Studies on the pathophysiology of the low urine pH in patients with uric acid stones

Uric acid in health and disease: From physiological functions to pathogenic mechanisms

Owing to renal reabsorption and the loss of uricase activity, uric acid (UA) is strictly maintained at a higher physiological level in humans than in other mammals, which provides a survival advantage during evolution but increases susceptibility to certain diseases such as gout. Although monosodium urate (MSU) crystal precipitation has been detected in different tissues of patients as a trigger for disease, the pathological role of soluble UA remains controversial due to the lack of causality in the clinical setting. Abnormal elevation or reduction of UA levels has been linked to some of pathological status, also known as U-shaped association, implying that the physiological levels of UA regulated by multiple enzymes and transporters are crucial for the maintenance of health. In addition, the protective potential of UA has also been proposed in aging and some diseases. Therefore, the role of UA as a double-edged sword in humans is determined by its physiological or non-physiological levels. In this review, we summarize biosynthesis, membrane transport, and physiological functions of UA. Then, we discuss the pathological involvement of hyperuricemia and hypouricemia as well as the underlying mechanisms by which UA at abnormal levels regulates the onset and progression of diseases. Finally, pharmacological strategies for urate-lowering therapy (ULT) are introduced, and current challenges in UA study and future perspectives are also described.

Asymptomatic hyperuricaemia in chronic kidney disease: mechanisms and clinical implications

Asymptomatic hyperuricaemia (HU) is considered a pathogenic factor in multiple disease contexts, but a causative role is only proven for the crystalline form of uric acid in gouty arthritis and urate nephropathy. Epidemiological studies document a robust association of HU with hypertension, cardiovascular disease (CVD) and CKD progression, but CKD-related impaired uric acid (UA) clearance and the use of diuretics that further impair UA clearance likely accounts for these associations. Interpreting the available trial evidence is further complicated by referring to xanthine oxidase inhibitors as urate-lowering treatment, although these drugs inhibit other substrates, so attributing their effects only to HU is problematic. In this review we provide new mechanistic insights into the biological effects of soluble and crystalline UA and discuss clinical evidence on the role of asymptomatic HU in CKD, CVD and sterile inflammation. We identify research areas with gaps in experimental and clinical evidence, specifically on infectious complications that represent the second common cause of death in CKD patients, referred to as secondary immunodeficiency related to kidney disease. In addition, we address potential therapeutic approaches on how and when to treat asymptomatic HU in patients with kidney disease and where further interventional studies are required.

Influence of Age and Geography on Chemical Composition of 98043 Urinary Stones from the USA

Background

Urolithiasis is a growing issue globally, but it is heterogeneous, with a different epidemiology and pathophysiology for each different stone composition.

Objective

The purpose of this study is to describe the incidence of urinary stones in the USA from 2016 to 2019 by chemical composition and to investigate the influence of age and geography on these stone types.

Design, setting, and participants

We obtained compositional analyses for all urinary stones submitted to a national laboratory over an approximately 3-yr period.

Outcome measurements and statistical analysis

Data collected included the chemical constituents of a stone, patient age, and geographical origin. We describe the incidence of each stone type by frequency. Statistical testing was performed to determine the influence of age and geographical region on overall incidence of each stone composition.

Results and limitations

In total, 99 908 specimens were analyzed. When pure stones were ordered by frequency, we found that the most common stone type was calcium oxalate (CaOx) (79.2%), followed by uric acid (UA; 14.3%), calcium phosphate (CaPO₄; 3.7%), cystine (0.51%), drug induced (0.12%), and magnesium ammonium phosphate (0.04%). CaOx, UA, and CaPO₄ were often mixed with one another. Among CaOx stones, the plurality (28.0%) was made of pure calcium oxalate monohydrate (COM), and only 0.002% was pure calcium oxalate dihydrate. There was an overall association between stone composition and both geographical distribution and age (p < 0.001).

Conclusions

CaOx stones comprise the majority of urinary stones in the USA, of which almost 28% were pure COM. Additionally, age and geographical region are significantly associated with variations in stone composition.

Patient summary

We evaluated the incidence of urinary stones in the USA based on their chemical composition. The most common stone type was calcium oxalate, the majority of which was pure calcium oxalate monohydrate. We also found age and geographical region to be significantly associated with variations in stone composition.

Association of Urine Findings with Metabolic Syndrome Traits in a Population of Patients with Nephrolithiasis

Background

The odds of nephrolithiasis increase with more metabolic syndrome (MetS) traits. We evaluated associations of metabolic and dietary factors from urine studies and stone composition with MetS traits in a large cohort of stone-forming patients.

Methods

Patients >18 years old who were evaluated for stones with 24-hour urine collections between July 2009 and December 2018 had their records reviewed retrospectively. Patient factors, laboratory values, and diagnoses were identified within 6 months of urine collection and stone composition within 1 year. Four groups with none, one, two, and three or four MetS traits (hypertension, obesity, dyslipidemia, and diabetes) were evaluated. Trends across groups were tested using linear contrasts in analysis of variance and analysis of covariance.

Results

A total of 1473 patients met the inclusion criteria (835 with stone composition). MetS groups were 684 with no traits, 425 with one trait, 211 with two traits, and 153 with three or four traits. There were no differences among groups for urine volume, calcium, or ammonium excretion. There was a significant trend (P<0.001) for more MetS traits being associated with decreasing urine pH, increasing age, calculated dietary protein, urine uric acid (UA), oxalate, citrate, titratable acid phosphate, net acid excretion, and UA supersaturation. The ratio of ammonium to net acid excretion did not differ among the groups. After adjustment for protein intake, the fall in urine pH remained strong, while the upward trend in acid excretion was lost. Calcium oxalate stones were most common, but there was a trend for more UA (P<0.001) and fewer calcium phosphate (P=0.09) and calcium oxalate stones (P=0.01) with more MetS traits.

