-ATPase B1 Subunit Polymorphism p.E161K Associates with Impaired Urinary Acidification in Recurrent Stone Formers

Mutations in the vacuolar-type H(+)-ATPase B1 subunit gene ATP6V1B1 cause autosomal-recessive distal renal tubular acidosis (dRTA). We previously identified a single-nucleotide polymorphism (SNP) in the human B1 subunit (c.481G>A; p.E161K) that causes greatly diminished pump function in vitro To investigate the effect of this SNP on urinary acidification, we conducted a genotype-phenotype analysis of recurrent stone formers in the Dallas and Bern kidney stone registries. Of 555 patients examined, 32 (5.8%) were heterozygous for the p.E161K SNP, and the remaining 523 (94.2%) carried two wild-type alleles. After adjustment for sex, age, body mass index, and dietary acid and alkali intake, p.E161K SNP carriers had a nonsignificant tendency to higher urinary pH on a random diet (6.31 versus 6.09; P=0.09). Under an instructed low-Ca and low-Na diet, urinary pH was higher in p.E161K SNP carriers (6.56 versus 6.01; P<0.01). Kidney stones of p.E161K carriers were more likely to contain calcium phosphate than stones of wild-type patients. In acute NH4Cl loading, p.E161K carriers displayed a higher trough urinary pH (5.34 versus 4.89; P=0.01) than wild-type patients. Overall, 14.6% of wild-type patients and 52.4% of p.E161K carriers were unable to acidify their urine below pH 5.3 and thus, can be considered to have incomplete dRTA. In summary, our data indicate that recurrent stone formers with the vacuolar H(+)-ATPase B1 subunit p.E161K SNP exhibit a urinary acidification deficit with an increased prevalence of calcium phosphate-containing kidney stones. The burden of E161K heterozygosity may be a forme fruste of dRTA.

Secondary osteoporosis: pathophysiology & diagnosis

Osteoporosis is a skeletal disease characterized by decreased bone mass and microarchitectural changes in bone tissue that increase the susceptibility to fracture. Secondary osteoporosis is loosely defined as low bone mineral density or increased risk of fragility fracture caused by any factor other than aging or postmenopausal status. The purpose of this review is to discuss the current understanding of the pathophysiology and contribution to fracture risk of many of the more common causes of secondary osteoporosis, as well as diagnostic considerations, outlined by organ system. While not comprehensive, included are a wide array of diseases, conditions, and medications that have been associated with bone loss and susceptibility to fractures. The hope is to highlight the importance to the general clinician of screening for and treating the osteoporosis in these patients, so to limit the resultant increased morbidity associated with fractures.

Metabolic evaluation of urinary lithiasis: what urologists should know and do

INTRODUCTION

Urolithiasis is a complex medical entity and regroups several different types of stones, each caused by a multitude of dietary imbalances or metabolic anomalies. In order to better assess the stone-forming patient, urologists should be competent in performing a thorough metabolic work-up.

MATERIALS AND METHODS

We reviewed the litterature in order to provide an appropriate overview of the various components of the metabolic evaluation, including stone analysis, biochemistry tests, and urine collection.

CONCLUSION

Performing a metabolic evaluation allows precise intervention in order to treat and mainly prevent stone disease.

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 interventions for preventing complications in idiopathic hypercalciuria

BACKGROUND

Idiopathic hypercalciuria is an inherited metabolic abnormality that is characterised by excessive amounts of calcium excreted in the urine by people whose calcium serum levels are normal. Morbidity associated with idiopathic hypercalciuria is chiefly related to kidney stone disease and bone demineralisation leading to osteopenia and osteoporosis. Idiopathic hypercalciuria contributes to kidney stone disease at all life stages; people with the condition are prone to developing oxalate and calcium phosphate kidney stones. In some cases, crystallised calcium can be deposited in the renal interstitium, causing increased calcium levels in the kidneys. In children, idiopathic hypercalciuria can cause a range of comorbidities including recurrent macroscopic or microscopic haematuria, frequency dysuria syndrome, urinary tract infections and abdominal and lumbar pain. Various dietary interventions have been described that aim to decrease urinary calcium levels or urinary crystallisation.

