Age, Body Mass Index, and Gender Predict 24-Hour Urine Parameters in Recurrent Idiopathic Calcium Oxalate Stone Formers

Incidence of metabolic syndrome in patients with unilateral or bilateral staghorn renal stones and its impact on percutaneous nephrolithotomy outcomes

BACKGROUND

To evaluate the incidence of metabolic syndrome (MetS) in patients with unilateral and bilateral staghorn calculi (SC) and evaluate the impact on the outcome of percutaneous nephrolithotomy (PCNL).

METHODS

The clinical data of patients who underwent PCNL for the treatment of SC between 2019 and 2022 were retrospectively reviewed. SC was divided into unilateral and bilateral. The incidence of MetS was compared between the patients with unilateral SC and the patients with bilateral SC, and the impact on the outcome of PCNL was assessed.

RESULTS

A total of 1778 patients underwent PCNL between 2019 and 2022. After screening computed tomography, 379 patients were confirmed to have SC, finally, leaving 310 patients with follow-up and complete data to be included in the study. Eighty-four had bilateral SC and 226 had unilateral SC. The patients with bilateral SC had a significantly higher body mass index and higher rates of complete staghorn stones and metabolic syndrome. Higher body mass index, hypertension, diabetes mellitus, hyperlipidaemia, and MetS were present in 62.58%, 44.84%, 21.94%, 60.65% and 27.42% of all patients, respectively. The number of MetS components remained significantly associated with bilateral SC. Specifically, when the number of MetS components increases from 0 to 3-4, the likelihood of developing bilateral staghorn calculi increases by 21.967 times. Eighty-five patients with MetS( +) had a higher rate of overall complications (number (N)(%), 29 (34.12) vs.33 (14.46), P < 0.001) and a comparable stone-free rate to 225 MetS(-) patients. Multivariable analysis confirmed that hyperlipidaemia (P = 0.044, odds ratio [OR] = 1.991, 95% confidence interval [CI] 1.020-3.888) and MetS (P = 0.005, OR = 2.427, 95% CI 1.316-4.477) were independent risk factors for overall complications.

CONCLUSIONS

MetS is correlated with the formation of bilateral SC and is the main predictor for complications of PCNL especially for low-grade complications (I-II).

Establishment and application of a nomogram diagram for predicting calcium oxalate stones in patients with urinary tract stones

This retrospective study aims to examine the correlation between calcium oxalate (CaOx) stones and common clinical tests, as well as urine ionic composition. Additionally, we aim to develop and implement a personalized model to assess the accuracy and feasibility of using charts to predict calcium oxalate stones in patients with urinary tract stones. A retrospective analysis was conducted on data from 960 patients who underwent surgery for urinary stones at the First Affiliated Hospital of Soochow University from January 1, 2010, to December 31, 2022. Among these patients, 447 were selected for further analysis based on screening criteria. Multivariate logistic regression analysis was then performed to identify the best predictive features for calcium oxalate stones from the clinical data of the selected patients. A prediction model was developed using these features and presented in the form of a nomogram graph. The performance of the prediction model was assessed using the C-index, calibration curve, and decision curve, which evaluated its discriminative power, calibration, and clinical utility, respectively. The nomogram diagram prediction model developed in this study is effective in predicting calcium oxalate stones which is helpful in screening and early identification of high-risk patients with calcium oxalate urinary tract stones, and may be a guide for urologists in making clinical treatment decisions.

Surgical Artificial Intelligence: Endourology

Endourology is ripe with information that includes patient factors, laboratory tests, outcomes, and visual data, which is becoming increasingly complex to assess. Artificial intelligence (AI) has the potential to explore and define these relationships; however, humans might not be involved in the input, analysis, or even determining the methods of analysis. Herein, the authors present the current state of AI in endourology and highlight the need for urologists to share their proposed AI solutions for reproducibility outside of their institutions and prepare themselves to properly critique this new technology.