Conclusions

Stone-forming patients with MetS have a defined pattern of metabolic and dietary risk factors that contribute to an increased risk of stone formation, including higher acid excretion, largely the result of greater protein intake, and lower urine pH.

Features of urine S100B and its ability to rule out intracranial hemorrhage in patients with head trauma: a prospective trial

Purpose

Traumatic brain injury causes morbidity and mortality worldwide. S100B is the most documented emergency brain biomarker and its urine-assay might be advantageous because of easier sampling. The primary aim was to evaluate urine S100B’s ability to rule out intracranial hemorrhage. Secondary aims included S100B temporal pattern for 48 h post-trauma and chemical properties of urine that affect urine S100B.

Methods

Patients with head trauma were sampled for serum and urine S100B. Patients who were admitted for intracranial hemorrhage were sampled for 48 h to assess S100B-level, renal function, urine-pH, etc.

Results

The negative predictive value of serum S100B was 97.0% [95% confidence interval (CI) 89.5–99.2%] and that of urine S100B was 89.1% (95% CI 85.5–91.9%). The specificity of serum S100B was 34.4% (95% CI 27.7–41.6%) and that of urine was 67.1% (95% CI 59.4–74.1%). Urine-pH correlated strongly with urine S100B during the first 6-h post-trauma. Trend-analysis of receiver operator characteristics of S100B in serum, urine the arithmetic difference between serum and urine S100B showed the largest area under the curve for arithmetic difference, which had a negative predictive value of 93.1% (95% CI 89.1–95.8%) and a specificity of 71.8% (95% CI 64.4–78.4%).

Conclusion

This study cannot support ruling out intracranial hemorrhage with urine S100B. Urine-pH might affect urine S100B and merits further studies. Serum and urine S100B have poor concordance and interchangeability. The arithmetic difference had a slightly better area under the curve and can be worth exploring in certain subgroups.

Prognostic value of spot testing urine pH as a novel marker in patients with ST-segment elevation myocardial infarction

Aim: To investigate the relationship between urinary pH (UpH) and clinical outcome in patients with ST-segment elevation myocardial infarction undergoing percutaneous coronary intervention. Methods: Data of 2081 patients with ST-segment elevation myocardial infarction were analyzed, including UpH. Patients were divided into UpH <6.0, 6.0≤ UpH <7.0 and UpH ≥7.0 based on UpH level. The primary outcome was in-hospital all-cause mortality and major adverse clinical events. Results: The incidence of in-hospital clinical outcomes was significantly higher in low UpH group. Multivariate analysis found low UpH (<6.0) was an independent predictor of in-hospital all-cause mortality (OR: 2.85) and major adverse clinical events (OR: 2.39). A Kaplan-Meier analysis showed long-term all-cause mortality was also significantly higher in low UpH group. The multivariate cox analysis demonstrated that low UpH was an independent predictor of long-term all-cause mortality (HR: 2.57). Conclusion: Low UpH is a simple, accessible and powerful marker of poor clinical outcomes in such patients.

Anthropometric variables, physical activity and dietary intakes of patients with uric acid nephrolithiasis

To evaluate anthropometric variables, energy expenditure by physical activity and nutrient intake of uric acid stone formers (UA-RSFs) compared to non-forming subjects (C). The study included 33 consecutive male patients with a diagnosis of "pure" stones of anhydrous uric acid at infrared spectroscopy and 49 male control subjects with no history of urinary stones. A personal interview was conducted including questionnaires for physical activity and dietary intakes. Anthropometric parametric and blood pressure were measured. Mean age, weight, height, waist circumference, body mass index, systolic and diastolic blood pressure values, dietary energy, carbohydrate intake, lipid intake, dietary acid load, time spent for different physical activities and total energy expenditure for physical activity were not different in UA-RSFs with respect to C. Mean dietary protein (76.2 ± 19.6 vs 65.4 ± 14.7 g/day, P = 0.006) and ethanol intake (10.4 ± 8.8 vs 4.1 ± 8.6, P = 0.002) were higher in UA-RSFs than in C. History of renal disease, heart disease and treatment with thiazides or allopurinol were more frequent and mean serum glucose and triglycerides (104 ± 12 vs 97 ± 11 mg/dl, P = 0.043) (172 ± 77 vs 123 ± 52 mg/dl, P = 0.023) were higher in UA-RSFs. Metabolic syndrome was more frequent in UA-RSFs (57% vs 39%) but not significant (p = 0.09). Increased dietary animal protein (and ethanol) intake can act as co-factors for uric acid stone formation although a more complex and not fully elucidated metabolic background can have an even more crucial role in the pathogenesis of this disease even in the absence of overweight.

Epidemiology, pathophysiology, and management of uric acid urolithiasis: A narrative review

An in-depth comprehension of the epidemiology as well as pathophysiology of uric acid urolithiasis is important for the identification, treatment, and prophylaxis of calculi in these patients. Persistently low urinary pH, hyperuricosuria, and low urinary volume are the most important factors in pathogenesis of uric acid urolithiasis. Other various causes of calculus formation comprises of chronic diarrhea, renal hyperuricosuria, insulin resistance, primary gout, extra purine in the diet, neoplastic syndromes, and congenital hyperuricemia. Non-contrast-enhanced computed tomography is the radiologic modality of choice for early assessment of patients with renal colic. Excluding situations where there is acute obstruction, rising blood chemistry, severe infection, or unresolved pain, the initial management ought to be medical dissolution by oral chemolysis since this method has proved to be effective in most of the cases.

