Moderate salt restriction in primary aldosteronism improves bone metabolism through attenuation of urinary calcium and phosphate losses

Abstract

Introduction

Primary aldosteronism (PA) is considered the most common endocrine cause of secondary hypertension.¹ Patients with PA feature aldosterone excess and inappropriately high dietary intake of sodium chloride (salt) with consecutive cardiovascular end-organ damage.² Primary aldosteronism also leads to an increased risk of osteoporosis with vertebral fractures,^3,4 with the mechanisms not being entirely clear. Aldosterone-producing adenomas may also secrete excessive amounts of cortisol.⁵ Excessive cortisol secretion is recognized as an independent risk factor for osteoporosis, also in the context of PA.⁴

Primary aldosteronism leads to an impaired taste perception for sodium,⁶ thereby promoting a sodium-rich diet. Higher sodium intake in turn is associated with higher urinary calcium^7-9 and probably promotes osteoporosis.¹⁰ Indeed, population-wide data and data from postmenopausal females suggest that the risk for osteoporosis increases with higher dietary salt consumption.^10,11 The established association of dietary salt intake and urinary calcium excretion also led to the recommendation of salt restriction measures to prevent the occurrence of kidney stones.¹²

Patients with PA frequently harbor hypercalciuria and secondary hyperparathyroidism¹³ as well as low vitamin D3 levels.^14,15 Even kidney stones seem to occur at a higher frequency than in the general population.¹⁶

The prevalence of osteoporosis is greatest in postmenopausal females.¹⁷ Female patients with PA typically are diagnosed at a mean age of 51, coinciding with the onset of menopause, with the onset of arterial hypertension 11 years prior.¹⁸ Such a prolonged exposure to the bone health-compromising conditions of PA may amplify the risk of developing osteoporosis in later life. Due to the high prevalence of PA, the disease may, therefore, contribute to a substantial number of cases with osteoporosis. Given that osteoporosis-related fractures carry a high morbidity and mortality,¹⁹ efforts to identify subgroups at increased risk and to provide preventive measures are warranted.

Our group and others recently demonstrated that restriction of salt intake in patients with PA is associated with recovery of renin activity, improvement of end-organ damage, and blood pressure control.^20,21 This is true even for patients receiving subtype-specific PA therapy.^21,22

As the majority of patients with PA (65%) harbors bilateral, ie, non-resectable disease,²³ strategies to minimize bone loss with continuing hormone excess are necessary. The number of medically treated patients is increased further by patients with unilateral disease opting to forego surgery. Japanese registry data suggest that this subgroup may encompass 19% of unilateral cases.²⁴ Salt restriction in adjunct to MRA may, thus, be a relevant option for up to 70% of patients with PA.

Recent data highlighted the issue of misclassification due to false high aldosterone concentrations returned by immunoassays for aldosterone used in confirmatory testing. Due to the lower aldosterone concentrations in patients with bilateral disease in comparison to patients with unilateral disease, the former are particularly prone to this misclassification.²⁵ On the other hand, it should be kept in mind that 20% of patients with a negative confirmatory test after screening positive will progress to overt PA over time.²⁶

While salt restriction is recommended for patients with PA not undergoing surgery,²⁷ the impact of this intervention on patients’ bone health is unknown.

We here report a post hoc analysis of the Salt CONNtrol trial²¹ with respect to osteoprotective effects of salt restriction in PA. We demonstrate that salt restriction leads to biochemical evidence of increased bone formation most likely by reduced urinary losses of both calcium and phosphate.

Methods

Study protocol and basic cohort characteristics

The present study was a post hoc analysis of the Salt CONNtrol, an investigator-initiated nonrandomized open label single arm intervention trial that subjected 41 patients with PA on stable MRA therapy to a 12-week salt restriction. Detailed information on the Salt CONNtrol trial has been published earlier.²¹ The study is registered as a clinical trial (ID DRKS00026030).