Drivers of calcium oxalate stone formation in the octogenarian population

INTRODUCTION

American Urological Association (AUA) guidelines suggest metabolic testing via 24-h urine studies in high-risk, interested first-time stone formers, and recurrent stone formers. If metabolic testing is not available or otherwise not feasible, clinicians may need to utilize empiric therapy. Debility and social barriers, particularly in the elderly population, may limit the practicality of metabolic testing, and therefore, empiric therapy is of particular importance. The aim of this study is to identify whether unique urinary metabolic abnormality profiles exist for octogenarians with calcium oxalate kidney stones, as this may guide empiric stone prevention therapy more precisely in this population.

MATERIALS AND METHODS

Patients with calcium oxalate stones from a single academic kidney stone center in New York, NY, were retrospectively identified in our prospectively managed database. Patient data, including demographic, clinical information, and baseline 24-h urine studies, were collected before initiating any treatment. Subjects were stratified by age (≤ 40, 41-59, 60-79, and ≥ 80 years) to compare the metabolic urinary abnormality profiles between octogenarians and other age groups. Subgroup analyses were also performed to compare results by gender and by the presence of underlying kidney dysfunction. Comparative statistical analysis was carried out using Chi-square tests, Mann-Whitney U tests, and t-tests where appropriate.

RESULTS

Hypocitraturia, low urine pH, and low urine volume were most common in older patients, particularly in octogenarians. Hypercalciuria, hypernatriuria, and hyperuricosuria were more apparent in younger groups.

CONCLUSION

With increasing age, hypocitraturia, low urine pH, and low urine volume were more prevalent on 24-h urine metabolic testing. We hypothesize increased comorbidity, including medical renal disease, polypharmacy, and dehydration are possible factors contributing to this unique profile. We suggest that empiric therapy targeted towards this profile is important in very elderly stone formers in whom 24-h urine testing may not be possible. Increased hydration, increased fruit and vegetable intake, and low-dose alkali therapy are easy measures to accomplish this.

Establishment of a new predictive model for the recurrence of upper urinary tract stones

PURPOSE

To construct a nomogram for evaluation of the recurrence risk of upper urinary tract stones in patients.

METHODS

We retrospectively reviewed the clinical data of 657 patients with upper urinary tract stones and divided them into stone recurrence group and non-recurrence group. Blood routine, urine routine, biochemical, and urological CT examinations were searched from the electronic medical record, relevant clinical data were collected, including age, BMI, stones number and location, maximum diameter, hyperglycemia, hypertension, and relevant blood and urine parameters. The Wilcoxon rank-sum test, independent sample t test, and Chi-square test were used to preliminarily analyze the data of the two groups, then LASSO and logistic regression analysis were used to find out the significant difference indicators. Finally, R software was used to draw a nomogram to construct the model, and ROC curve was drawn to evaluate the sensitivity and specificity.

RESULTS

The results showed that multiple stones (OR: 1.832, 95% CI 1.240-2.706), bilateral stones (OR: 1.779, 95% CI 1.226-2.582), kidney stones (OR: 3.268, 95% CI 1.638-6.518), and kidney ureteral stones (OR: 3.375, 95% CI 1.649-6.906) were high risk factors. And the stone recurrence risk was positively correlated with creatinine (OR: 1.012, 95% CI 1.006-1.018), urine pH (OR: 1.967, 95% CI 1.343-2.883), Apo B (OR: 4.189, 95% CI 1.985-8.841) and negatively correlated with serum phosphorus (OR: 0.282, 95% CI 0.109-0.728). In addition, the sensitivity and specificity of the prediction model were 73.08% and 61.25%, diagnosis values were greater than any single variable.

CONCLUSION

The nomogram model can effectively evaluate the recurrence risk of upper urinary stones, especially suitable for stone postoperative patients, to help reduce the possibility of postoperative stone recurrence.

Epidemiology of Kidney Stones

In the past two decades, major breakthroughs that improve our understanding of the pathophysiology and therapy of kidney stones (KS) have been lacking. The disease continues to be challenging for patients, physicians, and healthcare systems alike. In this context, epidemiological studies are striving to elucidate the worldwide changes in the patterns and the burden of the disease and identify modifiable risk factors that contribute to the development of kidney stones. Our expanding knowledge of the epidemiology of kidney stones is of paramount importance and largely upgrades the modern management of the disease. In this paper, we review the variables affecting prevalence and incidence, including age, gender, race, ethnicity, occupation, climate, geography, systemic diseases, diabetes, vascular disease, chronic kidney disease, and dietary risk factors relevant to kidney stones.