Biochemical and dietary factors of uric acid stone formation

The aim of this study was to compare the clinical characteristics of "pure" uric acid renal stone formers (UA-RSFs) with that of mixed uric acid/calcium oxalate stone formers (UC-RSFs) and to identify which urinary and dietary risk factors predispose to their formation. A total of 136 UA-RSFs and 115 UC-RSFs were extracted from our database of renal stone formers. A control group of 60 subjects without history of renal stones was considered for comparison. Data from serum chemistries, 24-h urine collections and 24-h dietary recalls were considered. UA-RSFs had a significantly (p = 0.001) higher body mass index (26.3 ± 3.6 kg/m²) than UC-RSFs, whereas body mass index of UA-RSFs was higher but not significantly than in controls (24.6 ± 4.7) (p = 0.108). The mean urinary pH was significantly lower in UA-RSFs (5.57 ± 0.58) and UC-RSFs (5.71 ± 0.56) compared with controls (5.83 ± 0.29) (p = 0.007). No difference of daily urinary uric acid excretion was observed in the three groups (p = 0.902). Daily urinary calcium excretion was significantly (p = 0.018) higher in UC-RSFs (224 ± 149 mg/day) than UA-RSFs (179 ± 115) whereas no significant difference was observed with controls (181 ± 89). UA-RSFs tend to have a lower uric acid fractional excretion (0.083 ± 0.045% vs 0.107+/-0.165; p = 0.120) and had significantly higher serum uric acid (5.33 ± 1.66 vs 4.78 ± 1.44 mg/dl; p = 0.007) than UC-RSFs. The mean energy, carbohydrate and vitamin C intakes were higher in UA-SFs (1987 ± 683 kcal, 272 ± 91 g, 112 ± 72 mg) and UC-SFs (1836 ± 74 kcal, 265 ± 117, 140 ± 118) with respect to controls (1474 ± 601, 188 ± 84, 76 ± 53) (p = 0.000). UA-RSFs should be differentiated from UC-RSFs as they present lower urinary pH, lower uric acid fractional excretion and higher serum uric acid. On the contrary, patients with UC-RSFs show urinary risk factors more similar to those for calcium oxalate stones. The dietary approach in patients forming uric acid stones should be reconsidered with more attention to the quantity and quality of carbohydrate intake.

[Epidemiology of urolithiasis in Belgium on the basis of a morpho-constitutional classification]

Urolithiasis is a common condition, with a prevalence of ∼10% and a male/female ratio above 1 according to large national series. Various types of urinary stones have been described upon their mineral content and/or their morphology. Hence, a combined morpho-constitutional (M-C) classification has been proposed. In order to detail the prevalence of urolithiasis in general and of each M-C type in particular upon age and gender in Belgium, we retrospectively studied the database of a reference center for urolithiasis analysis. Between 2010 and 2013, 2195 stones were characterized. We excluded 45 non-biological stones and 281 stones, which originated from outside the study zone. Among 1869 stones, 1293 (69.2%) affected men. Prevalence peak of urolithiasis was observed between 50-60 years of age in both genders. The M-C analysis was available for 1854 stones (99.2%): multiple morphological types were concomitantly identified in 49.3%. In the whole population, the main mineral constituent was whewellite (54.4%), mainly organized as type Ia (94%). Weddellite was found in 19.8%, with an equal distribution between types IIa and IIb. Uric acid was the 3rd most frequent constituent in man, with a similar distribution between IIIa and IIIb. Phosphate was uncommon in man (8.2%), but frequent in woman (26.6%) with a type IVa1 organization. Prevalence of M-C types changes with aging, i.e. decrease of weddellite and increase of whewellite and uric acid in both genders. This retrospective analysis of a single-center database of urinary stones helps characterize the M-C epidemiology of urolithiasis in Belgium.

Preventive and therapeutic effects of sodium bicarbonate on melamine-induced bladder stones in mice

The actual preventive and therapeutic effects of alkalinizing urine on melamine-induced bladder stones (cystolith) are not completely known. Using an ideal model, two experiments were conducted in Balb/c mice. The mice were fed a normal diet in controls and a melamine diet in the other groups. The first day was set as experiment-day 1. In "Experiment 1", either low-/mid-/high-dose sodium bicarbonate (SB) or sterile water was administered by intragastric perfusion (once daily) to the mice for 14 days. Relative to the model group, the mean pH of the urine in the SB groups was significantly elevated at 3 h after SB administration, with a significant decrease in cystolith incidence on experiment-day 14. In "Experiment 2", on experiment-day 12, the melamine diet was replaced by a normal diet in 4 groups with melamine withdrawal (MW). Meanwhile, either mid-/high-dose SB or sterile water was administered by intragastric perfusion (once) to the mice in the corresponding groups. On experiment-day 12, after an additional 8 h, the cystolith incidence was significantly reduced in the high-SB, MW + mid-SB and MW + high-SB groups than in the model group. In conclusion, low urinary pH is one of the main determinants of the formation of melamine-associated stones, urinary alkalinization can be achieved by a proper dose of oral SB, and SB acts to prevent and treat melamine-induced cystoliths in mice.

Comorbidities in patients with crystal diseases and hyperuricemia

Crystal arthropathies are among the most common causes of painful inflammatory arthritis. Gout, the most common example, has been associated with cardiovascular and renal disease. In recent years, evidence for these associations and those involving other comorbidities, such as the metabolic syndrome, have emerged, and the importance of asymptomatic hyperuricemia has been established. In this review, an update on evidence, both experimental and clinical, is presented, and associations between hyperuricemia, gout, and several comorbidities are described. Causality regarding calcium pyrophosphate arthropathy and associated comorbidities is also reviewed.

Metabolic syndrome, obesity and kidney stones

OBJECTIVES

To give a comprehensive and focused overview on the current knowledge of the causal relations of metabolic syndrome and/or central obesity with kidney stone formation.

METHODS

Previous reports were reviewed using PubMed, with a strict focus on the keywords (single or combinations thereof): urolithiasis, nephrolithiasis, kidney stones, obesity, metabolic syndrome, bariatric surgery, calcium oxalate stones, hyperoxaluria, insulin resistance, uric acid stones, acid-base metabolism.

RESULTS

Obesity (a body mass index, BMI, of >30 kg/m(2)) affects 10-27% of men and up to 38% of women in European countries. Worldwide, >300 million people are estimated to be obese. Epidemiologically, a greater BMI, greater weight, larger waist circumference and major weight gain are independently associated with an increased risk of renal stone formation, both for calcium oxalate and uric acid stone disease.