OBJECTIVES

Our objectives were to assess the efficacy, effectiveness and safety of dietary interventions for preventing complications in idiopathic hypercalciuria (urolithiasis and osteopenia) in adults and children, and to assess the benefits of dietary interventions in decreasing urological symptomatology in children with idiopathic hypercalciuria.

SEARCH METHODS

We searched the Cochrane Renal Group's Specialised Register (23 April 2013) through contact with the Trials' Search Co-ordinator using search terms relevant to this review. Studies contained in the Specialised Register are identified through search strategies specifically designed for CENTRAL, MEDLINE and EMBASE.

SELECTION CRITERIA

We included all randomised controlled trials (RCTs) and quasi-RCTs that investigated dietary interventions aimed at preventing complications of idiopathic hypercalciuria, compared with placebo, no intervention, or other dietary interventions regardless of route of administration, dose or amount.

DATA COLLECTION AND ANALYSIS

Studies were assessed for inclusion and data extracted using a standardised data extraction form. We calculated risk ratios (RR) for dichotomous outcomes and mean differences (MD) for continuous outcomes, both with 95% confidence intervals (CI).

MAIN RESULTS

We included five studies (379 adult participants) that investigated a range of interventions. Lack of similarity among interventions investigated meant that data could not be pooled. Overall, study methodology was not adequately reported in any of the included studies. There was a high risk of bias associated with blinding (although it seems unlikely that outcomes measures were unduly influenced by lack of intervention blinding), random sequence generation and allocation methodologies were unclear in most studies, but selective reporting bias was assessed as low.One study (120 participants) compared a low calcium diet with a normal calcium, low protein, low salt diet for five years. There was a significant decrease in numbers of new stone recurrences in those treated with the normal calcium, low protein, low salt diet (RR 0.77, 95% CI 0.61 to 0.98). This diet also led to a significant decrease in oxaluria (MD 78.00 µmol/d, 95% CI 26.48 to 129.52) and the calcium oxalate relative supersaturation index (MD 1.20 95% CI 0.21 to 2.19).One study (210 participants) compared a low salt, normal calcium diet with a broad diet for three months. The low salt, normal calcium diet decreased urinary calcium (MD -45.00 mg/d, 95% CI -74.83 to -15.17) and oxalate excretion (MD -4.00 mg/d, 95% CI -6.44 to -1.56).A small study (17 participants) compared the effect of dietary fibre as part of a low calcium, low oxalate diet over three weeks, and found that although calciuria levels decreased, oxaluria increased. Phyllanthus niruri plant substrate intake was investigated in a small subgroup with hypercalciuria (20 participants); there was no significant effect on calciuria levels occurred after three months of treatment.A small cross-over study (12 participants) evaluating the changes in urinary supersaturation indices among patients who consumed calcium-fortified orange juice or milk for one month found no benefits for participants.None of the studies reported any significant adverse effects associated with the interventions.

AUTHORS' CONCLUSIONS

Long-term adherence (five years) to diets that feature normal levels of calcium, low protein and low salt may reduce numbers of stone recurrences, decrease oxaluria and calcium oxalate relative supersaturation indexes in people with idiopathic hypercalciuria who experience recurrent kidney stones. Adherence to a low salt, normal calcium level diet for some months can reduce calciuria and oxaluria. However, the other dietary interventions examined did not demonstrate evidence of significant beneficial effects.No studies were found investigating the effect of dietary recommendations on other clinical complications or asymptomatic idiopathic hypercalciuria.

The characteristics of the stone and urine composition in Chinese stone formers: primary report of a single-center results

OBJECTIVE

To assess urine composition in Chinese patients with urolithiasis.

