The effects of acid on bone

Bone mineral density and growth changes in patients with distal renal tubular acidosis after two-years treatment with a new alkalizing drug (ADV7103)

BACKGROUND AND OBJECTIVES

ADV7103 is a new prolonged-release treatment for distal renal tubular acidosis (dRTA), containing potassium citrate and potassium bicarbonate. Since acidosis may affect bone mineral contents, the effects of ADV7103 on bone mineral density (BMD) and growth in patients with dRTA over 24 months were evaluated.

PATIENTS AND METHODS

Thirty patients (24 paediatric patients and 6 adults) were included in an open-label extension study after a phase II/III trial. BMD, measured by densitometry, was assessed at baseline and at 24 months. Growth was evaluated throughout the study. Plasma bicarbonate, parathyroid hormone, 25-hydroxy vitamin D, 1,25-dihydroxy vitamin D, bone alkaline phosphatase, calciuria and citraturia, were also determined. Safety and treatment compliance were evaluated as well.

RESULTS

After 24 months of treatment with ADV7103, mean spine BMD z-score values significantly increased as compared with baseline (p=0.024). In adults, spine and whole-body densitometry z-scores showed a significant correlation with plasma bicarbonate levels (rS=0.82 and rS=0.97, respectively, p<0.005). There was an increase>0.5 units in z-scores for height and weight in 18% and 36% of the paediatric patients, respectively. With treatment, plasma bicarbonate concentration and calciuria at the different visits were normal in 69-86% and 93-96% patients, respectively. Only nine treatment-related gastrointestinal AEs of mild/moderate severity, were reported in five patients.

CONCLUSIONS

Two years of ADV7103 treatment improved growth and increased spine BMD. These results suggest that control of acidosis by ADV7103 treatment improves bone parameters.

Comprehensive Associations between Acidosis and the Skeleton in Patients with Kidney Disease

SIGNIFICANCE STATEMENT

Renal osteodystrophy (ROD) contributes substantially to morbidity in CKD, including increased fracture risk. Metabolic acidosis (MA) contributes to the development of ROD, but an up-to-date skeletal phenotype in CKD-associated acidosis has not been described. We comprehensively studied associations between acidosis and bone in patients with CKD using advanced methods to image the skeleton and analyze bone-tissue, along with biochemical testing. Cross-sectionally, acidosis was associated with higher markers of bone remodeling and female-specific impairments in cortical and trabecular bone quality. Prospectively, acidosis was associated with cortical expansion and trabecular microarchitectural deterioration. At the bone-tissue level, acidosis was associated with deficits in bone mineral content. Future work investigating acidosis correction on bone quality is warranted.

BACKGROUND

Renal osteodystrophy is a state of impaired bone quality and strength. Metabolic acidosis (MA) is associated with alterations in bone quality including remodeling, microarchitecture, and mineralization. No studies in patients with CKD have provided a comprehensive multimodal skeletal phenotype of MA. We aim to describe the structure and makeup of bone in patients with MA in the setting of CKD using biochemistry, noninvasive imaging, and histomorphometry.

METHODS

The retrospective cross-sectional analyses included 180 patients with CKD. MA was defined as bicarbonate ≤22 mEq/L. We evaluated circulating bone turnover markers and skeletal imaging with dual energy x-ray absorptiometry and high-resolution peripheral computed tomography. A subset of 54 participants had follow-up. We assessed associations between baseline and change in bicarbonate with change in bone outcomes. Histomorphometry, microCT, and quantitative backscatter electron microscopy assessed bone biopsy outcomes in 22 participants.

RESULTS

The mean age was 68±10 years, 54% of participants were male, and 55% were White. At baseline, acidotic subjects had higher markers of bone turnover, lower areal bone mineral density at the radius by dual energy x-ray absorptiometry, and lower cortical and trabecular volumetric bone mineral density and impaired trabecular microarchitecture. Over time, acidosis was associated with opposing cortical and trabecular effects: cortical expansion but trabecular deterioration. Bone-tissue analyses showed reduced tissue mineral density with increased heterogeneity of calcium distribution in acidotic participants.

CONCLUSIONS

MA is associated with multiple impairments in bone quality. Future work should examine whether correction of acidosis improves bone quality and strength in patients with CKD.

Preclinical and Clinical Evidence of Effect of Acid on Bone Health

Acid can have ill effect on bone health in the absence of frank clinical acidosis but affecting the bone mioneral matrix and bone cells via complex pathways botyh ascute;y and chronically. While the reaction of bone to an acid load is conserved in evolution and is adaptive, the capacity can be overwhelmed resulting in dire consequences. The preclinical an clincl evidence of the acdi effect on bone is very convincing and the clinical evidence in both association and interventiopn studies are also quite credible, The adverse effects of acid on bone is underappreoicated, under-investigated, and the potential benefits of alkali therapy is not generrally known.

Dietary Acid Load and Bone Health: A Systematic Review and Meta-Analysis of Observational Studies

Findings on the association between dietary acid load (DAL) and bone health are conflicting. This study aimed to summarize available studies on the association between DAL and risk of fractures or bone mineral density (BMD) in adults. Online databases including PubMed, Scopus, and Embase were searched for relevant studies published up to June 2021, using pertinent keywords. We identified observational studies (cohort, case-control, and cross-sectional) investigating the association between DAL and risk of fractures or BMD, then selected studies following these reported criteria: RRs with corresponding 95% CIs for the relationship between DAL and fracture risk; correlation coefficients for the association between DAL and BMD; and mean ± SD of BMD values across the categories of DAL. Overall, 17 studies with 80545 individuals were included. There was no significant relationship between the PRAL and fracture risk (Pooled RR: 1.18; 95% confidence interval 0.98 to 1.41, I² = 60.6%). Moreover, a similar association was observed between the NEAP and fracture risk (Pooled RR: 1.41, 95% CI: 0.79 to 2.52, I² = 54.1%). The results of five studies from four publications revealed no significant association between dietary PRAL score and femoral and spinal BMD (WMD femoral = −0.01, 95% confidence interval: −0.02 to 0.01, I² = 76.5%; WMD spinal = −0.01, 95% CI: −0.03 to 0.01, I² = 56.7%). However, being in the highest category of NEAP was significantly associated with a lower femoral and spinal BMD (WMD femoral = −0.01, 95% CI: −0.02 to −0.00, I² = 82.1%; WMD spinal = −0.02, 95% CI: −0.03 to −0.01, I² = 93%). It was showed that adopting diets high in acidity was not associated with risk of fractures. We also found a significant negative relationship between NEAP and BMD. However, DAL based on PRAL was not associated with BMD.

Oral Acid Load Down-Regulates Fibroblast Growth Factor 23

Increased dietary acid load has a negative impact on health, particularly when renal function is compromised. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that is elevated during renal failure. The relationship between metabolic acidosis and FGF23 remains unclear. To investigate the effect of dietary acid load on circulating levels of FGF23, rats with normal renal function and with a graded reduction in renal mass (1/2 Nx and 5/6 Nx) received oral NH4Cl for 1 month. Acid intake resulted in a consistent decrease of plasma FGF23 concentrations in all study groups when compared with their non-acidotic control: 239.3 ± 13.5 vs. 295.0 ± 15.8 pg/mL (intact), 346.4 ± 19.7 vs. 522.6 ± 29.3 pg/mL (1/2 Nx) and 988.0 ± 125.5 vs. 2549.4 ± 469.7 pg/mL (5/6 Nx). Acidosis also decreased plasma PTH in all groups, 96.5 ± 22.3 vs. 107.3 ± 19.1 pg/mL, 113.1 ± 17.3 vs. 185.8 ± 22.2 pg/mL and 504.9 ± 75.7 vs. 1255.4 ± 181.1 pg/mL. FGF23 showed a strong positive correlation with PTH (r = 0.877, p < 0.0001) and further studies demonstrated that acidosis did not influence plasma FGF23 concentrations in parathyroidectomized rats, 190.0 ± 31.6 vs. 215 ± 25.6 pg/mL. In conclusion, plasma concentrations of FGF23 are consistently decreased in rats with metabolic acidosis secondary to increased acid intake, both in animals with intact renal function and with decreased renal function. The in vivo effect of metabolic acidosis on FGF23 appears to be related to the simultaneous decrease in PTH.

