Abstract
This observational, cross-sectional, epidemiological analysis investigated relationships of kidney function to urine calcium and other variables. The analyses targeted two population-based samples of adults (Gubbio study and Moli-sani study: n = 3508 and 955, respectively). Kidney function was assessed as estimated glomerular filtration rate (eGFR). Calcium/creatinine ratio (Ca/Cr) was used as index of urinary calcium in timed overnight urine under fed condition (Gubbio study), morning urine after overnight fast (Gubbio study), and first-void morning urine (Moli-sani study). Moli-sani study included also data for glomerular filtered calcium load, tubular calcium handling, and serum phosphorus, parathyroid hormone, 1,25-dihydroxyvitamin D, calcium, and 25-hydroxyvitamin D. eGFR positively and independently related to Ca/Cr (p < 0.001). In multivariate analyses, eGFR lower by 10 mL/min × 1.73 m2 related to overnight urine Ca/Cr lower by 14.0 mg/g in men and 17.8 mg/g in women, to morning urine Ca/Cr lower by 9.3 mg/g in men and 11.2 mg/g in women, and to first-void urine Ca/Cr lower by 7.7 mg/g in men and 9.6 mg/g in women (p < 0.001). eGFR independently related to glomerular filtered calcium load (p < 0.001) and did not relate to tubular calcium handling (p ≥ 0.35). In reduced eGFR only (<90 mL/min × 1.73 m2), low urine Ca/Cr independently related to low serum 1,25-dihydroxyvitamin D (p = 0.002) and did not relate to hyperphosphatemia, high serum parathyroid hormone, or hypocalcemia (p ≥ 0.14). Population-based data indicated consistent associations of lower kidney function with lower urine calcium due to reduction in glomerular filtered calcium. In reduced kidney function, relative hypocalciuria associated with higher prevalence of low serum 1,25-dihydroxyvitamin D.
Highlights
The term ‘chronic kidney disease—mineral and bone disorder’ (CKD–MBD) is used to define a complex syndrome secondary to the chronic decline in kidney function that encompasses the progressive appearance of biochemical, skeletal, and cardiovascular abnormalities [1]
Men and women differed for anthropometry, urine and serum creatinine, estimated glomerular filtration rate (eGFR), urine Ca, urine creatinine, and Ca/creatinine ratio
In the Gubbio study dataset, Ca/creatinine ratio was higher in overnight urine than morning urine after overnight fast
Summary
The term ‘chronic kidney disease—mineral and bone disorder’ (CKD–MBD) is used to define a complex syndrome secondary to the chronic decline in kidney function that encompasses the progressive appearance of biochemical, skeletal, and cardiovascular abnormalities [1]. Two studies reported data regarding the relationship between kidney function and urine Ca [3,4]. The present study analyzed two sets of epidemiological data to investigate the relationship of kidney function with urinary Ca in general population. It reports the consistence of results under fed and fasting conditions, and in different population samples, the relationship of kidney function with classical indices of kidney Ca handling, and the possible use of urine Ca as proxy of biochemical abnormalities typical of CKD-MBD
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