METHODS

Five hundred seven Chinese patients with urolithiasis from our center in southern China were included in this study. Analysis of stone composition was performed using infrared spectrometry. From all patients, 24-hour urine samples were collected for analysis of urinary variables. Some ion activity product risk indices were also calculated.

RESULTS

The major stone constituents in the 507 analyzed stones were as follows: calcium oxalate (78.3%), infection stone components (14.6%), uric acid (3.6%), and calcium phosphate (3.4%). Only 1 stone was composed of cystine (0.2%). Of all patients, 504 (99.4%) had 1 or several urinary metabolic abnormalities. Hypocitraturia was recorded in 93.9%, high sodium excretion in 58.6%, small urine volume in 45.6%, hyperoxaluria in 31.0%, hypercalciuria in 26.0%, hyperuricosuria in 19.3%, and hyperphosphaturia in 2.8%. Moreover, high sodium excretion was more frequent in men than women (59.2% vs 49.3%, P = .027), whereas hypercalciuria was more common in women (34.5% vs 20.4%, P <.001). High levels of urine sodium (187.7 ± 86.9 vs 179.8 ± 107.7 mmol/24h, P = .038) and phosphate (18.26 ± 8.36 vs 15.69 ± 11.14 mmol/24h, P <.001) were found in men than in women. Infection stones were significantly (P <.004) more common in women. Compared with noninfection stone formers, the occurrence of hypomagnesuria (P = .040) was more common in patients with infection stones.

CONCLUSION

The results of urinary risk factors for stone formation in this study might serve as a basis for design of recurrence prevention. It is of interest to note that some of the demonstrated abnormalities differ from that in reports from other countries.

Association of a reduction in central obesity and phosphorus intake with changes in urinary albumin excretion: the PREMIER study

BACKGROUND

Excess adiposity and dietary factors may be important determinants of urinary albumin excretion (UAE).

STUDY DESIGN

Observational analysis of PREMIER, a randomized trial designed to lower blood pressure using behavioral interventions (counseling on weight loss, healthy diet, and exercise).

SETTING & PARTICIPANTS

481 participants with normal kidney function who provided adequate 24-hour urine collections at baseline and 6 months.

PREDICTORS

Change in waist circumference; 24-hour urine sodium, potassium, and phosphorus excretion; and protein intake estimated from urea nitrogen.

OUTCOMES & MEASUREMENTS

The primary outcome was change in log-transformed 24-hour UAE over 6 months.

RESULTS

After 6 months, the proportion of individuals with UAE ≥10 mg/d decreased from 18.7% to 12.7% (P < 0.001). Changes in mean waist circumference (-4.2 ± 6.6 [SD] cm), 24-hour excretion of sodium (-28.2 ± 71.7 mmol/d), potassium (+8.4 ± 27.8 mmol/d), phosphorus (-27.7 ± 314.1 mg/d), and protein intake (-1.7 ± 19.4 g/d) were observed. After adjustment for relevant covariates, the following variables were associated significantly with reduction in ln(UAE) in separate models: decrease in waist circumference (P = 0.001), decrease in 24-hour urine phosphorus excretion (P < 0.001), and decrease in protein intake (P = 0.01). In a multivariable model including these 3 predictors, decreases in waist circumference (P = 0.002) and 24-hour urine phosphorus excretion (P = 0.03), but not change in protein intake (P = 0.5), remained associated significantly with reduction in ln(UAE). These associations remained significant even after adjustment for changes in blood pressure and insulin resistance. Baseline UAE and metabolic syndrome modified the relationship of waist circumference with ln(UAE); specifically, individuals with higher UAE and baseline metabolic syndrome experienced greater reductions in ln(UAE) from decreases in waist circumference.

LIMITATIONS

Observational study with potential for confounding.

CONCLUSIONS

In adults with normal kidney function, decreases in waist circumference and 24-hour urine phosphorus excretion are associated with reductions in UAE. These findings support the rationale for clinical trials to determine whether reducing dietary phosphorus intake or waist circumference could prevent chronic kidney disease or slow its progression.