A facile aqueous production of bisphosphonated-polyelectrolyte functionalized magnetite nanoparticles for pH-specific targeting of acidic-bone cells

Bone malignancy treatment is being hindered due to the insufficient selectivity of therapeutic nanoparticles towards malignant bone sites. Polyelectrolyte functionalized magnetic nanoparticles having dually specific pH-sensing ability and bisphosphonate moieties, can be an effective solution for selective targeting of bone malignancies. First, polyelectrolyte was prepared via N-carboxycitraconyzation of chitosan (NCCS) followed by successive functionalization with alendronic acid (AL) and fluorescein isothiocyanate (FITC). Then, Fe₃O₄-NCCS-FITC-AL nanoparticles were synthesized by a facile one-step microwave-assisted aqueous method via in situ surface functionalization. The formation, crystal structure, and surface conjugation of Fe₃O₄ nanoparticles with polyelectrolytic stabilizer were confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analyses. Synthesized Fe₃O₄-NCCS-FITC-AL nanoparticles were superparamagnetic, colloidally stable and highly hemocompatible under physiological conditions. Moreover, at pH 5.0, Fe₃O₄-NCCS-FITC-AL nanoparticles formed a precipitate due to inversion of their surface charge. This pH-dependent charge-inversion drastically changed the interactions with erythrocytes and bones. Selective membranolysis of erythrocytes occurred at pH 5.0. The designed nanoparticles showed enough potential for selective targeting of pathological bone sites in early-stage magnetofluorescent imaging and as a therapeutics carrier to treat malignant bone diseases.

Synthesis of a bisphosphonated polyelectrolytic stabilizer to in situ fabricate and functionalize Fe₃O₄ nanoparticles and their pH-dependent hemolysis and bone-cell adhesion.

The Association of Gut Microbiota With Osteoporosis Is Mediated by Amino Acid Metabolism: Multiomics in a Large Cohort

CONTEXT

Several small studies have suggested that the gut microbiome might influence osteoporosis, but there is little evidence from human metabolomics studies to explain this association.

OBJECTIVE

This study examined the association of gut microbiome dysbiosis with osteoporosis and explored the potential pathways through which this association occurs using fecal and serum metabolomics.

METHODS

We analyzed the composition of the gut microbiota by 16S rRNA profiling and bone mineral density using dual-energy X-ray absorptiometry in 1776 community-based adults. Targeted metabolomics in feces (15 categories) and serum (12 categories) were further analyzed in 971 participants using ultra-high-performance liquid chromatography coupled to tandem mass spectrometry.

RESULTS

This study showed that osteoporosis was related to the beta diversity, taxonomy, and functional composition of the gut microbiota. The relative abundance of Actinobacillus, Blautia, Oscillospira, Bacteroides, and Phascolarctobacterium was positively associated with osteoporosis. However, Veillonellaceae other, Collinsella, and Ruminococcaceae other were inversely associated with the presence of osteoporosis. The association between microbiota biomarkers and osteoporosis was related to levels of peptidases and transcription machinery in microbial function. Fecal and serum metabolomics analyses suggested that tyrosine and tryptophan metabolism and valine, leucine, and isoleucine degradation were significantly linked to the identified microbiota biomarkers and to osteoporosis, respectively.

CONCLUSION

This large population-based study provided robust evidence connecting gut dysbiosis, fecal metabolomics, and serum metabolomics with osteoporosis. Our results suggest that gut dysbiosis and amino acid metabolism could be targets for intervention in osteoporosis.

Effects of veverimer on serum bicarbonate and physical function in diabetic patients with chronic kidney disease and metabolic acidosis: subgroup analysis from a randomized, controlled trial

Background

Metabolic acidosis is a complication of chronic kidney disease (CKD) that increases risk of CKD progression, and causes bone demineralization and muscle protein catabolism. Patients with diabetes are prone to metabolic acidosis and functional limitations that decrease quality of life. Veverimer, an investigational, non-absorbed polymer that binds and removes gastrointestinal hydrochloric acid, is being developed as treatment for metabolic acidosis. This post hoc subgroup analysis evaluated effects of veverimer on metabolic acidosis and physical function among patients with diabetes.

Methods

This was a Phase 3, multicenter, randomized, blinded, placebo-controlled trial in 196 patients with CKD (estimated glomerular filtration rate 20–40 mL/min/1.73 m²) and metabolic acidosis who were treated for up to 1 year with veverimer or placebo.

Results

At Week 52, veverimer-treated patients with diabetes (n = 70), had a significantly greater increase in mean serum bicarbonate than the placebo group (n = 57) (4.4 versus 2.9 mmol/L, P < 0.05). Patient-reported limitations of physical function on the Kidney Disease and Quality of Life-Physical Function Domain (e.g. walking several blocks and climbing a flight of stairs) improved significantly in the veverimer versus placebo group (+12.5 versus +0.3, respectively, P < 0.001) as did objective physical performance on the repeated chair stand test (P < 0.0001).

Conclusions

Few interventions for patients with diabetes and CKD have successfully improved quality of life or physical functioning. Our study demonstrated that veverimer effectively treated metabolic acidosis in patients with diabetes and CKD, and significantly improved how these patients felt and functioned.

Global Variations in the Mineral Content of Bottled Still and Sparkling Water and a Description of the Possible Impact on Nephrological and Urological Diseases

Kidney stone disease (KSD) is a complex disease. Besides the high risk of recurrence, its association with systemic disorders contributes to the burden of disease. Sufficient water intake is crucial for prevention of KSD, however, the mineral content of water might influence stone formation, bone health and cardiovascular (CVD) risk. This study aims to analyse the variations in mineral content of bottled drinking water worldwide to evaluate the differences and describes the possible impact on nephrological and urological diseases. The information regarding mineral composition (mg/L) on calcium, bicarbonate, magnesium, sodium and sulphates was read from the ingredients label on water bottles by visiting the supermarket or consulting the online shop. The bottled waters in two main supermarkets in 21 countries were included. The evaluation shows that on a global level the mineral composition of bottled drinkable water varies enormously. Median bicarbonate levels varied by factors of 12.6 and 57.3 for still and sparkling water, respectively. Median calcium levels varied by factors of 18.7 and 7.4 for still and sparkling water, respectively. As the mineral content of bottled drinking water varies enormously worldwide and mineral intake through water might influence stone formation, bone health and CVD risk, urologists and nephrologists should counsel their patients on an individual level regarding water intake.

Results of a Gene Panel Approach in a Cohort of Patients with Incomplete Distal Renal Tubular Acidosis and Nephrolithiasis

BACKGROUND

Distal renal tubular acidosis (dRTA) is characterized by an impairment of urinary acidification resulting in metabolic acidosis, hypokalemia, and inappropriately elevated urine pH. If not treated, this chronic condition eventually leads to nephrocalcinosis, nephrolithiasis, impaired renal function, and bone demineralization. dRTA is a well-defined entity that can be diagnosed by genetic testing of 5 genes known to be disease-causative. Incomplete dRTA (idRTA) is defined as impaired urinary acidification that does not lead to overt metabolic acidosis and therefore can be diagnosed if patients fail to adequately acidify urine after an ammonium chloride (NH4Cl) challenge or furosemide and fludrocortisone test. It is still uncertain whether idRTA represents a distinct entity or is part of the dRTA spectrum and whether it is caused by mutations in the same genes of overt dRTA.

METHODS

In this cross-sectional study, we investigated a group of 22 stone formers whose clinical features were suspicious of idRTA. They underwent an NH4Cl challenge and were found to have impaired urinary acidification ability. These patients were then analyzed by genetic testing with sequencing of 5 genes: SLC4A1, ATP6V1B1, ATP6V0A4, FOXI1, and WDR72.

RESULTS

Two unrelated individuals were found to have two different variants in SLC4A1 that had never been described before.

CONCLUSIONS

Our results suggest the involvement of other genes or nongenetic tubular dysfunction in the pathogenesis of idRTA in stone formers. However, genetic testing may represent a cost-effective tool to recognize, treat, and prevent complications in these patients.

Distal Renal Tubular Acidosis: ERKNet/ESPN Clinical Practice Points

Distal renal tubular acidosis (dRTA) is characterised by an impaired ability of the distal tubule to excrete acid, leading to metabolic acidosis. Associated complications include bone disease, growth failure, urolithiasis and hypokalaemia. Due to its rarity, there is a limited evidence to guide diagnosis and management, however, available data strongly suggest that metabolic control of the acidosis by alkali supplementation can halt or revert almost all complications. Despite this, cohort studies show that adequate metabolic control is present in only about half of patients, highlighting problems with treatment provision or adherence. With these clinical practice points the authors, part of the working groups tubulopathies in the European Rare Kidney Disease Reference network (ERKnet) and inherited kidney diseases of the European Society for Paediatric Nephrology (ESPN) aim to provide guidance for the management of patients with dRTA to facilitate adequate treatment and establish an initial best practice standard against which treatment of patients can be audited.