CONCLUSIONS

There are two distinct metabolic conditions accounting for kidney stone formation in patients with metabolic syndrome/central obesity. (i) Abdominal obesity predisposes to insulin resistance, which at the renal level causes reduced urinary ammonium excretion and thus a low urinary pH; the consequence is a greater risk of uric acid stone formation. (ii) Bariatric surgery, the only intervention that facilitates significant weight loss in morbidly obese people, carries a greater risk of calcium oxalate nephrolithiasis. The underlying pathophysiological mechanisms are profound enteric hyperoxaluria due to intestinal binding of calcium by malabsorbed fatty acids, and severe hypocitraturia due to soft or watery stools, which lead to chronic bicarbonate losses and intracellular metabolic acidosis.

[Changes in kidney stones type according to sex and age in Tunisian patients]

BACKGROUND

Studies that evaluate the effect of age and gender on the stone composition are scarce. The aim of this study is to highlight the modification of epidemiological characteristics of this pathology according to patients' sex and age.

PATIENTS AND METHODS

We studied 1269 urolithiasic patients ranging from 6 months to 92 years old and known as having urinary stones (752 males and 517 females). Stone analysis was performed respectively using a stereomicroscope and infrared spectroscopy to determine, respectively, the morphological type and molecular composition of each.

RESULTS

The annual average incidence of new stone formation was 30.25 per 100,000 inhabitants. In 1041 patients (81%), calculi were located in the upper urinary tract. Children and old man were more affected by bladder stone. Calcium oxalate monohydrate remains the most frequent stone component even if its frequency fell according to age (63.2% in teenagers and 40.6% in elderly [p<0.05]) in favour of the increase of uric acid stones (3,5% in teenagers and 41.5% in elderly [p<0.05]). Struvite stones were rare (3.5%) and more frequent in children.

CONCLUSION

The analysis of these data shows that urinary stones in Tunisia are tending to evolve in the same direction as in industrialized countries.

Possible Relationship between Metabolic Syndrome Traits and Nephrolithiasis: Incidence for 15 Years According to Gender

PURPOSE

To analyze the independent effect of metabolic syndrome (MS) on nephrolithiasis (NL) despite differences in gender compared with the known lithogenic factors.

MATERIALS AND METHODS

From 1995 to 2009, 40,687 Koreans were enrolled in the study and observed for the development of NL at a health promotion center. The examination included anthropometric and biochemical measurements as well as kidney ultrasonography. A student's t-test or chi-square test was used to characterize the participants and a standard Cox proportional hazards model was used to calculate the adjusted odds ratio of lithogenic risk factors in the NL model.

RESULTS

The mean age of the study cohort was 44.9 years (range, 13-100 years), and 22,540 (55.4%) of the cohort was male. The incidence of NL was 1.5% (609 participants), with males exhibiting a higher incidence than females (1.9% vs 1.0%, p<0.01). Among the total cohort, MS as well as each trait of MS were risk factors for NL. In males, high body mass index (BMI), high blood pressure, and abnormal glucose metabolism were significant lithogenic factors, whereas in females, lithogenic factors included only high BMI and abnormal glucose metabolism.

CONCLUSIONS

MS is a significant lithogenic factor compared with other lithogenic factors. There was a correlated change in the prevalence of MS and NL and MS traits in Korea.

Uric Acid Nephrolithiasis: A Systemic Metabolic Disorder

Uric acid nephrolithiasis is characteristically a manifestation of a systemic metabolic disorder. It has a prevalence of about 10% among all stone formers, the third most common type of kidney stone in the industrialized world. Uric acid stones form primarily due to an unduly acid urine; less deciding factors are hyperuricosuria and a low urine volume. The vast majority of uric acid stone formers have the metabolic syndrome, and not infrequently, clinical gout is present as well. A universal finding is a low baseline urine pH plus insufficient production of urinary ammonium buffer. Persons with gastrointestinal disorders, in particular chronic diarrhea or ostomies, and patients with malignancies with a large tumor mass and high cell turnover comprise a less common but nevertheless important subset. Pure uric acid stones are radiolucent but well visualized on renal ultrasound. A 24 h urine collection for stone risk analysis provides essential insight into the pathophysiology of stone formation and may guide therapy. Management includes a liberal fluid intake and dietary modification. Potassium citrate to alkalinize the urine to a goal pH between 6 and 6.5 is essential, as undissociated uric acid deprotonates into its much more soluble urate form.

Metabolic syndrome, urine pH, and time-dependent risk of nephrolithiasis in Korean men without hypertension and diabetes

OBJECTIVE

To examine the independent effect of metabolic syndrome (MS) on nephrolithiasis (NL) even with changes in MS status over time.

METHODS

From 2002-2003, 3872 men who were reexamined annually or biannually until 2009 were enrolled in the analysis and observed for development of NL. The examination included anthropometric measurements, biochemical measurement, and kidney ultrasonography (US). A standard Cox proportional hazards model and a time-dependent Cox model were used to calculate the adjusted hazard ratio in the NL model.

RESULTS

After adjusting for age, baseline glomerular filtration rate, and uric acid level, MS at baseline was associated with a significantly increased risk of NL (HR, 1.771; 95% confidence interval, 1.157-2.711). MS over time as a time-dependent variable also predicted the development of NL (HR, 1.678; 95% CI, 1.151-2.447) after adjusted baseline covariate. After adjustment for potential confounding factors, there was a significant stepwise increase in risk of NL, with each additional MS trait compared with those with no traits of MS at baseline and follow-up. As the numbers of MS traits at baseline and follow-up increased, the urine pH of participants at baseline and follow-up decreased significantly (P <.01). The prevalence of NL in participants with continual MS (6.6%) was higher than those with resolved MS, and continual MS was an independent factor to predict NL.

CONCLUSION

Our findings suggest that MS is significantly associated with increased risk of developing urine acidification, even with changes in status of MS observed during follow-up.