Update on the evaluation of repeated stone formers

Office management of stone disease is an important component of a urologist's practice. Evaluation should include analysis of stone composition, 24-hour urine studies, identification of modifiable risk factors, and targeted dietary, lifestyle, and/or medical therapy. A sizeable portion of investigated etiologies and risk factors for stone disease have centered on the complex interplay between obesity, diabetes, and other disease states that comprise the metabolic syndrome. Alternatives to traditional preventive therapy, such as probiotics and various fruit juices, are still being studied but may prove useful adjuncts to traditional preventive therapy, where the mainstays remain increased fluid intake, dietary modification, and pharmacologic therapy. Future studies on preventive therapy of urolithiasis are likely to focus on strategies to increase compliance, cost-effectiveness, and systems-based implementation.

Dietary zinc intake and kidney stone formation: evaluation of NHANES III

AIMS

We aimed to determine whether there is an association between dietary zinc intake (DZI) and prevalent kidney stone disease defined as self-report of any previous episode of kidney stone.

METHODS

We examined The Third National Health and Nutrition Examination Survey (NHANES III), a large US population-based cross-sectional study, and used logistic regression analyses to determine the independent association between DZI and prevalent kidney stone disease.

RESULTS

A total of 15,444 men and women over 18 years of age were eligible for analysis. Among them, 710 participants reported a history of kidney stones. Stone formers tended to have higher DZI than non-stone formers among NHANES III participants, though this difference did not reach statistical significance (p = 0.1). Multivariate adjusted logistic regression analysis revealed that higher DZI (log transformed) was associated with a significantly increased risk of kidney stone disease (odds ratio, OR = 1.41, 95% confidence interval, CI: 1.10-1.81, p = 0.01). After categorizing zinc intake into three groups, those with highest DZI (>15 mg/day) were also associated with a significantly increased risk of kidney stone disease, compared to those with lower DZI (<7 mg/day; OR = 1.70, 95% CI: 1.13-2.57, p = 0.01).

CONCLUSIONS

Our study suggests that higher DZI is associated with increased risk of kidney stone disease. Future prospective studies are needed to clarify the causal relationship between zinc intake and kidney stone formation.

Net endogenous acid production is associated with a faster decline in GFR in African Americans

Increased acid excretion may promote renal injury. To evaluate this in African Americans with hypertensive nephrosclerosis, we studied the association between the net endogenous acid production and progression of kidney disease in 632 patients in the AASK trial. Protein and potassium intakes were estimated from 24 h urea nitrogen and potassium excretion, and used to estimate net endogenous acid production, averaged over 2 years, approximating routine intake. The link between net endogenous acid production and the I(125)iothalamate glomerular filtration rate (iGFR) and time to end-stage renal disease or doubling of serum creatinine was analyzed using mixed models and Cox proportional hazards regressions. The trend in higher net endogenous acid production was significantly associated with a faster decline in iGFR over a median of 3.2 years. After adjustment for age, body mass index, baseline iGFR, urine protein-to-creatinine ratio, and randomized treatment group, the trend in higher net endogenous acid production remained significantly associated with a faster decline in iGFR at a rate of 1.01 ml/min per 1.73 m(2) per year faster in the highest compared to the lowest quartile. However, in time-to-event analyses over a median of 7.7 years, the adjusted hazard ratio (1.10) for composite renal events per 25 mEq/day higher net endogenous acid production was not significant. Hence, our findings implicate endogenous acid production as a potential modifiable risk factor for progressive kidney disease.