The study protocol was approved by the ethics committee of the University of Munich. The study was conducted in accordance with the principles of the Declaration of Helsinki.

In brief, the intervention consisted in nutritional counseling on salt restriction that was reinforced by a smartphone app. The app provided information on seasoning alternatives to salt as well as the salt content of common foods in addition to incentives by unlocking challenges if weekly goals for salt restriction were met. Every 4 weeks, patients could come forward with questions concerning their efforts of salt restriction.

Diagnosis of PA, subtype determination, and PA-directed medical therapy

In line with Endocrine Society guidelines,²⁸ patients received a 4 h saline infusion test to confirm PA while on non-interfering blood pressure medication. Aldosterone concentrations for screening and confirmation were measured by immunoassay (Liaison® Aldosterone, DiaSorin, Saluggia, Italy). After receiving a diagnosis of PA, 39/41 patients were subjected to adrenal venous sampling which confirmed non-lateralized PA in 36 patients. Adrenal venous sampling (AVS) results returned inconclusive for 3 patients. Based on the AVS results and on patient preferences, all of the 41 patients opted for medical management of their disease and were started on MRA therapy with a run-in phase of at least 4 weeks prior to the first study visit. From 4 weeks before study start until the end of the study, MRA dosage was not modified.

Laboratory analysis

As described previously,²¹ blood sampling was done between 8 and 9 Am in fasting state. Concentrations of BAP, CTX-1, and PINP were measured by chemiluminescence immunoassay (CLIA). A panel of 15 steroids was determined by liquid chromatography mass spectrometry (LC/MS).²¹ Patients collected duplicate 24 h urinary samples before and after salt restriction. Urinary electrolyte excretion was determined from each sample with the mean of the duplicate determination serving as the representative value for each study time point.

Statistical analysis

All numerical values are expressed as median ± interquartile range (IQR). Paired variables were compared by Wilcoxon matched-pairs signed rank test. Two-tailed probability values of <5% were considered statistically significant. Statistical analysis was performed using GraphPad Prism 9.5 (GraphPad Software Inc., San Diego, USA).

Results

Cohort characteristics

As shown in Table 1, the cohort was characterized by a fair distribution of both sexes. Circa 50% of females were postmenopausal as defined by a serum FSH level equal or above the upper limit of normal of the assay (≥25.8 U/L). Five patients received hormone replacement or used a hormonal contraceptive. Sixteen patients were on vitamin D supplementation. Dosages of any medication, including MRA, did not change throughout the study period. Duration of MRA therapy was 9 ± 37.5 months. Before the start of salt restriction, 3 fractures had occurred of which one could be interpreted as possibly osteoporosis-related (radius fracture). No additional fractures occurred during the study period.

Principal biochemical changes upon salt restriction

As also described before, patients were able to reduce their urinary 24 h sodium excretion from 156.1 ± 57.0 to 89.6 ± 34.8 mmol/24 h which was paralleled by an antihypertensive effect from an initial 130 ± 8 to a final 121 ± 9 mmHg systolic blood pressure.²¹ In the current analysis, we noted a highly significant reduction in 24 h urinary calcium excretion (from 4.5 ± 2.2 to 3.5 ± 1.7 mmol/24 h after 12 weeks, P < .001). Similarly, phosphate excretion also was reduced significantly. In line with this, serum phosphate levels increased, while serum calcium concentrations remained unaltered.

On the other hand, concentrations of the bone turnover marker bone-specific alkaline phosphatase (BAP) increased. Levels of C-terminal telopeptide of type I collagen (CTX-I) and N-terminal propeptide of type I procollagen (PINP) as additional markers of turnover remained unaltered (Table 1).