Distal renal tubular acidosis: a systematic approach from diagnosis to treatment

Renal tubular acidosis (RTA) comprises a group of disorders in which excretion of hydrogen ions or reabsorption of filtered HCO₃ is impaired, leading to chronic metabolic acidosis with normal anion gap. In the current review, the focus is placed on the most common type of RTA, Type 1 RTA or Distal RTA (dRTA), which is a rare chronic genetic disorder characterized by an inability of the distal nephron to secrete hydrogen ions in the presence of metabolic acidosis. Over the years, knowledge of the molecular mechanisms behind acid secretion has improved, thereby greatly helping the diagnosis of dRTA. The primary or inherited form of dRTA is mostly diagnosed in infancy, childhood, or young adulthood, while the acquired secondary form, as a consequence of other disorders or medications, can happen at any age, although it is more commonly seen in adults. dRTA is not as “benign” as previously assumed, and can have several, highly variable long-term consequences. The present review indeed reports and summarizes both clinical symptoms and diagnosis, long-term outcomes, genetic inheritance, epidemiology and current treatment options, with the aim of shedding more light onto this rare disorder. Being a chronic condition, dRTA also deserves attention in the transition between pediatric and adult nephrology care, and as a rare disease it has a place in the European and Italian rare nephrological diseases network.

Effects of acidosis on the structure, composition, and function of adult murine femurs

Physiologic pH is maintained in a narrow range through multiple systemic buffering systems. Metabolic Acidosis (MA) is an acid-base disorder clinically characterized by a decrease in systemic pH and bicarbonate (HCO₃^-) levels. Acidosis affects millions annually, resulting in decreased bone mineral density and bone volume and an increased rate of fracture. We developed an adult murine model of diet-induced metabolic acidosis via graded NH₄Cl administration that successfully decreased systemic pH over a 14 day period to elucidate the effects of acidosis on the skeletal system. Blood gas analyses measured an increase in blood calcium and sodium levels indicating a skeletal response to 14 days of acidosis. MA also significantly decreased femur ultimate strength, likely due to modifications in bone morphology as determined from decreased microcomputed tomography values of centroid distance and area moment of inertia. These structural changes may be caused by aberrant remodeling based on histological data evidencing altered OCL activity in acidosis. Additionally, we found that acidosis significantly decreased bone CO₃ content in a site-specific manner similar to the bone phenotype observed in human MA. We determined that MA decreased bone strength thus increasing fracture risk, which is likely caused by alterations in bone shape and compounded by changes in bone composition. Additionally, we suggest the temporal regulation of cell-mediated remodeling in MA is more complex than current literature suggests. We conclude that our model reliably induces MA and has deleterious effects on skeletal form and function, presenting similarly to the MA bone phenotype in humans.

The therapeutic importance of acid-base balance

Baking soda and vinegar have been used as home remedies for generations and today we are only a mouse-click away from claims that baking soda, lemon juice, and apple cider vinegar are miracles cures for everything from cancer to COVID-19. Despite these specious claims, the therapeutic value of controlling acid-base balance is indisputable and is the basis of Food and Drug Administration-approved treatments for constipation, epilepsy, metabolic acidosis, and peptic ulcers. In this narrative review, we present evidence in support of the current and potential therapeutic value of countering local and systemic acid-base imbalances, several of which do in fact involve the administration of baking soda (sodium bicarbonate). Furthermore, we discuss the side effects of pharmaceuticals on acid-base balance as well as the influence of acid-base status on the pharmacokinetic properties of drugs. Our review considers all major organ systems as well as information relevant to several clinical specialties such as anesthesiology, infectious disease, oncology, dentistry, and surgery.

Citrate Supplementation Restores the Impaired Mineralisation Resulting from the Acidic Microenvironment: An In Vitro Study

Chronic metabolic acidosis leads to bone-remodelling disorders based on excessive mineral matrix resorption and inhibition of bone formation, but also affects the homeostasis of citrate, which is an essential player in maintaining the acid-base balance and in driving the mineralisation process. This study aimed to investigate the impact of acidosis on the osteogenic properties of bone-forming cells and the effects of citrate supplementation in restoring the osteogenic features impaired by the acidic milieu. For this purpose, human mesenchymal stromal cells were cultured in an osteogenic medium and the extracellular matrix mineralisation was analysed at the micro- and nano-level, both in neutral and acidic conditions and after treatment with calcium citrate and potassium citrate. The acidic milieu significantly decreased the citrate release and hindered the organisation of the extracellular matrix, but the citrate supplementation increased collagen production and, particularly calcium citrate, promoted the mineralisation process. Moreover, the positive effect of citrate supplementation was observed also in the physiological microenvironment. This in vitro study proves that the mineral matrix organisation is influenced by citrate availability in the microenvironment surrounding bone-forming cells, thus providing a biological basis for using citrate-based supplements in the management of bone-remodelling disorders related to chronic low-grade acidosis.

Dietary acid-base load and its association with risk of osteoporotic fractures and low estimated skeletal muscle mass

BACKGROUND/OBJECTIVES

Age-related decline in skeletal muscle mass and strength, loss of bone density, and increased risk of osteoporotic fractures are important public health issues. Systemic acid-base balance is affected by dietary intake and may be relevant to these conditions. We therefore investigated associations of dietary acid-base load with skeletal muscle mass, bone density status, and fracture risk.

SUBJECTS/METHODS

We analysed the European Prospective Investigation into Cancer and Nutrition-Norfolk cohort of >25,000 individuals, 39-79 years at baseline. Potential renal acid load (PRAL) was calculated from 7-day food diary data. As a proxy for skeletal muscle mass, we estimated fat-free mass from bioelectrical impedance analysis and scaled this for BMI (FFM_BMI). Bone density status was assessed by heel-bone broadband ultrasound attenuation (BUA), and fracture rates were obtained from health-care records. Multivariable regression was used to test musculoskeletal outcomes across sex-specific quintiles of PRAL.

RESULTS

PRAL in quintiles was negatively associated with FFM_BMI in men (n = 6350, p < 0.001) and women (n = 7989, p < 0.001), with quintile 5 vs 1 differences of -1.5% and -3.2% (both p < 0.001). PRAL was also negatively associated with BUA in women (n = 8312, p = 0.016; quintile 5 vs 1 difference -1.5%, p = 0.024). The combined hazard of hip, wrist and spine fractures (mean ± SD follow-up 17.9 ± 4.9 years) was higher with increasing quintiles of PRAL in men (610 fractures; n = 11,511; p = 0.013) and women (1583 fractures; n = 13,927; p = 0.009), with quintile 5 vs 1 hazard ratios of 1.33 (95% CI: 1.03-1.72, p = 0.029) and 1.21 (95% CI: 1.03-1.42, p = 0.022), but associations were not consistent for all fractures sites and age groups tested.

CONCLUSIONS

This study provides strong evidence, albeit observational, for a negative association between PRAL and musculoskeletal health in middle to older age men and women, and thus supports the rationale for a less acidic dietary load.

C-terminal telopeptide of type I collagen, osteocalcin, alkaline phosphatase, and parathyroid hormone in healthy and hospitalized foals

Hypocalcemia is a common finding in critically ill equine patients. Parathyroid hormone (PTH) helps to maintain calcium homeostasis in hypocalcemic patients by promoting renal calcium reabsorption and bone resorption. Increased serum PTH concentrations have been reported in critically ill people and animals, including horses and foals. It is unknown whether increased secretion of PTH is associated with markers of bone turnover in hospitalized foals. The goals of this study were to measure markers of bone resorption (C-terminal telopeptide of type I collagen [CTX-I]) and bone formation (osteocalcin [OCN]; alkaline phosphatase [ALP]) and to determine their association with PTH concentrations, disease severity, and mortality in hospitalized foals. This prospective, multicenter, cross-sectional study was conducted on 75 newborn foals ≤3 d old divided into hospitalized (n = 65; 41 septic; 24 sick nonseptic) and healthy (n = 10) groups. Blood samples were collected on admission to measure serum CTX-I, OCN, and PTH concentrations and ALP activity. Data were analyzed by nonparametric methods and univariate logistic regression. Serum CTX-I and PTH concentrations were significantly higher, whereas OCN concentrations were lower, in septic compared with healthy foals (P < 0.05). Serum ALP activity was not different between groups; however, it was lower in hospitalized and septic foals with low OCN concentrations (P < 0.05). In hospitalized foals, PTH concentrations were positively correlated with CTX-I concentrations and inversely associated with ALP activity (P < 0.05). High CTX-I and low OCN concentrations were associated with disease severity (P < 0.05). Hospitalized nonsurviving foals had significantly lower OCN concentrations compared with survivors (P < 0.05), but CTX-I concentrations were not associated with survival. Hospitalized foals with PTH concentrations >12.4 pmol/L were more likely to die (OR = 1.5; 95% CI = 1.1-4.16; P < 0.05). Elevated PTH and CTX-I together with reduced OCN concentrations and ALP activity in sick foals indicates that bone resorption is increased during critical illness, which may be a compensatory mechanism to correct hypocalcemia or reflect a response to systemic inflammation and metabolic imbalances. Bone resorption could negatively impact skeletal development in the growing foal. Low OCN and high PTH concentrations were predictors of nonsurvival in hospitalized foals.