Estimated net endogenous acid production and serum bicarbonate in African Americans with chronic kidney disease

BACKGROUND AND OBJECTIVES

Metabolic acidosis may contribute to morbidity and disease progression in patients with chronic kidney disease (CKD). The ratio of dietary protein, the major source of nonvolatile acid, to dietary potassium, which is naturally bound to alkali precursors, can be used to estimate net endogenous acid production (NEAP). We tested the association between estimated NEAP and serum bicarbonate in patients with CKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

NEAP was estimated among 462 African American adults with hypertensive CKD using published equations: NEAP (mEq/d) = -10.2 + 54.5 (protein [g/d]/potassium [mEq/d]). Dietary protein and potassium intake were estimated from 24-hour urinary excretion of urea nitrogen and potassium, respectively. All of the eligible measurements during follow-up were modeled using generalized linear regression clustered by participant and adjusted for demographics, 24-hour urinary sodium, kidney function, and selected medications.

RESULTS

Higher NEAP was associated with lower serum bicarbonate in a graded fashion (P trend < 0.001). Serum bicarbonate was 1.27 mEq/L lower among those in the highest compared with the lowest quartile of NEAP (P < 0.001). There was a greater difference in serum bicarbonate between the highest and lowest quartiles of NEAP among patients with stage 4/5 CKD (-2.43 mEq/L, P < 0.001) compared with those with stage 2/3 disease (-0.77 mEq/L, P = 0.01; P-interaction = 0.02).

CONCLUSIONS

Reducing NEAP, through reduction of dietary protein and increased intake of fruits and vegetables, may prevent metabolic acidosis in patients with CKD.

Evidence for metabolic origin of absorptive hypercalciuria Type II

The objective of this retrospective data analysis was to test the hypothesis that absorptive hypercalciuria Type II (AH-II) is a less severe variant of absorptive hypercalciuria Type I (AH-I), a common cause of calcareous stones. 24-h urinary calcium obtained on constant metabolic diets was retrieved from several data sources, including those of the authors and another group. On a low calcium diet (10 mmol calcium), 35 patients with AH-II were compared with 70 non-stone formers (NSF) and 76 patients with AH-I. On a high calcium diet (25 mmol calcium/day), 10 patients with AH-II were compared with 35 NSF and 32 with AH-I. On a low calcium diet for all participants, 24-h urinary calcium in AH-II (4.13 ± 0.63 mmol/day) was significantly higher than in NSF (3.06 ± 1.17 mmol/day), but significantly lower than in AH-I (6.11 ± 1.14 mmol/day) (p < 0.001). In a smaller subset, fractional intestinal calcium absorption in AH-II (65.0 ± 11.1%) was intermediate between NSF (50.0 ± 6.4%) and AH-I (71.0 ± 6.7%) (p < 0.001 between AH-II and other groups). On a high calcium diet, the rise in urinary calcium in AH-II was significantly higher than in NSF, but not as marked as in AH-I. Estimated calcium balance in AH-II was similar to NSF, but significantly more positive than AH-I. In conclusion, AH-II shares with AH-I the same metabolic disturbance(s) stimulating intestinal absorption and renal excretion of calcium but to a lesser degree. Bone might be spared in AH-II.

Sodium restriction as initial conservative treatment for urinary stone disease

PURPOSE

Sodium restriction is widely recommended to prevent urinary stone recurrence. However, the effect of urinary sodium excretion has not been fully evaluated. We investigated the relationship between urinary sodium, urinary metabolite excretion and the risk of recurrence in urinary stone formers.

MATERIALS AND METHODS

Selected for study were 798 stone formers with no evidence of metabolic disorders as the cause of abnormal urinary solutes. We analyzed the relationship between urinary sodium and other metabolic parameters by gender. Values were adjusted by covariates according to correlation status. Patients were divided into stone formers with hypernatriuresis or normal natriuresis (less than 220 mEq daily) and urinary parameters were compared. Kaplan-Meier analysis was done to determine the cumulative incidence of recurrent stones by urinary sodium. Patients were considered recurrence-free at a minimum followup of 3 years without incidence.

