Introduction

Osteoporosis is a primary or secondary progressive bone disease associated with many factors. Its incidence and prevalence have been increasing, and there are different risk factors that can be attributed to this increase. Osteoporosis is diagnosed based on the T-score [<−2.5 standard deviation (SD)] from measuring the bone mineral density by dual energy X-ray absorptiometry. Although it affects more women since estrogen is an important risk factor, osteoporosis has become more common in men in recent years [1, 2]. In secondary osteoporosis, the presence of idiopathic hypercalciuria is striking, which is also the most common metabolic abnormality in patients with calcium nephrolithiasis [3–6]. Hypercalciuria, which is urine calcium excretion higher than 260 mg in 24 h, is now subcategorized as absorptive hypercalciuria and fasting hypercalciuria, since the previous classification suggested by Pak is controversial [7]. The presence of fasting hypercalciuria has been observed in a significant percentage of patients with bone mineral density loss and calcium stones [8], which must be taken into account not only in this particular group of patients, but also in patients without lithiasis. In children, hypercalciuria has been observed to be an important factor determining bone growth and development [9–11]. Taken together, it appears that hypercalciuria may be present in a significant percentage of patients with bone mineral density loss. It is unclear, however, whether hypercalciuria together with other factors may favor the development of kidney calcium stones. The objective of this study was to analyze in a group of patients with bone mineral density loss, the differences in metabolic parameters in the blood and urine based on whether osteopenia (−1 to −2.5 standard deviation) or osteoporosis (<−2.5 standard deviation) was present.

Materials and methods

Study group

We conducted a cross-sectional study from January 1, 2013 to December 31, 2013 that included a total of 67 patients between ages 18–70 years with loss of bone mineral density and without a history of nephrolithiasis, who were treated in the Endocrinology and Rheumatology department. This group of patients was a part of a larger study that included patients with recurrent calcium nephrolithiasis and those without nephrolithiasis or loss of bone mineral density. In the current study, a subanalysis was performed in the group of patients with only the loss of bone mineral density.

  • Inclusion criteria Men and women between ages 18 and 70 years with bone mineral density loss and without calcium stones.

  • Exclusion criteria Patients with ages under 18 years or over 70 years, congenital bone disease, congenital renal disease, hyperparathyroidism, inflammatory bowel disease, renal tubular acidosis, treatment with bisphosphonates, hormone replacement therapy, treatment with thiazides, treatment with potassium citrate, treatment with corticosteroids or treatment with calcium and vitamin D.

The group of 67 patients with bone mineral density loss was divided into either the osteopenic or osteoporotic group:

  • Group 1: 40 patients (22 men and 18 women) with osteopenia (T-score between −1 and −2.5 SD).

  • Group 2: 27 patients (13 men and 14 women) with osteoporosis (T-score <−2.5 SD).

*Menopause was defined as the absence of menstruation after 12 months of amenorrhea.

Methodology

In all patients, medical history and physical examination were performed, and weight, height and body mass index were determined. At baseline, plain abdominal radiography and/or intravenous urography and renal ultrasound were done to rule out kidney stones.

Biochemical analysis of blood and urine along with measurement of the bone density of the hip and lumbar spine was also performed but at a later time point.

  • Parameters measured in blood: creatinine, uric acid, calcium, phosphorus, intact parathyroid hormone (iPTH), alkaline phosphatase, osteocalcin** and β-crosslaps***.

  • **Determined by chemiluminescence assay in an automatic analyzer using LIAISON-Osteocalcin (DIASORIN) (Roche Diagnostic).

  • ***Determined by electrochemiluminescence immunoassay (ECLIA) using Elecsys in a MODULAR ANALYTICS E170 automatic analyzer.

  • Parameters measured in fasting urine: calcium, creatinine, oxalate, citrate, uric and calcium/creatinine ratio. After 8 h of nocturnal fasting, low intake of protein (0.8–1 g/weight kg/day) and salt (3–5 g/day) was recommended.

  • Parameters measured in 24 h urine: diuresis, creatinine, uric acid, calcium, citrate, oxalate and calcium/creatinine ratio.

  • Bone density: measured by dual energy X-ray absorptiometry (Hologic QDR 4500).

Statistical analysis

Statistical analysis of the results was conducted using Student’s t test for qualitative–quantitative variables, Chi-square test for qualitative variables, odds ratio for analysis of binary logistic regression results, and using a confidence interval of 95 % for multivariate analysis. Pearson correlation test or failing Spearman rho test was used for linear correlation analysis between quantitative variables. Normality of the variables was checked using Kolmogorov–Smirnov test and analysis of variance with Levene test. We considered results to be statistically significant if p ≤ 0.05. Analyses were conducted with SPSS 17.0 for Windows.

Ethical considerations

All patients read and signed the informed consent to participate in the study. This study was approved by the Ethics Committee of the University Hospital San Cecilio and funded by the Fundación Progreso y Salud (Junta de Andalucía, Spain).

