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PubMed1994

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

Calcium (Ca) absorption was measured by the balance method and the dual tracer isotopic method in 462 studies of adult women. Results for both fractional absorption and net Ca balance were similar using the 2 methods. Slightly higher values (mean difference, 0.031 +/- 0.092) were found for fractional absorption measured by the balance method than by the isotopic method. No differences in fractional absorption between the methods were seen at a fractional absorption less than 25%. Variability in the data was greater from the balance than the isotopic method. Endogenous fecal Ca excretion was directly related to Ca intake, with an estimated value of 70-80 mg/day for a Ca intake less than 200 mg/day. These findings support the usefulness and accuracy of isotope-based measures of mineral absorption. No evidence is found from these or previous data to suggest that the process of isotopic equilibration falsely increases estimates of absorption or endogenous excretion in tracer studies. Isotopic techniques allow studies of calcium absorption in diverse populations and evaluation of unique aspects of mineral metabolism not accessible through other techniques.

Absorption↗
PubMed1989

One-day test using stable isotopes to measure true fractional calcium absorption.

True calcium absorption can only be measured by the time-consuming and expensive metabolic balance method and is not predicted well by measuring the fractional absorption (FA) of radiocalcium from a fixed calcium carrier. We describe here a 1 day method for measuring the actual fraction of calcium absorbed from the habitual diet (true fractional calcium absorption, TFCA) using stable isotopes of calcium. Oral and intravenous isotopes were administered with each of the three daily meals, and then the ratio of the two isotopes in the urine was measured by mass spectroscopy. In 12 subjects, TFCA determined from stable Ca correlated well with TFCA measured by the balance method (r = 0.71, P less than 0.01), and the mean values were not different (0.26 and 0.26). In contrast, no significant correlation was found between FA and TFCA. The weak relationship between FA and TFCA underscores the importance of tracing dietary calcium rather than a fixed calcium carrier in tests of calcium absorption. Using the new method, TFCA was inversely related to dietary calcium (r = -0.45, P less than 0.05), demonstrating that it could detect physiological changes in calcium absorption. Thus, this test has two important advantages: (1) it provides a simple way to measure TFCA and true fractional calcium absorption (the product of TFCA and dietary calcium), the physiologically relevant variables, and (2) because there is no radiation exposure, the test can be used in pregnant women and children, the isotopes can be prepared in advance, and several isotopes can be used simultaneously.

Administration, Oral↗
PubMed1992

Developmental changes in calcium kinetics in children assessed using stable isotopes.

Total exchangeable calcium pool size (TEP) and bone calcium accretion rate (Vo+) were measured using stable isotopes in healthy children and young adults. 42Ca or 46Ca was given intravenously to 10 children aged 10 months to 14 years and 3 women aged 23-33 years. Calcium kinetic parameters were determined using a two- or three-exponential curve of the resultant serum and urine tracer excesses. These data were compared with previously reported (radiotracer) kinetic studies of 21 children and 5 adults without known bone disease. Current results are comparable to those previously obtained, and the data from all studies were analyzed together. Total Vo+ was significantly greater in children aged 3-16 years than in adults (2.8 +/- 1.6 versus 0.7 +/- 0.2 g/day, p less than 0.01). Both TEP and Vo+ were significantly correlated to age independently of variations in body weight (p less than 0.01 for each). The ratio ko+ = Vo+/TEP was greater in children than adults (0.36 +/- 0.15 versus 0.12 +/- 0.03 day-1, p less than 0.001). These data demonstrate increased bone flow of calcium associated with increases in exchangeable calcium pools in children compared to adults. Vo+ and TEP may be maximum in early adolescence, associated with peak rates of net calcium accretion. The use of stable isotopes permits the safe evaluation of calcium kinetics in patients of all ages.

Adolescent↗
PubMed2000

Isotopic evidence for variations in the marine calcium cycle over the Cenozoic.

Significant variations in the isotopic composition of marine calcium have occurred over the last 80 million years. These variations reflect deviations in the balance between inputs of calcium to the ocean from weathering and outputs due to carbonate sedimentation, processes that are important in controlling the concentration of carbon dioxide in the atmosphere and, hence, global climate. The calcium isotopic ratio of paleo-seawater is an indicator of past changes in atmospheric carbon dioxide when coupled with determinations of paleo-pH.

Animals↗
PubMed2003

A novel dual radio- and stable-isotope method for measuring calcium absorption in humans: comparison with the whole-body radioisotope retention method.

BACKGROUND: Dietary calcium absorption can be determined only with the use of isotope techniques. Currently used isotope techniques require exclusive equipment or are not true tracer approaches. OBJECTIVE: The objective was to compare a dual-isotope method combining radioisotopes and stable isotopes with a whole-body radioisotope retention method for measuring calcium absorption. DESIGN: Seven healthy adults aged 21-27 y consumed a test meal containing 63 +/- 14 (macro x +/- SD) mg Ca together with a water solution of (47)Ca (0.11 MBq). One hour after ingestion, 18 mg (44)Ca was administered intravenously. All feces and urine were collected for 5 and 6 d, respectively. Calcium absorption was estimated from whole-body retention of the radioisotope 12 times over 3 wk after ingestion and from the excretion of (47)Ca and (44)Ca in a 24-h urine sample collected on day 2. (44)Ca in urine was determined by inductively coupled plasma mass spectrometry. RESULTS: Mean (+/- SD) calcium absorption was 75 +/- 9% with the dual-isotope method and was 74 +/- 8% with the whole-body radioisotope retention method. There was a high degree of agreement between the methods. CONCLUSION: The dual-isotope method is a valid approach for measuring calcium absorption from a single meal.

Administration, Oral↗
PubMed1964

STRONTIUM AND CALCIUM UPTAKE BY THE GREEN ALGA, OOCYSTIS EREMOSPHAERIA.

The uptake of calcium and strontium by a green unicellular alga cultured in media containing these elements in concentrations found in nature is directly proportional to the concentration in the media. Variation in the concentration of either element has a slight inverse influence on the uptake of the other. The increase of strontium uptake when the calcium concentration is very low indicates that strontium is substituted for calcium when calcium is limited and suggests that the alga requires either element.

Calcium↗