Impact of urinary calcium excretion on kidney, bone, and cardiovascular systems in patients with bone biopsy proven osteoporosis: a longitudinal long-term follow-up study

The impact of urine calcium on kidney, bone, and cardiovascular systems in osteoporosis is not well-known. In this 7-year-follow-up study, high urine calcium did not affect kidney function but increased risk of kidney stones, while low urine calcium increased cardiovascular diseases. Maintaining normal urine calcium is beneficial for bone health.

PURPOSE

Hypercalciuria is common in patients with osteoporosis. However, the long-term effect of urinary calcium excretion (UCaE) on patients' health is not well-examined. The current study aims to assess the impact of UCaE on kidney, bone, and cardiovascular outcomes in patients with bone biopsy proven osteoporosis.

METHODS

Longitudinal study of all patients with osteoporosis who underwent bone biopsy and 24-h urine collection between 2008 and 2015 in the University of Kentucky. DXA scans, serum markers, kidney function, and cardiovascular events were recorded until last clinic visit in 2021. Exclusion criteria were secondary osteoporosis or conditions that might substantially impact UCaE. The significant results in univariate analysis were confirmed in multi-variable regression models involving clinically important covariates that might impact patients' outcomes.

RESULTS

Study included 230 patients with mean follow-up of 7.2 ± 2.9 years. The mean age was 61 years, and the mean eGFR at baseline was 85 ± 19 ml/min/1.73 m². Low bone turnover (LBT) was present in 57% and high bone turnover (HBT) in 43% of patients. Hypercalciuria was found in one-third of patients with no difference between LTB and HTB. UCaE correlated positively with eGFR but did not affect the rate of eGFR decline over time. Higher UCaE predicted kidney stones development. We observed U-shaped effect of UCaE on bone health. Hypercalciuria predicted loss of BMD at all sites, but also hypocalciuria was associated with higher loss in total hip BMD. Upper limb fractures were the most observed fractures, and their incidence was higher in patients with hyper- or hypo-calciuria. Lower UCaE independently predicted development of major adverse cardiac events (MACE) and cardiovascular disease (CVD).

CONCLUSION

UCaE correlated with eGFR but it did not affect the change of eGFR over time. Patients with normal UCaE had lower incidence of upper limb fractures and less reduction in BMD. Low UCaE predicted MACE and CVD.

Management of osteoporosis in patients with chronic kidney disease

Patients with CKD have a 4-fivefold higher rate of fractures. The incidence of fractures increases with deterioration of kidney function. The process of skeletal changes in CKD patients is characterized by compromised bone strength because of deterioration of bone quantity and/or quality. The fractures lead to a deleterious effect on the quality of life and higher mortality in patients with CKD. The pathogenesis of bone loss and fracture is complex and multi-factorial. Renal osteodystrophy, uremic milieu, drugs, and systemic diseases that lead to renal failure all contribute to bone damage in CKD patients. There is no consensus on the optimal diagnostic method of compromised bone assessment in patients with CKD. Bone quantity and mass can be assessed by dual-energy x-ray absorptiometry (DXA) or quantitative computed tomography (QCT). Bone quality on the other side can be assessed by non-invasive methods such as trabecular bone score (TBS), high-resolution bone imaging methods, and invasive bone biopsy. Bone turnover markers can reflect bone remodeling, but some of them are retained by kidneys. Understanding the mechanism of bone loss is pivotal in preventing fracture in patients with CKD. Several non-pharmacological and therapeutic interventions have been reported to improve bone health. Controlling laboratory abnormalities of CKD-MBD is crucial. Anti-resorptive therapies are effective in improving BMD and reducing fracture risk, but there are uncertainties about safety and efficacy especially in advanced CKD patients. Accepting the prevalent of low bone turnover in patients with advanced CKD, the osteo-anabolics are possibly promising. Parathyroidectomy should be considered a last resort for intractable cases of renal hyperparathyroidism. There is a wide unacceptable gap in osteoporosis management in patients with CKD. This article is focusing on the updated management of CKD-MBD and osteoporosis in CKD patients. Chronic kidney disease deteriorates bone quality and quantity. The mechanism of bone loss mainly determines pharmacological treatment. DXA and QCT provide information about bone quantity, but assessing bone quality, by TBS, high-resolution bone imaging, invasive bone biopsy, and bone turnover markers, can guide us about the mechanism of bone loss.