Machine Learning Models to Predict 24 Hour Urinary Abnormalities for Kidney Stone Disease

OBJECTIVES

To help guide empiric therapy for kidney stone disease, we sought to demonstrate the feasibility of predicting 24-hour urine abnormalities using machine learning methods.

METHODS

We trained a machine learning model (XGBoost [XG]) to predict 24-hour urine abnormalities from electronic health record-derived data (n=1,314). The machine learning model was compared to a logistic regression model [LR]. Additionally, an ensemble (EN) model combining both XG and LR models was evaluated as well. Models predicted binary 24-hour urine values for volume, sodium, oxalate, calcium, uric acid and citrate; as well as a multiclass prediction of pH. We evaluated performance using area under the receiver operating curve (AUC-ROC) and identified predictors for each model.

RESULTS

The XG model was able to discriminate 24-hour urine abnormalities with fair performance, comparable to LR. The XG model most accurately predicted abnormalities of urine volume (accuracy=98%, AUC-ROC=0.59), uric acid (69%, 0.73) and elevated urine sodium (71%, 0.79). The LR model outperformed the XG model alone in prediction of abnormalities of urinary pH (AUC-ROC of 0.66 vs 0.57) and citrate (0.69 vs.0.64). The EN model most accurately predicted abnormalities of oxalate (accuracy=65%, ROC-AUC=0.70) and citrate (65%, 0.69) with overall similar predictive performance to either XG or LR alone. Body mass index, age, and gender were the three most important features for training the models for all outcomes.

CONCLUSIONS

Urine chemistry prediction for kidney stone disease appears to be feasible with machine learning methods. Further optimization of the performance could facilitate dietary or pharmacologic prevention.

Development and validation of a nomogram for risk prediction of nephrolithiasis recurrence in patients with primary hyperparathyroidism

BACKGROUND

Nephrolithiasis is a common complication of primary hyperparathyroidism (PHPT), and the recurrence of nephrolithiasis in patients with PHPT is also an urgent concern. What is worse, there is a scarcity of recommended evaluation to predict the risk of nephrolithiasis recurrence in patients with PHPT. This study was aimed to develop and validate a nomogram to facilitate risk assessment in patients with PHPT.

METHODS

A total of 197 patients with PHPT were retrospectively included in this study from September 2016 to August 2021. Patients' demographic data, blood test parameters, urinalysis, stone parameters, and surgical intervention were collected. Extracted variables were submitted to a least absolute shrinkage and selection operator (LASSO) regression model. A nomogram was built and validated according to the area under the curve (AUC) value, calibration curve, and decision curve analysis.

RESULTS

According to the LASSO regression and logistic regression analyses, five predictors were derived from 22 variables: creatinine, uric acid, bilateral stone, multiplicity, and surgery. The AUC and concordance index of the training cohort and validation cohort were 0.829 and 0.856, and 0.827 and 0.877, respectively. The calibration curve analysis and the decision curve analysis showed that the nomogram had an adequate prediction accuracy.

CONCLUSION

We built a useful nomogram model to predict the risk of nephrolithiasis recurrence in patients with PHPT. This would assist clinicians to provide appropriate advices and managements for these patients.

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.

Risk factors for high-dose methotrexate-induced nephrotoxicity

High-dose methotrexate (MTX) is widely used for the treatment of hematological malignancies. Despite the application of routine supportive care measures, such as intensive fluid hydration and urine alkalinization, nephrotoxicity is still a problem. The present study aimed to evaluate the risk factors for MTX-induced nephrotoxicity. We retrospectively reviewed 88 patients who received a regimen consisting of high-dose MTX (1000 mg/m²) and cytosine arabinoside between 2006 and 2018. Nephrotoxicity (≥ grade 2) was observed in 11 patients. Nephrotoxicity was observed only in patients with a high MTX concentration. Other than the MTX concentration, the serum uric acid level and urine pH at day 1 were associated with nephrotoxicity. A multivariate analysis revealed that urine pH was an independent risk factor for MTX-induced nephrotoxicity. Urine pH < 7.0 at day 1 was a significant risk factor for nephrotoxicity (odds ratio, 8.05; 95% confidence interval 1.95-33.3) and was also a predictor of delayed MTX elimination at 72 h after injection. Among pre-treatment factors, a low serum calcium level predicted urine pH < 7.0 at day 1. In conclusion, the present study suggests that low urine pH at day 1 is an independent risk factor for MTX-induced nephrotoxicity.