The negative effect of unloading exceeds the bone-sparing effect of alkaline supplementation: a bed rest study

Potassium bicarbonate was administrated to an already alkaline diet in seven male subjects during a 21-day bed rest study and was able to decrease bed rest induced increased calcium excretion but failed to prevent bed rest-induced bone resorption.

INTRODUCTION

Supplementation with alkali salts appears to positively influence calcium and bone metabolism and, thus, could be a countermeasure for population groups with an increased risk for bone loss. However, the extent to which alkalization counteracts acid-induced bone resorption or whether it merely has a calcium and bone maintenance effect is still not completely understood. In the present study, we hypothesized that additional alkalization to an already alkaline diet can further counteract bed rest-induced bone loss.

METHODS

Seven healthy male subjects completed two parts of a crossover designed 21-day bed rest study: bed rest only (control) and bed rest supplemented with 90 mmol potassium bicarbonate (KHCO₃) daily.

RESULTS

KHCO₃supplementation during bed rest resulted in a more alkaline status compared to the control intervention, demonstrated by the increase in pH and buffer capacity level (pH p = 0.023, HCO₃p = 0.02, ABE p = 0.03). Urinary calcium excretion was decreased during KHCO₃ supplementation (control 6.05 ± 2.74 mmol/24 h; KHCO₃ 4.87 ± 2.21 mmol/24 h, p = 0.03); whereas, bone formation was not affected by additional alkalization (bAP p = 0.58; PINP p = 0.60). Bone resorption marker UCTX tended to be lower during alkaline supplementation (UCTX p = 0.16).

CONCLUSIONS

The more alkaline acid-base status, achieved by KHCO₃ supplementation, reduced renal calcium excretion during bed rest, but was not able to prevent immobilization-induced bone resorption. However, advantages of alkaline salts on bone metabolism may occur under acidic metabolic conditions or with respect to the positive effect of reduced calcium excretion within a longer time frame.

TRIAL REGISTRATION

Trial number: NCT01509456.

Nephrolithiasis secondary to inherited defects in the thick ascending loop of henle and connecting tubules

Twin and genealogy studies suggest a strong genetic component of nephrolithiasis. Likewise, urinary traits associated with renal stone formation were found to be highly heritable, even after adjustment for demographic, anthropometric and dietary covariates. Recent high-throughput sequencing projects of phenotypically well-defined cohorts of stone formers and large genome-wide association studies led to the discovery of many new genes associated with kidney stones. The spectrum ranges from infrequent but highly penetrant variants (mutations) causing mendelian forms of nephrolithiasis (monogenic traits) to common but phenotypically mild variants associated with nephrolithiasis (polygenic traits). About two-thirds of the genes currently known to be associated with nephrolithiasis code for membrane proteins or enzymes involved in renal tubular transport. The thick ascending limb of Henle and connecting tubules are of paramount importance for renal water and electrolyte handling, urinary concentration and maintenance of acid-base homeostasis. In most instances, pathogenic variants in genes involved in thick ascending limb of Henle and connecting tubule function result in phenotypically severe disease, frequently accompanied by nephrocalcinosis with progressive CKD and to a variable degree by nephrolithiasis. The aim of this article is to review the current knowledge on kidney stone disease associated with inherited defects in the thick ascending loop of Henle and the connecting tubules. We also highlight recent advances in the field of kidney stone genetics that have implications beyond rare disease, offering new insights into the most common type of kidney stone disease, i.e., idiopathic calcium stone disease.

Hexokinase 2 as a novel selective metabolic target for rheumatoid arthritis

OBJECTIVES

Recent studies indicate that glucose metabolism is altered in rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS). Hexokinases (HKs) catalyse the first step in glucose metabolism, and HK2 constitutes the principal HK inducible isoform. We hypothesise that HK2 contributes to the synovial lining hypertrophy and plays a critical role in bone and cartilage damage.

METHODS

HK1 and HK2 expression were determined in RA and osteoarthritis (OA) synovial tissue by immunohistochemistry. RA FLS were transfected with either HK1 or HK2 siRNA, or infected with either adenovirus (ad)-GFP, ad-HK1 or ad-HK2. FLS migration and invasion were assessed. To study the role of HK2 in vivo, 108 particles of ad-HK2 or ad-GFP were injected into the knee of wild-type mice. K/BxN serum transfer arthritis was induced in HK2F/F mice harbouring Col1a1-Cre (HK2Col1), to delete HK2 in non-haematopoietic cells.

RESULTS

HK2 is particular of RA histopathology (9/9 RA; 1/8 OA) and colocalises with FLS markers. Silencing HK2 in RA FLS resulted in a less invasive and migratory phenotype. Consistently, overexpression of HK2 resulted in an increased ability to migrate and invade. It also increased extracellular lactate production. Intra-articular injection of ad-HK2 in normal knees dramatically increased synovial lining thickness, FLS activation and proliferation. HK2 was highly expressed in the synovial lining after K/BxN serum transfer arthritis. HK2Col1 mice significantly showed decreased arthritis severity, bone and cartilage damage.

CONCLUSION

HK2 is specifically expressed in RA synovial lining and regulates FLS aggressive functions. HK2 might be an attractive selective metabolic target safer than global glycolysis for RA treatment.

Incomplete Distal Renal Tubular Acidosis and Kidney Stones

Renal tubular acidosis (RTA) is comprised of a diverse group of congenital or acquired diseases with the common denominator of defective renal acid excretion with protean manifestation, but in adults, recurrent kidney stones and nephrocalcinosis are mainly found in presentation. Calcium phosphate (CaP) stones and nephrocalcinosis are frequently encountered in distal hypokalemic RTA type I. Alkaline urinary pH, hypocitraturia, and, less frequently, hypercalciuria are the tripartite lithogenic factors in distal RTA (dRTA) predisposing to CaP stone formation; the latter 2 are also commonly encountered in other causes of urolithiasis. Although the full blown syndrome is easily diagnosed by conventional clinical criteria, an attenuated forme fruste called incomplete dRTA typically evades clinical testing and is only uncovered by provocative acid-loading challenges. Stone formers (SFs) that cannot acidify urine of pH < 5.3 during acid loading are considered to have incomplete dRTA. However, urinary acidification capacity is not a dichotomous but rather a continuous trait, so incomplete dRTA is not a distinct entity but may be one end of a spectrum. Recent findings suggest that incomplete dRTA can be attributed to heterozygous carriers of hypofunctional V-ATPase. The value of incomplete dRTA diagnosis by provocative testing and genotyping candidate genes is a valuable research tool, but it remains unclear at the moment whether they alter clinical practice and needs further clarification. No randomized controlled trials have been performed in SFs with dRTA or CaP stones, and until such data are available, treatment of CaP stones are centered on reversing the biochemical abnormalities encountered in the metabolic workup. SFs with type I dRTA should receive alkali therapy, preferentially in the form of K-citrate delivered judiciously to treat the chronic acid retention that drives both stone formation and bone disease.

Acid Balance, Dietary Acid Load, and Bone Effects-A Controversial Subject

Modern Western diets, with higher contents of animal compared to fruits and vegetable products, have a greater content of acid precursors vs. base precursors, which results in a net acid load to the body. To prevent inexorable accumulation of acid in the body and progressively increasing degrees of metabolic acidosis, the body has multiple systems to buffer and titrate acid, including bone which contains large quantities of alkaline salts of calcium. Both in vitro and in vivo studies in animals and humans suggest that bone base helps neutralize part of the dietary net acid load. This raises the question of whether decades of eating a high acid diet might contribute to the loss of bone mass in osteoporosis. If this idea is true, then additional alkali ingestion in the form of net base-producing foods or alkalinizing salts could potentially prevent this acid-related loss of bone. Presently, data exists that support both the proponents as well as the opponents of this hypothesis. Recent literature reviews have tended to support either one side or the other. Assuming that the data cited by both sides is correct, we suggest a way to reconcile the discordant findings. This overview will first discuss dietary acids and bases and the idea of changes in acid balance with increasing age, then review the evidence for and against the usefulness of alkali therapy as a treatment for osteoporosis, and finally suggest a way of reconciling these two opposing points of view.