RESULTS

In the 492 men and 306 women mean ± SD age was 40.0 ± 11.4 and 45.4 ± 12.7 years, and mean body mass index was 23.9 ± 3.1 and 23.0 ± 3.0 kg/m(2), respectively. Using covariate adjusted partial correlation coefficients urinary sodium was noted to influence volume, pH, calcium, uric acid, oxalate and citrate in men, and volume, pH, calcium, uric acid and citrate in women (each p <0.05). At a median followup of 56.1 months 46 of 98 stone formers (46.9%) with normal natriuresis experienced stone recurrence vs 60 of 93 (64.5%) with hypernatriuresis. Patients with hypernatriuresis also had significantly decreased time to recurrence than those with normal natriuresis (log rank test p = 0.043).

CONCLUSIONS

Results show that urinary sodium is an important determinant of other stone forming parameters and of the risk of recurrent stones. These findings suggest that a sodium restricted diet should be the initial step when treating stone formers.

Diet to reduce mild hyperoxaluria in patients with idiopathic calcium oxalate stone formation: a pilot study

OBJECTIVES

To assess whether a normal-calcium, low-animal protein, low-salt diet is effective in reducing hyperoxaluria in idiopathic calcium oxalate nephrolithiasis compared with a traditional low-oxalate diet, routinely recommended by clinicians

METHODS

We treated 56 patients with idiopathic calcium oxalate stone formation who presented with mild hyperoxaluria (>40 mg/d) while consuming a free diet with a normal-calcium, low-animal protein, low-salt diet for a 3-month period. We compared the results obtained with this diet with those of a historical control group of 20 hyperoxaluric patients treated in the traditional way with a low-oxalate diet

RESULTS

After 3 months of therapy, the mean oxaluria level had decreased from 50.2 to 35.5 mg/d with the normal-calcium, low-animal protein, low-salt diet and from 45.9 to 40.2 mg/d with the traditional diet (adjusted difference between post-treatment mean value -7.3 mg/d, 95% confidence interval -12.3 to -2.2, P = .005)

CONCLUSIONS

The results suggest that a normal-calcium, low-animal protein, low-salt diet can reduce oxalate excretion in hyperoxaluric patients. This should encourage the undertaking of a randomized-control study to confer more solid evidence in support of our findings.

Medical stone management: 35 years of advances

PURPOSE

The urological community has had a vital role in the author's 35 years of research on the medical management of urolithiasis. The goal of this article is to review the progress made from the perspective of collaborating urologists and urological journals in which the findings were reported.

MATERIALS AND METHODS

The author's work appeared in 94 articles in urological journals, including 63 in The Journal of Urology, and in 28 other journals with collaborating urologists. Progress on various aspects of medical management of stone disease was reviewed based on these articles.

RESULTS

Pathophysiological exploration was performed by elucidating metabolic-dietary etiologies of hypocitraturia, separating hypercalciuria into 3 types, and linking gouty diathesis (uric acid stones) with obesity and insulin resistance. Physicochemical consequences of hypocitraturia were delineated and semi-empirical methods were developed to assess calcium salt saturation. Potassium-rich fruit juices differed from potassium-poor fruit juices and excessive salt intake increased the stone forming risk. Vital to diagnostic separation was a comprehensive analysis of urine for stone risk factors. As an example of selective treatment, potassium citrate was shown to be useful for controlling uric acid stones by urinary alkalinization as well as calcareous stones by hypercitraturia.

CONCLUSIONS

During the last 35 years much progress has been made on the pathophysiology of stone formation, crystallization of stone forming salts, diagnostic separation and prevention of stone recurrence. The author's contribution in this effort would not have been possible without the active participation and support of the urological community.