Impact of age and renal function on urine chemistry in patients with calcium oxalate kidney stones

Nephrolithiasis is associated with an increased risk of chronic kidney disease, and its incidence varies with age. However, little is known on the combined impact of aging and declining renal function on urinary risk factors for calcium oxalate stone formation. A retrospective analysis was performed on 24-h urine collections from 993 calcium oxalate stone-forming patients. We first tested for interactions between age and creatinine clearance on various urinary determinants of calcium oxalate nephrolithiasis, and then examined their separate and combined effects in univariable and multivariable analyses adjusting for demographic and biochemical covariates. We identified significant interactions between age and creatinine clearance in predicting 24-h urine pH, calcium, and citrate. In view of the small number of stone formers with low creatinine clearance, we limited further regression analyses to patients with creatinine clearance ≥ 60 mL/min. In multivariable analyses, urine citrate, oxalate, and total volume were positively correlated with age, whereas urine pH, citrate, calcium, oxalate, total volume, and RSR of calcium oxalate all significantly decreased with lower creatinine clearance. A decrease in creatinine clearance from 120 to 60 mL/min was associated with clinically significant decreases in the daily excretion rate of citrate (by 188 mg/day), calcium (by 33 mg/day), and oxalate (by 4 mg/day), and in RSR calcium oxalate (by 1.84). Age and creatinine clearance are significant and independent predictors of several urinary determinants of calcium oxalate nephrolithiasis. The impacts of aging and declining renal function should be considered during the management of calcium oxalate stone-forming patients.

Association of urinary sex steroid hormones with urinary calcium, oxalate and citrate excretion in kidney stone formers

BACKGROUND

Sex-specific differences in nephrolithiasis with respect to both distribution of prevalence and stone composition are widely described and may be influenced by sex hormones.

METHODS

We conducted a cross-sectional analysis of the relationship between 24-hour urinary sex hormone metabolites measured by gas chromatography-mass spectrometry with urinary calcium, oxalate and citrate excretion in a cohort of 628 kidney stone formers from a tertiary care hospital in Switzerland, taking demographic characteristics, kidney function and dietary factors into account.

RESULTS

We observed a positive association of urinary calcium with urinary testosterone and 17β-estradiol. Positive associations of urinary calcium with dehydroepiandrosterone, 5α-DH-testosterone, etiocholanolone, androsterone, and estriol were modified by net gastrointestinal alkali absorption or urinary sulfate excretion. As the only sex hormone, dehydroepiandrosterone was inversely associated with urinary oxalate excretion in adjusted analyses. Urinary citrate correlated positively with urinary testosterone. Associations of urinary citrate with urinary androsterone, 17β-estradiol and estriol were modified by urinary sulfate or sodium, or by sex.

CONCLUSIONS

Urinary androgens and estrogens are significantly associated with urinary calcium and citrate excretion, and associations are in part modified by diet. Our data furthermore reveal dehydroepiandrosterone as a novel factor associated with urinary oxalate excretion in humans.