Causal assessment of dietary acid load and bone disease: a systematic review & meta-analysis applying Hill's epidemiologic criteria for causality

Background

Modern diets have been suggested to increase systemic acid load and net acid excretion. In response, alkaline diets and products are marketed to avoid or counteract this acid, help the body regulate its pH to prevent and cure disease. The objective of this systematic review was to evaluate causal relationships between dietary acid load and osteoporosis using Hill's criteria.

Methods

Systematic review and meta-analysis. We systematically searched published literature for randomized intervention trials, prospective cohort studies, and meta-analyses of the acid-ash or acid-base diet hypothesis with bone-related outcomes, in which the diet acid load was altered, or an alkaline diet or alkaline salts were provided, to healthy human adults. Cellular mechanism studies were also systematically examined.

Results

Fifty-five of 238 studies met the inclusion criteria: 22 randomized interventions, 2 meta-analyses, and 11 prospective observational studies of bone health outcomes including: urine calcium excretion, calcium balance or retention, changes of bone mineral density, or fractures, among healthy adults in which acid and/or alkaline intakes were manipulated or observed through foods or supplements; and 19 in vitro cell studies which examined the hypothesized mechanism. Urine calcium excretion rates were consistent with osteoporosis development; however calcium balance studies did not demonstrate loss of whole body calcium with higher net acid excretion. Several weaknesses regarding the acid-ash hypothesis were uncovered: No intervention studies provided direct evidence of osteoporosis progression (fragility fractures, or bone strength as measured using biopsy). The supporting prospective cohort studies were not controlled regarding important osteoporosis risk factors including: weight loss during follow-up, family history of osteoporosis, baseline bone mineral density, and estrogen status. No study revealed a biologic mechanism functioning at physiological pH. Finally, randomized studies did not provide evidence for an adverse role of phosphate, milk, and grain foods in osteoporosis.

Conclusions

A causal association between dietary acid load and osteoporotic bone disease is not supported by evidence and there is no evidence that an alkaline diet is protective of bone health.

Infrared spectroscopic analysis of 5,248 urinary stones from Chinese patients presenting with the first stone episode

A series of 5,248 urinary stones was analyzed by Fourier transform infrared spectroscopy between 1999 and 2008. This study evaluated the percentage of each stone type and the association with sex and age in Chinese stone formers presenting with the first stone episode. The overall sex ratio (male:female) was 2.34:1. Results showed that the preponderant type of stone was calcium oxalate, followed by carbapatite, anhydrous uric acid, struvite and cystine. Struvite stones in this study accounted for a relatively low rate compared to that reported by others. Of 5,248 stones, only 38.1% had one component, 42.5% consisted of two components, and 20.4% consisted of three components. Our results also showed the higher percentage of carbapatite stones in females than in males and the increment of anhydrous uric acid stones with age. In addition, the percentage of calcium oxalate stones decreased with increase in the percentage of carbapatite stones over the period.

Difference in urinary stone components between obese and non-obese patients

The prevalence and incidence of urinary stone disease have been reported to be associated with body weight and body mass index (BMI). The aim of the study was to determine the difference in stone components among different BMI groups in patients with urolithiasis. Between Dec 2005 and Jan 2008, 907 urinary calculi were collected and analyzed by infrared spectroscopy. Most of the stones had been passed spontaneously, and some were collected during surgical manipulations. The data on patients' gender, age, BMI at diagnosis, and stone composition were collected. The patients were classified as normal weight (18.5≤ BMI <24), overweight (24≤ BMI <27), or obese (BMI ≥27). Of the 907 patients with urinary stone disease, 27.7% had normal weight, 33.5% were overweight, and 38.8% were obese. The prevalence of calcium oxalate stones in the normal weight, overweight, and obese groups were 23.1, 30.6, and 34.9%, respectively (P = 0.002), and the prevalence of uric acid stones in the different groups was 2.8, 7.2, and 7.7%, respectively (P = 0.002). The prevalence of calcium oxalate and uric acid stones, but not that of calcium phosphate stones, increased with body size. There was a significant correlation between BMI and uric acid stones in the overweight and obesity groups, with odds ratios of 3.28 and 4.35, respectively. The prevalence and incidence of urinary stone disease were found to be associated with BMI. The percentage of uric acid and calcium oxalate stones was higher in obese than in non-obese patients. There was no apparent difference in the prevalence of calcium phosphate stones between obese and non-obese patients.

Metabolic basis for low urine pH in type 2 diabetes

BACKGROUND AND OBJECTIVES

Type 2 diabetes is associated with excessively low urine pH, which increases the risk for uric acid nephrolithiasis. This study was conducted to assess the metabolic basis responsible for the excessive urinary acidity of individuals with type 2 diabetes.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Nine non-stone-forming patients who had type 2 diabetes and low urine pH and 16 age- and body mass index-matched non-stone-forming volunteers without type 2 diabetes were maintained on a constant metabolic diet for 7 days, and 24-hour urine was collected on the last 2 days of the diet.

RESULTS

Urine dietary markers (potassium, sulfate, phosphorus, and urea nitrogen) were not different between the two groups. Patients with type 2 diabetes exhibited a significantly lower 24-hour urine pH (5.45+/-0.27 versus 5.90+/-0.42; P<0.01) and higher net acid excretion (NAE; 57+/-12 versus 38+/-18 mEq/d; P<0.01) compared with control subjects. The proportion of NAE excreted as ammonium (NH4+/NAE) was significantly lower in patients with type 2 diabetes than in control subjects (0.70+/-0.12 versus 0.94+/-0.36; P<0.01); however, the greater NAE in patients with type 2 diabetes was not accounted for by the differences in unmeasured urinary anions.

CONCLUSIONS

The overly acidic urine in patients with type 2 diabetes persists after controlling for dietary factors, body size, and age. The lower pH is due to a combination of greater NAE and lower use of ammonia buffers in patients with diabetes, which predisposes them to uric acid urolithiasis.