Endocrine changes

In paired 15 parameter steroid profiles (LC-MS), significant changes in aldosterone and testosterone were observed (Table 1, Figure 1B). No significant longitudinal changes in 25-OH vitamin D3, LH, FSH, iPTH, eGFR, or TSH could be detected (Table 1). At 12 weeks, a significant increase in plasma ACTH was apparent. Urinary free cortisol excretion decreased numerically but missed the significance threshold (P = .056). Four patients showed evidence of secondary hyperparathyroidism at baseline, which normalized in 3 after salt restriction.

Figure 1.

A, Graphical summary of the components of multivariate analysis. Variables are presented as individual data points before and after the 12-week intervention next to a waterfall plot to visualize the changes in the particular variable. Plots for delta phosphate, delta iPTH, delta SBP, and delta urinary calcium also feature the multilinear regression coefficient (β) of the correlation with delta BAP. B, Steroid panel displayed as percent change after 12 weeks compared to baseline. Significant changes are highlighted in bold. *P < .05; **P < .01, Wilcoxon matched-pairs signed rank test.

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Multivariate analysis

Using multivariate analysis, we observed that the longitudinal change (delta) in BAP was independently correlated with delta serum phosphate (β = −2.538), delta iPTH (β = −0.098), delta systolic blood pressure (SBP) (β = 0.144), and delta calcium excretion (β = 0.602) (Figure 1A, Table 2).

In this cohort, the significant association between delta SBP and the improvement in BAP was not associated with the achieved reduction in sodium excretion.

Even though testosterone levels increased in the overall cohort, incorporating changes in testosterone into the multivariate analysis did not reveal an association with the change in BAP. Likewise, estradiol and biological sex could not be shown to be independent determinants of BAP increase.

An impact of autonomous cortisol secretion on the improvement of BAP, as measured by post-dexamethasone serum cortisol, could not be demonstrated in either bivariate or multivariate analysis.

Discussion

This study addressed the question whether salt restriction in an exquisitely salt sensitive population might have an additional benefit on bone health next to blood pressure. We observed that salt restriction was associated with a prominent decrease in 24 h urinary calcium and phosphate excretion, paralleled by an increase in serum phosphate. Most importantly, the electrolyte changes went along with a significant increase in BAP concentration. We could identify both classic (iPTH, phosphate, calcium excretion) as well as novel parameters (SBP) to be associated with this osteoanabolic effect. The efficacy of the degree of salt restriction was evident by the significant increase in both plasma renin and aldosterone,²⁹ most likely representing disinhibition of renin secondary to salt depletion.

In accordance with our findings, a study reported that mice exhibited an increase in urinary excretion of calcium and phosphate upon salt loading. The increase was even more pronounced when the diet was rich in sodium and low in potassium.³⁰ These experimental data are backed by a number of epidemiological studies which clearly documented the positive correlation between sodium intake and calciuria.^7-9

On the other hand, we could not observe a correlation between baseline sodium and calcium excretion. In a previous study, our group outlined a significant correlation between urinary sodium and calcium in therapy-naïve patients with PA.³¹ In the subgroup of patients with bilateral PA, 24 h urinary calcium decreased significantly after 1 year of MRA therapy despite sodium excretion being unaltered (rather showing a trend to increase). Of note, salt restriction in our cohort began after a stabilization of blood pressure under MRA therapy. The discrepancy to our previous observation may, thus, be due to a disconnect between sodium and calcium excretion because patients in the present study were already on MRA therapy. This may have led to changes in epithelial claudin expression as suggested by preclinical models^32,33 with possibly increased baseline calcium reabsorption compared to the previous cohort.

In the multivariate analysis, delta BAP was positively associated with a change in SBP and calciuria and negatively associated with serum phosphate and iPTH changes. An inverse correlation of serum phosphate and BAP has been described in a population-based study from Japan.³⁴ A possible underlying mechanism for this may be BAP-mediated phosphate accumulation in hydroxyapatite vesicles.³⁵

It is noteworthy that blood pressure is independently associated both with calcium excretion as well as the risk for osteoporotic fractures.^9,36 Mechanistically, inflammation, sympathetic tone, and vascular perturbations are thought to provide a link between calciuria, osteoporosis, and arterial hypertension.³⁷

We do acknowledge some weaknesses of our study: First, the sample size was limited, and patients were only recruited in a single site. Further, the results and conclusions must be held against a possible residual risk of bias due to the lack of a control group. The statistical test procedures were unadjusted considering the exploratory nature and the post hoc design of our study, therefore further trials are needed to confirm our findings. Also, the short observation period precluded the occurrence of hard end points such as fractures and mortality.