Uric Acid and Urate in Urolithiasis: The Innocent Bystander, Instigator, and Perpetrator

Uric acid is an end product of purine metabolism in human beings. An unusual and still unexplained phenomenon is that higher primates have relatively high uric acid levels in body fluids owing to a combination of absence of degradation and renal retention. The physiologic purpose of high uric acid levels still is enigmatic, but the pathobiologic burden is a variety of crystallopathies owing to the low aqueous solubility of uric acid such as gouty arthritis and acute uric acid nephropathy. In the urinary space, three distinct conditions result from chronic uric acid and/or urate precipitation. The first and most common variety is uric acid urolithiasis. In this condition, urate is a victim of a systemic metabolic disease in which increased acid load to the kidney is coupled with diminished urinary buffer capacity owing to defective ammonium excretion, resulting in titration of urate to its sparingly soluble protonated counterpart, uric acid, and the formation of stones. Uric acid is the innocent bystander of the crime. The second variety is hyperuricosuric calcium urolithiasis, in which uric acid confers lithogenicity via promotion of calcium oxalate precipitation by multiple mechanisms involving soluble, colloidal, and crystalline urate salts. Uric acid is the instigator of the crime. The third and least common condition involves urate as an integral part of the urolith as an ammonium salt driven by high ammonium and high urate concentrations in urine. Here, uric acid is one of the perpetrators of the crime. Both known and postulated pathogenesis of these three types of urolithiasis are reviewed and summarized.

CT-related parameters and Framingham score as predictors of spontaneous passage of ureteral stones ≤ 10 mm: results from a prospective, observational, multicenter study

To investigate the reliability of newly defined CT-related parameters and cardiovascular risk factors in groups adjusted for stone size and location to predict spontaneous stone passage (SP) of uncomplicated ureteral stones ≤ 10 mm. The data of 280 adult patients with solitary unilateral ureteral stones ≤ 10 mm in diameter in non-contrast computed tomography were prospectively recorded. All patients undergoing a four-week observation protocol with medical expulsive therapy using tamsulosin were divided into two groups according to SP or no SP. Demographic, clinical and radiological findings of these groups were recorded. Spontaneous stone passage was observed in 176 (62.9%) of the patients, whereas the SP rate was 57.6% for 118 upper ureteral stones and 66.7% for 162 lower ureteral stones. The SP rate was 13.3 times greater with ureteral wall thickness < 1.88 mm, 4.4 times greater with a ratio of ureter to stone diameter of < 1.24, 3.4 times greater with Framingham score of < 11.5%, 2 times greater with neutrophil lymphocyte ratio < 1.96, 1.9 times greater with ureteral diameter < 6.33 mm and 1.5 times greater with stone volume < 38.54 mm³. Lower levels of ureteral wall thickness, ratio of ureter to stone diameter, Framingham score, neutrophil lymphocyte ratio, ureteral diameter, stone volume and absence of hydronephrosis were found to be more successful predictors. We consider that the success rate can be increased by selection of the proper option (observation or active treatment) according to these predictors.

Urinary oxalate as a potential mediator of kidney disease in diabetes mellitus and obesity

PURPOSE OF REVIEW

Hyperoxaluria can cause kidney disease through multiple mechanisms, including tubular obstruction from calcium oxalate crystals, sterile inflammation, and tubular epithelial cell injury. Hyperoxaluria is also observed in individuals with diabetes mellitus and obesity, which are in turn risk factors for chronic kidney disease (CKD). Whether hyperoxaluria is a potential mediator of increased risk of CKD in diabetes mellitus and obesity is unknown.

RECENT FINDINGS

Individuals with diabetes have increased levels of plasma glyoxal (a protein glycation product) and glyoxylate, both of which are precursors for oxalate. Increased gut absorption of oxalate in obesity may be because of obesity-associated inflammation. A recent study in individuals with CKD found that higher 24 h urinary oxalate excretion was independently associated with increased risk of kidney disease progression, especially in individuals with diabetes and obesity.

SUMMARY

Both diabetes mellitus and obesity are associated with higher urinary oxalate excretion through distinct mechanisms. Hyperoxaluria could be a mechanism by which kidney disease develops in individuals with diabetes mellitus or obesity and could also contribute to progressive loss of renal function. Future research on pharmacologic or dietary measures to limit oxalate absorption or generation are required to test whether lowering urinary oxalate excretion is beneficial in preventing kidney disease development and progression in diabetes mellitus and obesity.

Sex differences in the therapy of kidney and ureteral stones

PURPOSE OF REVIEW

Better understanding of sex differences affecting urolithiasis may help us offer tailored treatment strategies to our patients.