Acid Load and Phosphorus Homeostasis in CKD

BACKGROUND

The kidneys maintain acid-base homeostasis through excretion of acid as either ammonium or as titratable acids that primarily use phosphate as a buffer. In chronic kidney disease (CKD), ammoniagenesis is impaired, promoting metabolic acidosis. Metabolic acidosis stimulates phosphaturic hormones, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23) in vitro, possibly to increase urine titratable acid buffers, but this has not been confirmed in humans. We hypothesized that higher acid load and acidosis would associate with altered phosphorus homeostasis, including higher urinary phosphorus excretion and serum PTH and FGF-23.

STUDY DESIGN

Cross-sectional.

SETTING & PARTICIPANTS

980 participants with CKD enrolled in the Chronic Renal Insufficiency Cohort (CRIC) Study.

PREDICTORS

Net acid excretion as measured in 24-hour urine, potential renal acid load (PRAL) estimated from food frequency questionnaire responses, and serum bicarbonate concentration < 22 mEq/L.

OUTCOME & MEASUREMENTS

24-hour urine phosphorus and calcium excretion and serum phosphorus, FGF-23, and PTH concentrations.

RESULTS

Using linear and log-linear regression adjusted for demographics, kidney function, comorbid conditions, body mass index, diuretic use, and 24-hour urine creatinine excretion, we found that 24-hour urine phosphorus excretion was higher at higher net acid excretion, higher PRAL, and lower serum bicarbonate concentration (each P<0.05). Serum phosphorus concentration was also higher with higher net acid excretion and lower serum bicarbonate concentration (each P=0.001). Only higher net acid excretion associated with higher 24-hour urine calcium excretion (P<0.001). Neither net acid excretion nor PRAL was associated with FGF-23 or PTH concentrations. PTH, but not FGF-23, concentration (P=0.2) was 26% (95% CI, 13%-40%) higher in participants with a serum bicarbonate concentration <22 versus ≥22 mEq/L (P<0.001). Primary results were similar if stratified by estimated glomerular filtration rate categories or adjusted for iothalamate glomerular filtration rate (n=359), total energy intake, dietary phosphorus, or urine urea nitrogen excretion, when available.

LIMITATIONS

Possible residual confounding by kidney function or nutrition; urine phosphorus excretion was included in calculation of the titratable acid component of net acid excretion.

CONCLUSIONS

In CKD, higher acid load and acidosis associate independently with increased circulating phosphorus concentration and augmented phosphaturia, but not consistently with FGF-23 or PTH concentrations. This may be an adaptation that increases titratable acid excretion and thus helps maintain acid-base homeostasis in CKD. Understanding whether administration of base can lower phosphorus concentrations requires testing in interventional trials.

Fibroblast-like synoviocyte metabolism in the pathogenesis of rheumatoid arthritis

An increasing number of studies show how changes in intracellular metabolic pathways alter tumor and immune cell function. However, little information about metabolic changes in other cell types, including synovial fibroblasts, is available. In rheumatoid arthritis (RA), fibroblast-like synoviocytes (FLS) are the most common cell type at the pannus–cartilage junction and contribute to joint destruction through their production of cytokines, chemokines, and matrix-degrading molecules and by migrating and invading joint cartilage. In this review, we show that these cells differ from healthy synovial fibroblasts, not only in their marker expression, proto-oncogene expression, or their epigenetic changes, but also in their intracellular metabolism. These metabolic changes must occur due to the stressful microenvironment of inflamed tissues, where concentrations of crucial nutrients such as glucose, glutamine, and oxygen are spatially and temporally heterogeneous. In addition, these metabolic changes will increase metabolite exchange between fibroblast and other synovial cells, which can potentially be activated. Glucose and phospholipid metabolism as well as bioactive lipids, including sphingosine-1-phosphate and lysophosphatidic acid, among others, are involved in FLS activation. These metabolic changes likely contribute to FLS involvement in aspects of immune response initiation or abnormal immune responses and strongly contribute to joint destruction.

Systematic Characterization of Dynamic Parameters of Intracellular Calcium Signals

Dynamic processes, such as intracellular calcium signaling, are hallmark of cellular biology. As real-time imaging modalities become widespread, a need for analytical tools to reliably characterize time-series data without prior knowledge of the nature of the recordings becomes more pressing. The goal of this study is to develop a signal-processing algorithm for MATLAB that autonomously computes the parameters characterizing prominent single transient responses (TR) and/or multi-peaks responses (MPR). The algorithm corrects for signal contamination and decomposes experimental recordings into contributions from drift, TRs, and MPRs. It subsequently provides numerical estimates for the following parameters: time of onset after stimulus application, activation time (time for signal to increase from 10 to 90% of peak), and amplitude of response. It also provides characterization of the (i) TRs by quantifying their area under the curve (AUC), response duration (time between 1/2 amplitude on ascent and descent of the transient), and decay constant of the exponential decay region of the deactivation phase of the response, and (ii) MPRs by quantifying the number of peaks, mean peak magnitude, mean periodicity, standard deviation of periodicity, oscillatory persistence (time between first and last discernable peak), and duty cycle (fraction of period during which system is active) for all the peaks in the signal, as well as coherent oscillations (i.e., deterministic spikes). We demonstrate that the signal detection performance of this algorithm is in agreement with user-mediated detection and that parameter estimates obtained manually and algorithmically are correlated. We then apply this algorithm to study how metabolic acidosis affects purinergic (P2) receptor-mediated calcium signaling in osteoclast precursor cells. Our results reveal that acidosis significantly attenuates the amplitude and AUC calcium responses at high ATP concentrations. Collectively, our data validated this algorithm as a general framework for comprehensively analyzing dynamic time-series.

Acidosis and Urinary Calcium Excretion: Insights from Genetic Disorders

Metabolic acidosis is associated with increased urinary calcium excretion and related sequelae, including nephrocalcinosis and nephrolithiasis. The increased urinary calcium excretion induced by metabolic acidosis predominantly results from increased mobilization of calcium out of bone and inhibition of calcium transport processes within the renal tubule. The mechanisms whereby acid alters the integrity and stability of bone have been examined extensively in the published literature. Here, after briefly reviewing this literature, we consider the effects of acid on calcium transport in the renal tubule and then discuss why not all gene defects that cause renal tubular acidosis are associated with hypercalciuria and nephrocalcinosis.

A neglected requirement for optimizing treatment of age-related osteoporosis: Replenishing the skeleton's base reservoir with net base-producing diets

Osteoporosis is a disorder of bone in which the mass of the bone is reduced and the bone's architecture at the microscopic level is disordered. Together those abnormalities predispose affected individuals to experience fractures despite only minimal trauma (i.e., fragility fractures). Age related osteoporosis is a common type of osteoporosis that occurs with aging in both men and women usually beginning after the age of peak bone mass. Research has found that the disorder can be partially reversed by reducing the net amount of acid that is produced when consuming typical Western diets. However, the amelioration that results has not been so dramatic or so consistent that physicians have adopted the procedure as part of the standard treatment for age-related osteoporosis. We propose that reducing the net acid load from the diet is not sufficient to reverse age related osteoporosis because it fails to supply base needed to restore the large amount of base in bone that had been lost by reacting with the net acid load of the diet that had been consumed for years or decades. Reducing the net acid load from the diet might be expected to have little ameliorative effect or merely slow the progression of the disorder. We hypothesize that both to restore osteoporotic bone to, or nearly to, its pre-disease state, as well as to eliminate the risk of fragility fractures, requires consuming diets that produce net amounts of base to restore the base lost from years to decades of consuming diets that produce net amounts of acid. We hypothesize also that the excess base and attendant subclinical metabolic alkalosis will both stimulate the cellular process of bone formation and suppress the cellular process of bone resorption, and thereby implement the restorative process.