Association of body mass index and urine pH in patients with urolithiasis

Increase in body size increases the risk of renal stone formation. The mechanism explaining this relationship remains unclear. Urine pH is one of the important factors for urinary stone formation. The purpose of this study was to determine whether there is an association between urine pH and body mass index (BMI) in patients with urolithiasis. Medical charts review that included 342 urinary stone formers (248 men and 94 women). Data obtained included patient sex, age, BMI, urine pH at diagnosis, and stone composition. The patients were classified as normal weight (18.5 <or= BMI < 24), overweight (24 <or= BMI < 27), or obese (BMI >or= 27). The mean urine pH of the normal body weight, overweight, and obese groups was 6.25, 6.14, and 6.00, respectively (P < 0.05). Urine pH is inversely related to BMI among patients with urolithiasis. Among patients with urolithiasis, higher BMI will have lower urine pH. This may explain why obesity is associated with an increased risk of nephrolithiasis. Weight loss should be explored as a potential treatment to prevent kidney stone formation. The prevention of urinary stone disease gives clinicians an additional reason to encourage weight reduction through diet.

Meta-analysis of the quantity of calcium excretion associated with the net acid excretion of the modern diet under the acid-ash diet hypothesis

BACKGROUND

The acid-ash diet hypothesis of osteoporosis suggests that acid from the modern diet causes a demineralization of the skeleton, and mobilized bone calcium is excreted. A systematic approach has not been used to summarize the findings of the numerous studies about the hypothesis.

OBJECTIVES

The purpose of this meta-analysis was to estimate the quantity of net acid excretion and calciuria associated with the modern diet, to assess the association between acid excretion and calcium excretion, and to assess the influence of urine preservatives on calcium measurement.

DESIGN

We systematically searched for trials of the acid-ash hypothesis and conducted a meta-analysis.

RESULTS

Twenty-five of 105 studies met the inclusion criteria. The estimated quantity of net acid excretion from the weighted average of the control diets from 11 studies was 47 mEq/d. The increase in urinary calcium with a change in renal net acid excretion depended on whether the urine was acidic or alkaline (P < 0.001). A significant linear relation was observed between net acid excretion and calcium excretion for both acidic and alkaline urine (P < 0.001). The estimated change in urine calcium associated with a change of 47 mEq of net acid excretion in acidic urine was 1.6 mmol/d (66 mg/d) of calcium.

CONCLUSION

Evidence suggests a linear association between changes in calcium excretion in response to experimental changes in net acid excretion. However, this finding is not evidence that the source of the excreted calcium is bone or that this calciuria contributes to the development of osteoporosis.

Extracorporeal shock wave lithotripsy of impacted radiolucent stone at the right pyeloureteric junction and oral dissolution therapy in a patient with transplanted liver: a case report

Extracorporeal shock wave lithotripsy (ESWL) is a safe, effective method to treat urinary lithiasis. The success rate in ESWL depends on stone location, size, number, and fragility as well as calceal anatomy and patency of the urinary tract. An association of calcineurin inhibitors and uric acid urolithiasis has been reported in renal allograft recipients, but the mechanism remains unknown. Herein we have reported the case of 68-year-old male patient who developed cryptogenic cirrhosis and underwent liver transplantation. Seven years after transplantation, the patient was admitted to the hospital with right renal colic. An 8.9-mm radiolucent stone at the pyeloureteric junction was associated with moderate consecutive hydronephrosis. The second stone was located in a lower renal calyx. After a failed attempt at retrograde ureteral stenting, we performed a percutaneous nephrostomy. Antegrade pyelography with following ESWL treatment resulted in disintegration of the obstructive stone at the pyeloureteric junction. Afterward, we performed antegrade placement of a double-J stent. Residual stones in the lower renal calyx were successfully treated with a 3-month course of oral intake of a dissolution agent-potassium sodium hydrogen citrate. In this case, we have discussed ESWL and oral dissolution therapy of radiolucent stones in a hydronephrotic right kidney, which resulted in stone-free disease after 3 months of combined therapy. There was neither clinical nor biochemical damage to the transplanted liver.

Diabetes and nephrolithiasis

Type 2 diabetes is associated with an increased risk of nephrolithiasis, specifically in the form of uric acid (UA) nephrolithiasis. Diabetic patients who produce uric stones exhibit a low urine pH, the key factor of UA crystallization. Production of such acidic urine appears to result from the insulin-resistant state characteristic of diabetes mellitus. Insulin resistance is also involved in the pathogenesis of primary UA nephrolithiasis observed in overweight subjects with the metabolic syndrome. Therefore, UA nephrolithiasis should be considered a possible manifestation of insulin resistance, as it already is for hyperuricemia. Occurrence of UA stones in a patient, especially if overweight or hypertensive, should prompt a search for components of the metabolic syndrome in order to implement therapeutic intervention aimed at preventing the development of type 2 diabetes and atherosclerotic complications. Reciprocally, diabetologists should be aware of the risk of UA stones in their patients.

Uric Acid and renal disease

The interrelationship between uric acid and renal disease is reviewed in a historical context. Four phases can be distinguished--the descriptions of uric acid stones and gravel in the eighteenth century, of chronically scarred kidneys containing urate crystals in the nineteenth, the appearance of the syndrome of acute urate nephropathy following tumour lysis in the mid twentieth century, and finally the realization that soluble urate affects both systemic and glomerular blood vessels, and may play a role in both hypertension and chronic renal damage.

Estimated urine pH and bladder cancer risk in a cohort of male smokers (Finland)

OBJECTIVE

Low urine pH may be an important risk factor for bladder cancer, although few studies have evaluated this association. We examined the relationship between estimated renal net acid excretion (NAE), an indirect measure of urine pH based on nutrient intake and anthropometry, and bladder cancer risk in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study.

METHODS

At baseline, 27,096 male smokers 50-69 years old completed a dietary questionnaire that assessed usual frequency of consumption and portion sizes for the previous 12 months, had height and weight measured, and provided a history of smoking. A total of 446 incident bladder cancer cases were identified during up to 17.4 years of follow-up.