RECENT FINDINGS

The incidence of urolithiasis is increasing and the male-to-female ratio has decreased from 3 : 1 to 1.3 : 1 between 1970 and 2000. In women, obesity has a larger effect on the risk of developing urolithiasis [odds ratio (OR) 1.35; 95% confidence interval (CI): 1.33-1.37] compared with men (OR 1.04; 95% CI: 1.02-1.06). Urolithiasis is a risk factor for coronary artery disease in men (risk ratio = 1.23; 95% CI: 1.02-1.49) and for stroke in women (risk ratio = 1.12; 95% CI: 1.03-1.21). Women tolerate cystoscopic stent removal and shock wave lithotripsy better than men. For shock wave lithotripsy menopaused women have reported lower visual analog scale scores than menstruating women (P < 0.001). Female sex was an independent predictor of stone impaction (OR 1.15; 95% CI: 1.03-1.27) and postoperative sepsis after ureteroscopy (OR 2.31; 95% CI: 1.14-4.37).

SUMMARY

The sex gap in urolithiasis is closing. The changing role of women in society and dietary habits can be responsible for this epidemiologic shift. Women show a higher threshold for pain during urologic procedures, but they suffer from infectious complications more than men. Urolithiasis is a risk factor for cardiovascular events in both sexes.

Metabolic evaluation in urolithiasis - study of the prevalence of metabolic abnormalities in a tertiary centre

Introduction

Metabolic abnormalities are one of the most important risk factors for urinary stone disease. Our objective was to determine the prevalence of metabolic abnormalities in patients referred to the urolithiasis outpatient clinic of a tertiary centre.

Material and methods

We performed a cross-sectional study evaluating 67 patients referred to the urolithiasis outpatient clinic. Metabolic evaluation was performed, including one 24-hour urine sample.

Results

Metabolic abnormalities could be identified in 92.5% patients. Almost a quarter of the patients had only one metabolic abnormality and 67.6% had more than one abnormality. The most prevalent metabolic abnormalities were hypercalciuria (54.5%), hyperoxaluria (34.7%) and hyperuricosuria (32.3%). Patients with hypercalciuria were older (54.7 vs. 47.8 years, p = 0.018) and family history of stone disease was significantly more frequent among patients with hyperoxaluria (71.4% vs. 28.6%, p = 0.013). There was a positive linear relationship between body mass index (BMI) and urinary calcium (r = 0.247, p = 0.048) and a negative linear relationship between BMI and urinary pH (r = −0.254, p = 0.046).

Conclusions

Given the high prevalence of metabolic abnormalities, metabolic evaluation should be performed in every patient with urolithiasis evaluated in a tertiary setting.

Mechanisms for falling urine pH with age in stone formers

One of the main functions of the kidney is to excrete an acid load derived from both dietary and endogenous sources, thus maintaining the pH of other fluids in the body. Urine pH is also of particular interest in stone formers, since it determines the presence of either calcium phosphate or uric acid content in stones. Others have noted in epidemiological studies a rise in incidence of low pH-dependent uric acid stones with age, coinciding with a decrease in the incidence of high pH-dependent phosphate stones. Taken together, these trends are suggestive of a longitudinal decline in urine pH in stone-forming patients, and, if true, this could explain the observed trends in stone incidence. We studied 7,891 stone formers, all of whom collected a 24-h urine sample and matching serum. Multivariate modeling revealed that urine pH did indeed fall with age and particularly between the ages of 20 and 50 yr old in both men and women. We sought to explain this trend through the inclusion of traditionally understood determinants of urine pH such as urinary buffers, estimates of glomerular filtration, and dietary acid load, but these, taken together, accounted for but a small fraction of the pH fall. Gastrointestinal anion absorption was the strongest predictor of urine pH in all age groups, as we have previously reported in middle-aged normal men and women. However, we found that, despite a decreasing urine pH, gastrointestinal anion absorption increased monotonically with age. In fact, after adjustment for gastrointestinal anion absorption, urine pH declined more markedly, suggesting that bicarbonate-producing anion absorption is regulated in a manner that offsets the decline of urine pH.