RANKL is a mediator of bone resorption in idiopathic hypercalciuria

BACKGROUND AND OBJECTIVES

This study aimed to determine the expression of osteoprotegerin, receptor activator of nuclear factor kappaB ligand, interleukin-1alpha, transforming growth factor-beta, and basic fibroblast growth factor in stone-forming patients with idiopathic hypercalciuria.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Immunohistochemical analysis was performed in undecalcified bone samples previously obtained from 36 transiliac bone biopsies of patients who had idiopathic hypercalciuria and whose histomorphometry had shown lower bone volume, increased bone resorption, and prolonged mineralization lag time.

RESULTS

Bone expression of receptor activator of nuclear factor kappaB ligand and osteoprotegerin was significantly higher in patients with idiopathic hypercalciuria versus control subjects. Transforming growth factor-beta immunostaining was lower in patients with idiopathic hypercalciuria than in control subjects and correlated directly with mineralization surface. Interleukin-1alpha and basic fibroblast growth factor staining did not differ between groups. Receptor activator of nuclear factor kappaB ligand bone expression was significantly higher in patients who had idiopathic hypercalciuria and exhibited higher versus normal bone resorption.

CONCLUSION

A higher expression of receptor activator of nuclear factor kappaB ligand in bone tissue suggests that increased bone resorption in patients with idiopathic hypercalciuria is mediated by receptor activator of nuclear factor kappaB ligand. Osteoprotegerin bone expression might have been secondarily increased in an attempt to counteract the actions of receptor activator of nuclear factor kappaB ligand. The low bone expression of transforming growth factor-beta could contribute to the delayed mineralization found in such patients.

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.

Medical management of urinary stone disease

Kidney stones occur in approximately 10% of patients in their lifetimes, and > 10 crystal types have been reported in the literature. After treatment, a subset of these patients will have recurrent calculi, leading to significant morbidity and potential for serious chronic renal disease. Detailed metabolic evaluation is indicated in patients at high risk for stone recurrence, as a reversible metabolic abnormality can be identified in > 90% of them. Once the patient's underlying physicochemical and physiologic derangements are defined, targeted medical therapy can be initiated in order to prevent growth of pre-existing stones and new stone formation. In this report, the author provides a comprehensive review of the presently available selective and nonselective pharmacologic treatments for stones. Several exciting investigational pharmaceutical agents for kidney stone prevention are also discussed. Although many of these agents are effective, there remain clinical scenarios in which existing medicines are insufficient.

Bone disease in idiopathic hypercalciuria

PURPOSE OF REVIEW

Decreased bone mineral density and increased prevalence of bone fractures have been found in patients with idiopathic hypercalciuria. The purpose of this review is to summarize the recent published evidence that supports a potential role of the bone, and its link to the kidney and intestine, in the pathogenesis of idiopathic hypercalciuria. The effects of hypercalciuria on bone and the implications for treatment are also reviewed.

RECENT FINDINGS

Evidence suggests that the incidence of a first fracture in kidney stone patients is fourfold higher than the control population. Support for the role of bone in the pathophysiology of hypercalciuria has been corroborated. New studies have detailed the effects of several cytokines - increased number and sensitivity of vitamin D receptors, and increased acid production - upon the bone acting cells. Similarly, recent clinical and experimental studies have suggested that genetic factors confer a predisposition to the formation of renal calcium stones and bone demineralization.

SUMMARY

Whether hypercalciuria is the result of a primary bone disorder, a consequence of a persisting negative calcium balance or a combination of both still remains to be determined. Nevertheless, bone status must be evaluated and followed up in patients with idiopathic hypercalciuria.

Henle loop basement membrane as initial site for Randall plaque formation

Since the early description of Randall plaques in 1937, studies of the pathogenesis of stone formation mainly focused on the chemistry involving salt precipitation and crystallization, rather than tubular and interstitial medullar mechanisms of calcium concentration and supersaturation. In 2003, Bushinsky published a suggestive and inspiring sequence of events aimed to show that the basement membrane of the thin limb of the loop of Henle can be the first site of nucleation, as recently shown by the impressive work by Evan et al. The aim of this minireview is to verify the consistency of the Evan and Bushinsky theory with the current literature in the field.