Increased bone density in mice lacking the proton receptor OGR1

Chronic metabolic acidosis stimulates cell-mediated calcium efflux from bone through osteoblastic prostaglandin E2-induced stimulation of receptor activator of NF-kB ligand leading to increased osteoclastic bone resorption. Osteoblasts express the proton-sensing G-protein-coupled receptor OGR1, which activates inositol phosphate-mediated intracellular calcium. Proton-induced osteoblastic intracellular calcium signaling requires ovarian cancer G-protein-coupled receptor 1 (OGR1), suggesting that OGR1 is the sensor activated during acidosis to cause bone resorption. Growing mice produce large amounts of metabolic acids, which must be buffered, primarily by bone, before excretion by the kidney. Here we tested whether lack of OGR1 inhibits proton-induced bone resorption by measuring bone mineral density by micro-computed tomography and histomorphometry in 8-week-old male OGR1(-/-) and C57/Bl6 wild type mice. OGR1(-/-) mice have normal skeletal development with no atypical gross phenotype. Trabecular and cortical bone volume was increased in tibiae and vertebrae from OGR1(-/-). There were increased osteoblast numbers on the cortical and trabecular surfaces of tibiae from OGR1(-/-) mice, increased endocortical and trabecular bone formation rates, and osteoblastic gene expression. Osteoclast numbers and surface were increased in tibiae of OGR1(-/-) mice. Thus, in rapidly growing mice, lack of OGR1 leads to increased bone mass with increased bone turnover and a greater increase in bone formation than resorption. This supports the important role of the proton receptor OGR1 in the response of bone to protons.

Ratiometric Organic Fibers for Localized and Reversible Ion Sensing with Micrometer-Scale Spatial Resolution

A fundamental issue in biomedical and environmental sciences is the development of sensitive and robust sensors able to probe the analyte of interest, under physiological and pathological conditions or in environmental samples, and with very high spatial resolution. In this work, novel hybrid organic fibers that can effectively report the analyte concentration within the local microenvironment are reported. The nanostructured and flexible wires are prepared by embedding fluorescent pH sensors based on seminaphtho-rhodafluor-1-dextran conjugate. By adjusting capsule/polymer ratio and spinning conditions, the diameter of the fibers and the alignment of the reporting capsules are both tuned. The hybrid wires display excellent stability, high sensitivity, as well as reversible response, and their operation relies on effective diffusional kinetic coupling of the sensing regions and the embedding polymer matrix. These devices are believed to be a powerful new sensing platform for clinical diagnostics, bioassays and environmental monitoring.

Dietary cation-anion difference and day length have an effect on milk calcium content and bone accretion of dairy cows

Milk and dairy products are an important source of Ca for humans. Recent studies have shown fluctuations in cow milk Ca content during the year in France, with high values in winter and with corn silage diets, and a decrease during May and June and with grass diets. The aim of this study was to identify the reasons for this seasonal decrease in milk Ca content by testing the effect of 2 levels of dietary cation-anion differences (DCAD; 0 mEq/kg of dry matter for DCAD 0 and 400 mEq/kg for DCAD 400) and 2 day lengths (8 h of light/d for short days: SD; and 16 h/d for long days: LD) on the Ca balances of dairy cows. The DCAD treatments were designed to mimic diets based either on corn silage or on herbage. The cows were only illuminated by solarium lights providing UVA and UVB. The trial was conducted according to 2 simultaneous replicates of a 4×4 Latin square design using 8 dairy cows averaging 103±44 d in milk with 4 periods of 14 d. Data were analyzed by ANOVA with a model including treatment, cow, and period effects. No significant interaction was found between day length and DCAD treatments. With DCAD 400 compared with DCAD 0, blood pH increased and plasma ionized Ca content decreased, whereas the plasma total Ca content did not differ between treatments. Milk Ca content, however, increased with DCAD 400 compared with DCAD 0, in relation to a decrease in the amount of Ca excreted in urine. The DCAD had no significant effect on protein and casein contents and DCAD 400 tended to decrease milk yield. This illustrates that the udder did not decrease Ca uptake from the blood at high DCAD even though DCAD 400 decreased the mammary availability of Ca by decreasing the proportion of blood ionized Ca. Milk Ca and casein contents were significantly lower with LD compared with SD, whereas day length had no effect on milk yield after 14 d of treatment. Bone accretion of cows increased when the Ca content of milk increased (i.e., with DCAD 400 compared with DCAD 0 and with SD compared with LD). This work suggests that long and sunny days could explain part of the seasonal decrease in milk Ca content in summer and refutes the hypothesis that low milk Ca contents at grazing could be due to the high DCAD of herbage.

The effects of dietary protein and amino acids on skeletal metabolism

Dietary protein is required for optimal skeletal growth and maturation. Although Recommended Dietary Allowances (RDAs) exist for global dietary protein intake, the level and sources of dietary protein that are optimal for skeletal health over the life continuum have not been established. This is partly due to the difficulty in quantifying the effects of variable levels of a nutrient's intake over a lifetime as well as the complex nature of the relationships between dietary protein and calcium economy. Areas of current uncertainty include the precise source and amount of dietary protein required for optimal skeletal accretion and maintenance of skeletal mass, as well as the site-specific effects of dietary protein. The cellular and molecular mechanisms that underpin the actions of dietary protein on mineral metabolism and skeletal homeostasis remain unclear. This review attempts to summarize recent data bearing on these questions.

The Effect of a Whey Protein Supplement on Bone Mass in Older Caucasian Adults

CONTEXT

It has been assumed that the increase in urine calcium (Ca) that accompanies an increase in dietary protein was due to increased bone resorption. However, studies using stable Ca isotopes have found that dietary protein increases Ca absorption without increasing bone resorption.

OBJECTIVE

The objective of the study was to investigate the impact of a moderately high protein diet on bone mineral density (BMD).

DESIGN

This was a randomized, double-blind, placebo-controlled trial of protein supplementation daily for 18 months.

SETTING

The study was conducted at two institutional research centers.

PARTICIPANTS

Two hundred eight older women and men with a body mass index between 19 and 32 kg/m(2) and a self-reported protein intake between 0.6 and 1.0 g/kg participated in the study.

INTERVENTION

Subjects were asked to incorporate either a 45-g whey protein or isocaloric maltodextrin supplement into their usual diet for 18 months.

MAIN OUTCOME MEASURE

BMD by dual-energy x-ray absorptiometry, body composition, and markers of skeletal and mineral metabolism were measured at baseline and at 9 and 18 months.

RESULTS

There were no significant differences between groups for changes in L-spine BMD (primary outcome) or the other skeletal sites of interest. Truncal lean mass was significantly higher in the protein group at 18 months (P = .048). C-terminal telopeptide (P = .0414), IGF-1 (P = .0054), and urinary urea (P < .001) were also higher in the protein group at the end of the study period. There was no difference in estimated glomerular filtration rate at 18 months.

CONCLUSION

Our data suggest that protein supplementation above the recommended dietary allowance (0.8 g/kg) may preserve fat-free mass without adversely affecting skeletal health or renal function in healthy older adults.

The acid-ash hypothesis revisited: a reassessment of the impact of dietary acidity on bone

The acid-ash hypothesis states that when there are excess blood protons, bone is eroded to provide alkali to buffer the net acidity and maintain physiologic pH. There is concern that with the typical Western diet, we are permanently in a state of net endogenous acid production, which is gradually reducing bone. While it is clear that a high acid-producing diet generates increased urinary acid and calcium excretion, the effect of diet does not always have the expected results on BMD, fracture risk and markers of bone formation and resorption, suggesting that other factors are influencing the effect of acid/alkali loading on bone. High dietary protein, sodium and phosphorus intake, all of which are necessary for bone formation, were thought to be net acid forming and contribute to low BMD and fracture risk, but appear under certain conditions to be beneficial, with the effect of protein being driven by calcium repletion. Dietary salt can increase short-term markers of bone resorption but may also trigger 1,25(OH)2D synthesis to increase calcium absorption; with low calcium intake, salt intake may be inversely correlated with BMD but with high calcium intake, salt intake was positively correlated with BMD. With respect to the effect of phosphorus, the data are conflicting. Inclusion of an analysis of calcium intake may help to reconcile the contradictory results seen in many of the studies of bone. The acid-ash hypothesis could, therefore, be amended to state that with an acid-producing diet and low calcium intake, bone is eroded to provide alkali to buffer excess protons but where calcium intake is high the acid-producing diet may be protective.

[Pharmaceutical therapy of bone metabolism disorders in chronic kidney disease mineral bone disorder (CKD-MBD) with special respect to antiresorptive substances]

Disturbances in bone and mineral turnover are common complications in patients with impaired renal function. Besides an increased risk for cardiovascular events they promote skeletal events, such as bone pain and fractures. Evidence for the antifracture efficacy of antiresorptive and osteoanabolic treatment strategies has only been demonstrated for patients with osteoporosis. The use of osteotropic drugs in patients with impaired renal function requires large randomized placebo-controlled trials.