RESULTS

In multivariate proportional hazards models, the relative risk (RR) for bladder cancer was 1.15 (95% confidence interval (CI)=0.86-1.55) for individuals in the highest (i.e., most acidic) versus the lowest (i.e., least acidic) NAE quintile (p=0.38). Among men who smoked for more than 45 years, there was a suggestion of increased risk with higher NAE levels (RR=1.72, 95% CI=0.96-3.10, p=0.08).

CONCLUSIONS

These findings do not indicate that urine pH is a major risk factor for bladder cancer, although certain subsets of individuals may be at increased risk.

Acid-base metabolism: implications for kidney stones formation

The physiology and pathophysiology of renal H+ ion excretion and urinary buffer systems are reviewed. The main focus is on the two major conditions related to acid-base metabolism that cause kidney stone formation, i.e., distal renal tubular acidosis (dRTA) and abnormally low urine pH with subsequent uric acid stone formation. Both the entities can be seen on the background of disturbances of the major urinary buffer system, NH3+ <--> NH4+. On the one hand, reduced distal tubular secretion of H+ ions results in an abnormally high urinary pH and either incomplete or complete dRTA. On the other hand, reduced production/availability of NH4+ is the cause of an abnormally low urinary pH, which predisposes to uric acid stone formation. Most recent research indicates that the latter abnormality may be a renal manifestation of the increasingly prevalent metabolic syndrome. Despite opposite deviations from normal urinary pH values, both the dRTA and uric acid stone formation due to low urinary pH require the same treatment, i.e., alkali. In the dRTA, alkali is needed for improving the body's buffer capacity, whereas the goal of alkali treatment in uric acid stone formers is to increase the urinary pH to 6.2-6.8 in order to minimize uric acid crystallization.

Uric acid stones following hepatic transplantation

We report the case of a 52 year old man with a history of insulin-requiring diabetes and hepatitis B with cirrhosis who received an orthotopic liver transplant. One year later he developed renal colic and was found to have a 3 mm stone at the left ureterovesical junction. Numerous other stones formed and infrared spectroscopy analysis demonstrated all to be composed of 100% uric acid. Urine collections demonstrated a low urine pH of 5.1 without hyperuricosuria. His stones were effectively prevented with potassium citrate therapy. Few incidence data are available for uric acid stone occurrence in solid organ recipients. Calcineurin inhibitors are thought to often cause hyperuricemia on the basis of decreased urate excretion. However, this effect would not be expected to cause hyperuricosuria nor uric acid stones. This class of drugs may also be associated with low urine pH, perhaps on the basis of hypoaldosteronism, but the contribution of such a syndrome to uric acid stone formation is not established.

Novel insights into the pathogenesis of uric acid nephrolithiasis

PURPOSE OF REVIEW

The factors involved in the pathogenesis of uric acid nephrolithiasis are well known. A low urinary pH is the most significant element in the generation of stones, with hyperuricosuria being a less common finding. The underlying mechanism(s) responsible for these disturbances remain poorly characterized. This review summarizes previous knowledge and highlights some recent developments in the pathophysiology of low urine pH and hyperuricosuria.

RECENT FINDINGS

Epidemiological and metabolic studies have indicated an association between uric acid nephrolithiasis and insulin resistance. Some potential mechanisms include impaired ammoniagenesis caused by resistance to insulin action in the renal proximal tubule, or substrate competition by free fatty acids. The evaluation of a large Sicilian kindred recently revealed a putative genetic locus linked to uric acid stone disease. The identification of novel complementary DNA has provided an interesting insight into the renal handling of uric acid, including one genetic cause of renal uric acid wasting.

SUMMARY

The recognition of metabolic, molecular, and genetic factors that influence urinary pH, and uric acid metabolism and excretion, will provide novel insights into the pathogenesis of uric acid stones, and open the way for new therapeutic strategies.

Dogmas and controversies in the handling of nitrogenous wastes: Excretion of nitrogenous wastes in human subjects

Two major nitrogenous waste products, urea and ammonium(NH4+), are produced in humans when proteins are oxidized, and in this manuscript their excretions are examined from two perspectives. First, the specific physiology of each nitrogenous waste is reviewed and the current dogmas summarized. Second, their excretions are considered in the context of integrative physiology, i.e. the need to ensure that the urine composition is appropriate to minimize the risk of kidney stone formation. After the latter analysis, weak links in our understanding of the overall physiology become apparent and a conundrum is defined. The conundrum for the excretion of urea focuses on the fact that urea is not an effective osmole in the medullary-collecting duct when vasopressin acts. As a result, it appears that urinary urea cannot prevent a large decline in the urine flow rate and thereby minimize the risk of forming kidney stones in electrolyte-poor urine. The conundrum for the excretion of NH4+ is: high rates of NH4+excretion require a low urine pH, yet a pH ∼6.0 must be maintained in order to reduce the risk of precipitating uric acid in the urine. Possible ways of resolving these conundrums require novel physiological interpretations.