On the other hand, the single center nature of our study allowed all measurements to be conducted at the same site, thereby facilitating a low methodological variability. Further, blood and urinary samples were acquired with procedural rigor to ensure circadian consistency. Urinary electrolytes were determined in duplicate measurements of 24 h urine collections, which is considered the gold standard.³⁸ BAP as bone turnover marker has been reported to correlate well with bone histopathologic parameters and therefore seems to be a good parameter to assess the efficacy of a bone-protective intervention.³⁹ Specifically, a study reported that the use of the osteoanabolic anti-sclerostin antibody romosozumab resulted in increases in blood bone alkaline phosphatase, suggesting that the increase seen in our cohort may also be interpreted as an osteoanabolic effect.⁴⁰ Since the observed increase in BAP correlated positively with the decrease in urinary calcium excretion, we believe that this reflects an overall beneficial, ie, osteoanabolic impact on bone health. However, the underlying physiology of this increase remains unresolved until future studies with parallel assessment of bone mineral density are completed.⁴¹

Our study suggests that salt restriction has an important impact on bone health. The positive effect on bone metabolism was visible already after 12 weeks. We suggest examining this approach in larger collectives and longer treatment intervals with parallel registration of fracture incidences and bone mineral density or microstructure. At the present time, due to the absence of observed adverse effects, salt restriction in adjunct to MRA therapy seems to be a benign and effective bone-protective intervention in patients with bilateral PA.

Acknowledgments

The study was only feasible due to the support of our clinical PA team, the Endocrine laboratory team in Munich as well as the participation of several volunteers from our clinic.

Funding

This work was supported by the Else Kröner-Fresenius Stiftung in support of the German Conn's Registry-Else-Kröner Hyperaldosteronism Registry (2013_A182, 2015_A171, and 2019_A104 to M.R.), the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 694913 to M.R.), by the Deutsche Forschungsgemeinschaft (DFG) within the CRC/Transregio 205/2, (project no. 314061271) “The Adrenal: Central Relay in Health and Disease” to C.A., H.S., and M.R., within the Clinician Scientist Program In Vascular MEdicine (PRIME) MA 2186/14-1 to H.S., Eva Luise und Horst Köhler Stiftung & Else Kröner-Fresenius-Stiftung (2019_KollegSE.03 to H.F.N.), and by the Förderprogramm für Forschung und Lehre (FöFoLe) Reg.-Nr 1051 to C.A.

Authors’ contributions

Holger Schneider (Conceptualization [lead], Data curation [equal], Formal analysis [lead], Funding acquisition [supporting], Investigation [lead], Methodology [equal], Project administration [equal], Visualization [lead], Writing—original draft [lead], Writing—review & editing [lead]), Denise Brüdgam (Data curation [lead], Investigation [equal], Resources [equal], Writing—original draft [supporting]), Hanna Nowotny (Investigation [supporting], Writing—original draft [supporting], Writing—review & editing [supporting]), Ralf Schmidmaier (Investigation [supporting], Writing—original draft [equal]), Martin Reincke (Funding acquisition [supporting], Investigation [equal], Writing—original draft [supporting]), and Christian Adolf (Conceptualization [equal], Data curation [equal], Formal analysis [supporting], Funding acquisition [lead], Investigation [equal], Methodology [equal], Project administration [lead], Supervision [lead], Writing—original draft [lead], Writing—review & editing [lead])

References

Author notes