Supplementing a low-protein diet with dibasic amino acids increases urinary calcium excretion in young women

Increasing dietary protein within a physiologic range stimulates intestinal calcium absorption, but it is not known if specific amino acids or dietary protein as a whole are responsible for this effect. Therefore, we selectively supplemented a low-protein (0.7 g/kg) diet with either the calcium-sensing receptor-activating amino acids (CaSR-AAAs) L-tryptophan, L-phenylalanine, and L-histidine, or the dibasic amino acids (DAAs) L-arginine and L-lysine, to achieve intakes comparable to the content of a high-protein diet (2.1 g/kg) and measured intestinal calcium absorption. Fourteen young women took part in a placebo-controlled, double-blind, crossover feeding trial in which each participant ingested a 6-d low-protein diet supplemented with CaSR-AAAs, DAAs, or methylcellulose capsules (control) after an 11-d adjustment period. All participants ingested all 3 diets in random order. Intestinal calcium absorption was measured between days 5 and 6 using dual-stable calcium isotopes ((42)Ca, (43)Ca, and (44)Ca). There was no difference in calcium absorption between the diet supplemented with CaSR-AAAs (22.9 ± 2.0%) and the control diet (22.3 ± 1.4%) (P = 0.64). However, calcium absorption tended to be greater during the DAA supplementation period (25.2 ± 1.4%) compared with the control diet period (22.3 ± 1.4%) (P < 0.10). Larger and longer clinical trials are needed to clarify the possible benefit of arginine and lysine on calcium absorption.

Effect of chronic metabolic acidosis on bone density and bone architecture in vivo in rats

Chronic metabolic acidosis (CMA) might result in a decrease in vivo in bone mass based on its reported in vitro inhibition of bone mineralization, bone formation, or stimulation of bone resorption, but such data, in the absence of other disorders, have not been reported. CMA also results in negative nitrogen balance, which might decrease skeletal muscle mass. This study analyzed the net in vivo effects of CMA's cellular and physicochemical processes on bone turnover, trabecular and cortical bone density, and bone microarchitecture using both peripheral quantitative computed tomography and μCT. CMA induced by NH4Cl administration (15 mEq/kg body wt/day) in intact and ovariectomized (OVX) rats resulted in stable CMA (mean Δ[HCO3(-)]p = 10 mmol/l). CMA decreased plasma osteocalcin and increased TRAP5b in intact and OVX animals. CMA decreased total volumetric bone mineral density (vBMD) after 6 and 10 wk (week 10: intact normal +2.1 ± 0.9% vs. intact acidosis -3.6 ± 1.2%, P < 0.001), an effect attributable to a decrease in cortical thickness and, thus, cortical bone mass (no significant effect on cancellous vBMD, week 10) attributed to an increase in endosteal bone resorption (nominally increased endosteal circumference). Trabecular bone volume (BV/TV) decreased significantly in both CMA groups at 6 and 10 wk, associated with a decrease in trabecular number. CMA significantly decreased muscle cross-sectional area in the proximal hindlimb at 6 and 10 wk. In conclusion, chronic metabolic acidosis induces a large decrease in cortical bone mass (a prime determinant of bone fragility) in intact and OVX rats and impairs bone microarchitecture characterized by a decrease in trabecular number.

Nutritional disturbance in acid-base balance and osteoporosis: a hypothesis that disregards the essential homeostatic role of the kidney

The nutritional acid load hypothesis of osteoporosis is reviewed from its historical origin to most recent studies with particular attention to the essential but overlooked role of the kidney in acid–base homeostasis. This hypothesis posits that foods associated with an increased urinary acid excretion are deleterious for the skeleton, leading to osteoporosis and enhanced fragility fracture risk. Conversely, foods generating neutral or alkaline urine would favour bone growth and Ca balance, prevent bone loss and reduce osteoporotic fracture risk. This theory currently influences nutrition research, dietary recommendations and the marketing of alkaline salt products or medications meant to optimise bone health and prevent osteoporosis. It stemmed from classic investigations in patients suffering from chronic kidney diseases (CKD) conducted in the 1960s. Accordingly, in CKD, bone mineral mobilisation would serve as a buffer system to acid accumulation. This interpretation was later questioned on both theoretical and experimental grounds. Notwithstanding this questionable role of bone mineral in systemic acid–base equilibrium, not only in CKD but even more in the absence of renal impairment, it is postulated that, in healthy individuals, foods, particularly those containing animal protein, would induce ‘latent’ acidosis and result, in the long run, in osteoporosis. Thus, a questionable interpretation of data from patients with CKD and the subsequent extrapolation to healthy subjects converted a hypothesis into nutritional recommendations for the prevention of osteoporosis. In a historical perspective, the present review dissects out speculation from experimental facts and emphasises the essential role of the renal tubule in systemic acid–base and Ca homeostasis.

Claudin expression in the bone-lining cells of female rats exposed to long-standing acidemia

Besides enhancing osteoclast-mediated bone resorption, chronic metabolic acidosis (CMA) induces mineral efflux across the epithelial-like bone membrane formed by bone-lining cells (inactive osteoblasts), possibly via the paracellular pathway. However, there was a compensatory mechanism that restricted bone loss in the late phase of CMA, and changes in the expression of claudins, which are tight junction proteins known to regulate epithelial barrier function, were therefore anticipated in bone-lining cells. Herein, primary rat osteoblasts were found to express several transcripts of claudins, i.e., claudin-5, -11, -14, -15 and -16. Their protein expressions in bone-lining cells were demonstrated by immunohistochemistry in decalcified tibial sections. After exposure to CMA induced by oral administration of 1.5% NH(4)Cl for 21 days, expression of claudin-14, which normally seals the paracellular space and restricts ion movement, was increased, whereas that of claudin-15 and -16 which form pores for ion transport were decreased. Expressions of claudin-5 and -11 were not changed by CMA. In conclusion, the bone-lining cells of rats exposed to CMA for 21 days upregulated an ion-restrictive claudin (i.e., claudin-14), while downregulating ion-permeable claudins (i.e., claudin-15 and -16). These cellular responses might be parts of a compensatory mechanism accounting for deceleration of bone loss in late CMA.

Postgraduate Symposium: Positive influence of nutritional alkalinity on bone health

There is growing evidence that consumption of a Western diet is a risk factor for osteoporosis through excess acid supply, while fruits and vegetables balance the excess acidity, mostly by providing K-rich bicarbonate-rich foods. Western diets consumed by adults generate approximately 50-100 mEq acid/d; therefore, healthy adults consuming such a diet are at risk of chronic low-grade metabolic acidosis, which worsens with age as a result of declining kidney function. Bone buffers the excess acid by delivering cations and it is considered that with time an overstimulation of this process will lead to the dissolution of the bone mineral content and hence to reduced bone mass. Intakes of K, Mg and fruit and vegetables have been associated with a higher alkaline status and a subsequent beneficial effect on bone health. In healthy male volunteers an acid-forming diet increases urinary Ca excretion by 74% and urinary C-terminal telopeptide of type I collagen (C-telopeptide) excretion by 19% when compared with an alkali (base-forming) diet. Cross-sectional studies have shown that there is a correlation between the nutritional acid load and bone health measured by bone ultrasound or dual-energy X-ray absorptiometry. Few studies have been undertaken in very elderly women (>75 years), whose osteoporosis risk is very pertinent. The EVAluation of Nutrients Intakes and Bone Ultra Sound Study has developed and validated (n 51) an FFQ for use in a very elderly Swiss population (mean age 80.4 (sd 2.99) years), which has shown intakes of key nutrients (energy, fat, carbohydrate, Ca, Mg, vitamin C, D and E) to be low in 401 subjects. A subsequent study to assess net endogenous acid production (NEAP) and bone ultrasound results in 256 women aged > or = 75 years has shown that lower NEAP (P=0.023) and higher K intake (P=0.033) are correlated with higher bone ultrasound results. High acid load may be an important additional risk factor that may be particularly relevant in very elderly patients with an already-high fracture risk. The latter study adds to knowledge by confirming a positive link between dietary alkalinity and bone health indices in the very elderly. In a further study to complement these findings it has also been shown in a group of thirty young women that in Ca sufficiency an acid Ca-rich water has no effect on bone resorption, while an alkaline bicarbonate-rich water leads to a decrease in both serum parathyroid hormone and serum C-telopeptide. Further investigations need to be undertaken to study whether these positive effects on bone loss are maintained over long-term treatment. Mineral-water consumption could be an easy and inexpensive way of helping to prevent osteoporosis and could be of major interest for long-term prevention of bone loss.