Fig. 1
figure 1

Points diagram in which we observe the positive, linear and significant relation between levels of beta-crosslaps and calciuria 24 h

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Fig. 2
figure 2

Points diagram in which we observe the positive, linear and significant relation between levels of beta-crosslaps and calcium/creatinine 24 h

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Results

The mean age of the patients included in the study was 52.9 ± 12.8 years in group 1 versus 50.3 ± 11.4 in group 2 without any statistically significant difference. No statistically significant difference was observed regarding sex between groups. There were 32 women out of the total 67 patients. Among the 32 women, 16 had menopause with osteopenia and 11 had menopause with osteoporosis, and the difference was not statistically significant (p = 0.43). When the different parameters studied from the blood and urine of patients in the two groups were evaluated, we observed that patients with osteoporosis had higher levels of osteocalcin and β-crosslaps compared to the osteopenia group, and this was statistically significant (see Table 1). Urine analysis of patients in group 2 showed increased excretion of calcium and oxalate in the 24 h urine samples compared to group 1, which was statistically significant, and the 24 h calcium/creatinine and fasting calcium/creatinine ratios were higher compared to group 1 (see Table 2). Calcium excretion in urine had a positive linear correlation with levels of sodium (R = 0.710; p = 0.0001) and urea (R = 0.778; p = 0.0001) in the urine.

Table 1 Value of the parameters in the studied variables in blood between two groups of patients (Group 1: osteopenia; Group 2: osteoporosis)
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Table 2 Urinary values in fasting and 24 h in the two groups studied
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Multivariate analysis of the following variables was performed: osteocalcin, β-crosslaps, fasting urine calcium/creatinine ratio, 24 h urine calcium/creatinine ratio, 24 h urinary calcium and 24 h oxaluria. The two variables independently associated with the occurrence of osteoporosis were fasting urine calcium/creatinine ratio (p = 0.03) and β-crosslaps levels (p = 0.03).

In the Pearson linear correlation analysis, the calcium excretion in 24 h urine was positively related with the levels of β-crosslaps (R = 0.258; p = 0.03) (Fig. 1); the 24 h urine calcium/creatinine and fasting urine calcium/creatinine Fig. 2 ratios showed positive linear correlation with the levels of β-crosslaps (R = 0.279; p = 0.02 and R = 0.282; p = 0.02, respectively).

Discussion

Calcium is an important and vital component for mineralization of the skeleton, and intrinsic or extrinsic alterations in bone metabolism can affect the blood and urine levels; therefore, it is critical to understand the role of hypercalciuria in bone health [12]. Furthermore, the determination of calcium in urine in patients with bone mineral density loss may have implications on medical treatment and monitoring: resulting in changes in management depending on the presence or absence of hypercalciuria [13]. As previously mentioned, while it is clear that the diagnosis of osteopenia or osteoporosis is made by bone densitometry, it may be useful to identify and measure different markers in the blood and urine of these patients. In a study by Ryan et al. [14], they noted that a group of patients with osteoporosis, hypercalciuria was also present. Studies by Cerdá et al. [15] and Peris et al. [16] detected alterations in iPTH or other markers of bone resorption. In our study, we observed that patients with osteoporosis showed higher levels of markers of bone turnover compared to patients with osteopenia; however, iPTH levels were not different between the two groups. Hence, the loss of bone density does not appear to have a direct relationship with this particular hormone in our group of patients. Other authors have analyzed the presence of hypercalciuria in women with osteoporosis, noting that about 40 % have hypercalciuria [17] and recommend testing for hypercalciuria as a part of a routine evaluation of patients with osteoporosis [18]. In the study presented here, the mean calcium excretion in 24 h urine was much higher in patients with osteoporosis versus patients with osteopenia. In addition, urinary calcium and β-crosslaps (bone resorption marker) were each an independent factor associated with osteoporosis after multivariate analysis. In addition to these findings, higher fasting and 24 h urine calcium/creatinine ratios were observed in patients with osteoporosis compared to patients with osteopenia. Some studies have postulated that there may be an increase in intestinal calcium absorption in women with osteoporosis and hypercalciuria [19], but this has not been confirmed. It seems clear that calcium metabolism is altered in patients with osteoporosis since increased levels of calcium is observed in the urine and has a positive linear relationship with levels of β-crosslaps, suggesting that with more bone resorption, calciuria is greater. This increase in urinary calcium has not produced an increase in the incidence of kidney stones. However, since hypercalciuria is a lithogenic risk factor [6], it may be important to determine the presence hypercalciuria in patients with osteoporosis. The main limitation of this study was the low number of patients. Another limitation was not taking into account the potential effects of food intake in the study group. However, our findings corroborates with the existing data in the literature in regard to the independent association of urinary calcium with osteoporosis. Calciuria could be altered in patients with incipient osteoporosis, independent of iPTH and vitamin D, probably in relation with bone intrinsic metabolism and in correlation with bone mineral density resorption markers, such as β-crosslaps. Usually, the treatment of osteoporosis is based on increasing oral calcium intake and vitamin D. This could lead to increase in calciuria in patients with previously unknown hypercalciuria. Hence, we recommend the evaluation and analysis of calciuria and fasting and 24 h urine calcium/creatinine ratios in patients with osteoporosis, as it may have possible therapeutic implications. Finally, low diet calcium intake results in increased bone mineral density loss and hypercalciuria, so we recommend a normal daily calcium intake (1000–1200 mg at day with natural sources).