Relationship between balance and dual tracer isotopic measurements of calcium absorption and excretion

Calcium isotope ratios in blood and urine: A new biomarker for the diagnosis of osteoporosis

We assessed the potential of Calcium (Ca) isotope fractionation measurements in blood (δ^44/42Ca_Blood) and urine (δ^44/42Ca_Urine) as a new biomarker for the diagnosis of osteoporosis. One hundred post-menopausal women aged 50 to 75 years underwent dual-energy X-ray absorptiometry (DXA), the gold standard for determination of bone mineral density. After exclusion of women with kidney failure and vitamin D deficiency (<25 nmol/l) 80 women remained in the study. Of these women 14 fulfilled the standard diagnostic criteria for osteoporosis based on DXA. Both the δ^44/42Ca_Blood (p < 0.001) and δ^44/42Ca_Urine (p = 0.004) values were significantly different in women with osteoporosis (δ^44/42Ca_Blood: −0.99 ± 0.10‰, δ ^44/42Ca_Urine: +0.10 ± 0.21‰, (Mean ± one standard deviation (SD), n = 14)) from those without osteoporosis (δ^44/42Ca_Blood: −0.84 ± 0.14‰, δ^44/42Ca_Urine: +0.35 ± 0.33‰, (SD), n = 66). This corresponded to the average Ca concentrations in morning spot urine samples ([Ca]_Urine) which were higher (p = 0.041) in those women suffering from osteoporosis ([Ca]_{Urine-Osteoporosis}: 2.58 ± 1.26 mmol/l, (SD), n = 14) than in the control group ([Ca]_Urine-_Control: 1.96 ± 1.39 mmol/l, (SD), n = 66). However, blood Ca concentrations ([Ca]_Blood) were statistically indistinguishable between groups ([Ca]_Blood, control: 2.39 ± 0.10 mmol/l (SD), n = 66); osteoporosis group: 2.43 ± 0.10 mmol/l (SD, n = 14) and were also not correlated to their corresponding Ca isotope compositions. The δ^44/42Ca_Blood and δ^44/42Ca_Urine values correlated significantly (p = 0.004 to p = 0.031) with their corresponding DXA data indicating that both Ca isotope ratios are biomarkers for osteoporosis. Furthermore, Ca isotope ratios were significantly correlated to other clinical parameters ([Ca]_Urine, ([Ca]_Urine/Creatinine)) and biomarkers (CRP, CTX/P1NP) associated with bone mineralization and demineralization. From regression analysis it can be shown that the δ^44/42Ca_Blood values are the best biomarker for osteoporosis and that no other clinical parameters need to be taken into account in order to improve diagnosis. Cut-off values for discrimination of subjects suffering from osteoporosis were − 0.85‰ and 0.16‰ for δ^44/42Ca_Blood and δ^44/42Ca_Urine, respectively. Corresponding sensitivities were 100% for δ^44/42Ca_Blood and ~79% for δ^44/42Ca_Urine. Apparent specificities were ~55% for δ^44/42Ca_Blood and ~71%. The apparent discrepancy in the number of diagnosed cases is reconciled by the different methodological approaches to diagnose osteoporosis. DXA reflects the bone mass density (BMD) of selected bones only (femur and spine) whereas the Ca isotope biomarker reflects bone Ca loss of the whole skeleton. In addition, the close correlation between Ca isotopes and biomarkers of bone demineralization suggest that early changes in bone demineralization are detected by Ca isotope values, long before radiological changes in BMD can manifest on DXA. Further studies are required to independently confirm that Ca isotope measurement provide a sensitive, non-invasive and radiation-free method for the diagnosis of osteoporosis.

Evaluation of an inexpensive calcium absorption index in healthy older men and women

OBJECTIVE

Calcium absorption is an important determinant of calcium retention and bone metabolism. However, most methods of measuring calcium absorption, including the well-established dual stable isotope method, are costly and cumbersome to implement. We evaluated whether an oral calcium tolerance test (OCTT), which involves measuring calcium excretion in a fasting 2-h urine collection and two 2-h collections following an oral calcium dose, may be a useful index of calcium absorption in older adults consuming a fixed calcium intake of 30 mmol/day.

DESIGN

After a 10-day metabolic diet containing 30 mmol/day of calcium, subjects had calcium absorption measured using the dual stable isotope method and the OCTT.

PARTICIPANTS

Eleven healthy subjects aged 54-74 years.

MEASUREMENTS

Fractional calcium absorption (FCA), calcium excretion in a fasting 2-h urine collection and two 2-h collections in response to a 10-mmol calcium dose (total intake 30 mmol/day).

RESULTS

Calcium excretion from several combinations of the urine collections was examined in relation to FCA. The most predictive of FCA was calcium excretion 4 h following the calcium dose. This measure was significantly correlated with FCA (r = 0.735, P = 0.010), fitting 54% of the variability in FCA.

CONCLUSION

Urinary calcium excretion during the 4 h after a 10-mmol calcium dose is a useful index of calcium absorption among older adults consuming recommended calcium intakes. This test is inexpensive, easy to implement and potentially useful in large clinical studies.

Higher serum 25-hydroxyvitamin D levels in school-age children are inconsistently associated with increased calcium absorption

CONTEXT

Increasing serum 25-hydroxyvitamin D (25-OHD) in adults may enhance calcium absorption (Ca-abs). There are few similar pediatric data leading to uncertainty about the optimal target for 25-OHD to maximize Ca-abs.

OBJECTIVE

Our objective was to evaluate the relationship between 25-OHD and Ca-abs in a large cohort of school-age children and adolescents.

DESIGN

We evaluated data from 439 Ca-abs measurements performed using dual-tracer stable isotope techniques conducted at our center over a 15-yr period in 251 healthy children, 4.9-16.7 yr of age.

RESULTS

Serum 25-OHD ranged from 28 to 197 nmol/liter (mean 85 +/- 2 nmol/liter) (sem). Total Ca-abs (intake times fractional absorption) were significantly correlated to 25-OHD in the whole population (r = 0.16, P = 0.001). This relationship was closer in the 197 studies in early puberty (Tanner 2 or 3, r = 0.35, P < 0.001) and not significant in pre- or late pubertal subjects. For the whole population, fractional Ca-abs adjusted for calcium intake were slightly but significantly higher at 25-OHD of 28-50 nmol/liter (0.344 +/- 0.019) compared with 25-OHD of 50-80 nmol/liter (0.280 +/- 0.014) or 25-OHD greater than 80 nmol/liter (0.297 +/- 0.015, P < 0.01 for each), suggesting adaptation to moderately low 25-OHD values.

CONCLUSION

There is no consistent pattern of relationship between 25-OHD and either fractional or total calcium absorption in school-age children. However, there appears to be a modest calcium absorptive response to higher 25-OHD during early puberty.