Early acceleration phase and late stationary phase of remodeling imbalance in long bones of male rats exposed to long-standing acidemia: a 10-month longitudinal study using bone histomorphometry

Chronic metabolic acidosis (CMA) is known to have a detrimental effect on bone metabolism as a result of accelerated bone resorption and impaired bone formation. Typically, a number of compensatory adaptations must have occurred which may help palliate negative calcium balance and acidemia, e.g., increased intestinal calcium and phosphorus absorption. The final outcome with respect to bone remodeling after exposure to CMA for several months was, therefore, elusive. Herein, we investigated bone changes in male rats fed 1.5% NH(4)Cl in drinking water for up to 10 months to induce CMA with plasma pH of 7.2-7.3. Significant decreases in bone mineral density and content were detected by dual-energy X-ray absorptiometry after 6 months of CMA, whereas histomorphometric analysis revealed a significant decrease in bone volume already at week 2 after CMA induction. Exposure to CMA longer than 2 weeks also decreased trabecular number, trabecular thickness, osteoblast surface, mineral apposition rate, and bone formation rate, while increasing trabecular separation, osteoclast surface, and eroded surface. Bone resorption was rapid during weeks 2-16 (acceleration phase) and thereafter persisted at a slower rate (stationary phase) until week 40. Furthermore, CMA markedly reduced the total calcium content in bone and enhanced urinary calcium excretion as measured by atomic absorption spectrophotometry. It could be concluded that, after exposure to a long-standing acidemia, the enhanced bone resorption and suppressed bone formation led to osteopenia throughout the 10-month period, with accelerated bone loss seen only during the first 6 months. Thereafter, the compensatory adaptations appeared to help stabilize bone mass at a subnormal level.

The composition of mineral waters sourced from Europe and North America in respect to bone health: composition of mineral water optimal for bone

The consumption of mineral waters is increasing in industrialised countries. High intakes of Ca and other alkalising cations as well as a low acid intake are beneficial to bone. We examined which components of mineral waters are conditioning their Ca content and their alkalinising power, in order to define the optimal profile. European mineral waters were randomly selected on the Internet: 100 waters with less than 200 mg Ca/l (9.98 mEq/l) and fifty with more than 200 mg/l, all with complete data for SO4, P, Cl, Na, K, Mg and Ca, and most also for HCO3. For comparison, forty North American mineral waters were randomly chosen. The potential renal acid load (PRAL) was calculated for each mineral water. North American waters did not reveal significant results because of their low mineralisation. We performed correlations between all eight components in order to explore the properties of the mineral waters. In the European waters, twenty-six out of twenty-eight correlations showed a P value of <or= 0.01. In waters with PRAL >0 (acidifying waters), PRAL was positively correlated with SO4, Ca, K and Mg (P < 0.001). In those with PRAL < 0 (alkalinising waters), PRAL was negatively correlated with HCO3, Na, Mg, Ca, K, Cl and SO4 (P < 0.001). SO4 and HCO3 were not found together in high quantities in the same water for geochemical reasons. A high Ca content is associated with either a high SO4 or a high HCO3 content. SO4 theoretically increases Ca excretion, while HCO3 and low PRAL values are associated with positive effects on bone. Therefore, the best waters for bone health are rich in both HCO3 and Ca, and by consequence low in SO4.

Low estimates of dietary acid load are positively associated with bone ultrasound in women older than 75 years of age with a lifetime fracture

Dietary acid load from Western diets may be a risk factor for osteoporosis. It can be estimated by net endogenous acid production (NEAP). No data currently exists for NEAP estimates and bone indices in the very elderly (i.e. > or = 75 y). The aim of this study was to determine the association between NEAP estimates by using the potential renal acid load (PRAL) equation and quantitative bone ultrasound (QUS) measurements at the heel [broadband ultrasound attenuation (BUA)] in Caucasian women. We assessed NEAP and QUS in 401 very elderly Swiss ambulatory women. We evaluated dietary intake and NEAP estimates with a validated FFQ. QUS was measured using Achilles (Lunar). We identified 2 subgroups: 256 women (80.6 y +/- 3; BUA, 96.8 dB/MHz) with a fracture history and the remaining 145 (79.9 y SD 2.9; BUA, 101.7 dB/MHz) without. Women who reported having suffered a fracture had lower BUA (P < 0.001) than nonfractured women but did not differ in nutrient intakes and NEAP. Lower NEAP (P = 0.023) and higher potassium intake (P = 0.033) were correlated with higher BUA, which remained significant even after adjustment for age, BMI, and osteoporosis treatment. BUA was positively correlated with calcium (P = 0.016) and BMI (P < 0.001). Women who reported no fractures had no significant correlations between nutrient intake, NEAP, and BUA. Low nutritional acid load was correlated with higher BUA in very elderly women with a fracture history. Although relatively weak compared with age and BMI, this association was significant and may be an important additional risk factor that might be particularly relevant in frail patients with an already high fracture risk.

Adynamic bone in patients with chronic kidney disease

Adynamic bone in patients with chronic kidney disease (CKD) is a clinical concern because of its potential increased risk for fracture and cardiovascular disease (CVD). Prevalence rates for adynamic bone are reportedly increased, although the variance for its prevalence and incidence is large. Differences in its prevalence are largely attributed to classification and population differences, the latter of which constitutes divergent groups of elderly patients having diabetes and other comorbidities that are prone to low bone formation. Most patients have vitamin D deficiency and the active form, 1,25-dihydroxyvitamin D, invariably decreases to very low levels during CKD progression. Fortunately, therapy with vitamin D receptor activators (VDRAs) appears to be useful in preventing bone loss, in part, by its effect to stimulate bone formation and in decreasing CVD morbidity, and should be considered as essential therapy regardless of bone turnover status. Future studies will depend on assessing cardiovascular outcomes to determine whether the risk/reward profile for complications related to VDRA and CKD is tolerable.

Phenotypic characteristics of bone in carbonic anhydrase II-deficient mice

Carbonic anhydrase II (CAII)-deficient mice were created to study the syndrome of CAII deficiency in humans including osteopetrosis, renal tubular acidosis, and cerebral calcification. Although CAII mice have renal tubular acidosis, studies that analyzed only cortical bones found no changes characteristic of osteopetrosis. Consistent with previous studies, the tibiae of CAII-deficient mice were significantly smaller than those of wild-type (WT) mice (28.7 +/- 0.9 vs. 43.6 +/- 3.7 mg; p < 0.005), and the normalized cortical bone volume of CAII-deficient mice (79.3 +/- 2.2%) was within 5% of that of WT mice (82.7 +/- 2.3%; p < 0.05), however, metaphyseal widening of the tibial plateau was noted in CAII-deficient mice, consistent with osteopetrosis. In contrast to cortical bone, trabecular bone volume demonstrated a nearly 50% increase in CAII-deficient mice (22.9 +/- 3.5% in CAII, compared to 15.3 +/- 1.6% in WT; p < 0.001). In addition, histomorphometry demonstrated that bone formation rate was decreased by 68% in cortical bone (4.77 +/- 1.65 microm3/microm2/day in WT vs. 2.07 +/- 1.71 microm3/microm2/day in CAII mice; p < 0.05) and 55% in trabecular bone (0.617 +/- 0.230 microm3/microm2/day in WT vs. 0.272 +/- 0.114 microm3/microm2/day in CAII mice; p < 0.05) in CAII-deficient mice. The number of osteoclasts was significantly increased (67%) in CAII-deficient mice, while osteoblast number was not different from that in WT mice. The metaphyseal widening and changes in the trabecular bone are consistent with osteopetrosis, making the CAII-deficient mouse a valuable model of human disease.

The effect of physical activity and its interaction with nutrition on bone health

Physical activity (PA) is a popular therapy for the prevention and treatment of bone loss and osteoporosis because it has no adverse side effects, it is low cost, and it confers additional benefits such as postural stability and fall prevention. Bone mass is regulated by mechanical loading, and is limited but not controlled by diet. The mechanism by which strain thresholds turn bone remodelling ‘on’ and ‘off ’ is known as the mechanostat theory. Research in animals has shown that optimal strains are dynamic, with a high change rate, an unusual distribution and a high magnitude of strain, but the results of randomized controlled trials in human subjects have been somewhat equivocal. In the absence of weight-bearing activity nutritional or endocrine interventions cannot maintain bone mass. Biochemical markers of bone turnover predict bone mass changes, and findings from our research group and others have shown that both acute and chronic exercise can reduce bone resorption. Similarly, Ca intervention studies have shown that supplementation can reduce bone resorption. Several recent meta-analytical reviews concur that changes in bone mass with exercise are typically 2–3%. Some of these studies suggest that Ca intake may influence the impact of PA on bone, with greater effects in Ca-replete subjects. Comparative studies between Asian (high PA, low Ca intake) and US populations (low PA, high Ca intake) suggest that PA may permit an adaptation to low Ca intakes. Whether Ca and PA interact synergistically is one of the most important questions unanswered in the area of lifestyle-related bone health research.