Changes in stone composition according to age and gender of patients: a multivariate epidemiological approach

Urinary stone incidence and composition have changed markedly over the past half-century in industrialized countries, in parallel with profound changes in living standards and dietary habits, with a dramatic increase in the incidence of calcium oxalate stones. However, studies evaluating the influence of age and gender on the distribution of the various types of urinary calculi are scarce. We report the results of a study based on 27,980 calculi (from 19,442 males and 8,538 females) analyzed by infrared spectroscopy between 1976 and 2001. The relationships between age and sex and stone composition were investigated using a multivariate approach, based on correspondence factor analysis (CFA). We found a male predominance for calcium oxalate and uric acid, a female preponderance for calcium phosphate and struvite stones, and an increasing prevalence of uric acid stones with age in both genders. CFA was able to reconstruct in blind the age curve from stone composition. The first two axes of the multidimensional classification, which correspond to age, included 86.9% of the total variance, indicating that age was the main factor involved in stone type. Superimposition of age classes and stone components showed a strong relationship between age and whewellite, weddellite, brushite, carbapatite, octacalcium phosphate and uric acid, while other substances (whitlockite, amorphous carbonated calcium phosphate, struvite, proteins, mucopolysaccharides, triglycerides or ammonium urate) appeared weakly related to age. In addition, CFA suggests the role of common lithogenic factors between weddellite, carbapatite and brushite, which clustered in the same area, whereas the various crystalline forms of phosphate stones segregated into two different clusters, suggesting distinct pathogenic factors. In conclusion, this study provides a picture of the present epidemiology of urinary stones in France. CFA helped to confirm: (1) an etiopathogenic distinction between weddellite and whewellite, (2) etiopathogenic associations between chemical compounds, which were only suspected on a clinical basis, and (3) suggested yet unrecognized associations, especially with respect to the heterogeneous group of phosphate stones.

Urinary glycosaminoglycans as risk factors for uric acid nephrolithiasis: case control study in a Sardinian genetic isolate

OBJECTIVES

To assess the clinical association between glycosaminoglycan (GAG) excretion and uric acid (UA) nephrolithiasis by measuring urinary GAG levels in a case-control study conducted in a Sardinian genetic isolate. Inhibitors of crystallization such as GAGs seem to be involved in kidney stone formation.

METHODS

Overnight (12-hour) urinary excretion of GAGs, calcium, oxalate, and UA were measured in urine samples from 60 patients who had formed at least one urinary stone (UA or mixed) and 52 healthy controls. The total GAG concentration was measured by a dye-binding assay, and the values were normalized against creatinine to obtain values in micrograms of GAG per milligram creatinine. Statistical analysis was performed using t tests and logistic regression analysis.

RESULTS

No significant difference was found between the two groups with respect to calcium and oxalate concentrations. Nonetheless, stone formers had significantly lower levels of GAGs (29.5 +/- 2.2 versus 36.4 +/- 3.9 microg/mg creatinine, P = 0.003) and greater levels of UA (385.11 +/- 38.2 versus 298.43 +/- 31.4 mg/12 hr, P = 0.0010) than did the normal controls.

CONCLUSIONS

We report that the lower excretion of GAGs in stone formers could impair their inhibitory activity on UA stone formation, and, as a consequence, it may represent a risk factor for this form of urolithiasis.

Unique pattern of urinary tract calculi in Australian Aboriginal children

Young Aboriginal children in remote regions of tropical and desert Australia are at risk of developing urate stones in their upper urinary tract from an early age. These radiolucent calculi were only recognized with the availability of ultrasound diagnosis and are not associated with anatomic anomalies or abnormal uric acid production/metabolism. Although these stones appear to resolve spontaneously after the weaning period, some result in ureteric obstruction and infection which may lead to renal damage. This pattern of urolithiasis differs from the usual global urolithiasis pattern of either endemic bladder stones in young children in developing countries or predominantly calcium-based stones in upper tracts of older children and adults in affluent industrialized countries, where upper tract urate stones account for only a minority of childhood urinary tract stones. Risk factors for urate stones are low urine output and acidic urine. An association between urolithiasis and carbohydrate intolerance leading to chronic acidosis has been suggested for Aboriginal children, but existing limited evidence does not support this as a major aetiological factor. Although further studies on the epidemiology, natural history and management of these urate stones are needed, we believe the focus should be on improving the known social and environmental risk factors of remote Aboriginal children during the weaning period which contribute to the unacceptably high prevalence of failure to thrive, diarrhoeal disease, environmental enteropathy, iron deficiency and urolithiasis.

Pathophysiology of uric acid nephrolithiasis

Humans although a predominantly ureotylic organism, has preserved the ability to excrete nitrogen as uric acid and ammonia. An imbalance between these two secondary modes of nitrogen excretion has resulted in uric acid precipitation in human urine. Uric acid nephrolithiasis can arise from diverse etiologies all with distinct underlying defects converging to one or more of three defects of hyperuricosuria, acidic urine pH, and low urinary volume, originating from secondary, genetic or heretofore undefined (idiopathic) causes. A subset of idiopathic uric acid nephrolithiasis (gouty diathesis) may be the "tip of the icebergp" of a broader systemic illness characterized by insulin resistance. A novel renal manifestation of insulin resistance is a mild defect in ammonium excretion, which is not severe enough to disturb acid-base homeostasis, but is sufficient to set up the chemical milieu for uric acid nephrolithiasis.

Facilitating an understanding of integrative physiology: emphasis on the composition of body fluid compartments

As a teaching exercise, we used deductive reasoning and a quantitative analysis to convert a number of facts into a series of concepts to facilitate an understanding of integrative physiology and shed light on the composition of the different body fluid compartments. The starting point was the central need to regenerate ATP to perform biologic work. Because a large quantity of O2 must be delivered to cells at a sufficiently high concentration to aid its diffusion into mitochondria, approximately one third of the O2 in inspired air was extracted; this led to a P(CO2) in arterial blood of 40 mmHg (1 mmHg = 133.322 Pa). Blood flow to individual organs must be adjusted precisely to avoid having too low or too high a P(O2) in mitochondria--the latter augments the formation of reactive O2 species. The extracellular fluid (ECF) bicarbonate concentration (E(HCO3)) must be high to minimize H+ buffering by proteins. This high E(HCO3) sets the ECF concentrations of ionized calcium (Ca2+) and inorganic phosphate (HPO4(2-)) because of solubility issues. Three features defined the intracellular fluid (ICF) volume and composition. First, expelling monovalent anions minimized its mass (volume). Second, controlling the tissue P(CO2) ensured a relatively constant net valence on intracellular proteins. Third, the range of ICF Ca2+ concentrations must both induce regulatory signals and avoid Ca3(PO4)2 formation. All the above were incorporated into the integrated response that optimized the capacity for vigorous exercise.