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PubMed1978

[Enteral absorption of high calcium doses determined by means of stable isotopes (author's transl)].

The calcium absorption was determined in 8 healthy test persons (average age 25.6 years) after oral administration of 500 mg and 1000 mg of this element. A double isotope method with enriched stable calcium isotopes was used which can be applied without any restriction as, unlike in radiotracer techniques, any exposure to ionizing radiation is avoided. The calcium administered orally was labelled with 48Ca, the CaCl2 injected intravenously with 46Ca. The determination of both isotopes in serum and urine samples was carried out by means of neutron activation analysis. Independent of the calcium dose given, an absorption of 30% was found. From the 46Ca content in the serum samples a mean value of 6.4 +/- 1.0 g calcium or 98.8 +/- 15.4 mg Ca/kg body weight was calculated for the 24 hr-exchangeable calcium body pool.

Adult↗
PubMed1984

A comparative study of calcium transients by isotopic tracer, metallochromic indicator, and intrinsic fluorescence in sarcoplasmic reticulum ATPase.

Comparative measurements were carried out in order to evaluate the significance of intrinsic fluorescence transients with respect to various steps of the catalytic and transport cycle of sarcoplasmic reticulum ATPase. The enzyme can acquire three levels of intrinsic fluorescence. Level 1 (lowest fluorescence) is observed in the absence of Ca2+ and ATP. Level 2 (highest fluorescence) is induced by Ca2+ through a sequential mechanism including two binding steps interspaced by an isomerization step. This transition occurs more rapidly in the presence of ATP and produces enzyme activation. Level 3 (slightly higher fluorescence than level 1) is observed immediately upon ATP binding (or phosphorylation with Pi) in the absence of Ca2+. When ATP is added to the enzyme X calcium complex, the enzyme is rapidly phosphorylated and the bound calcium is translocated to a position which is protected from La3+ added to the medium. This initial phenomenon is followed by a slow isomerization of the phosphoenzyme which is revealed by a decrease of fluorescence intensity and produces calcium release inside the vesicles before hydrolytic cleavage of the phosphoenzyme. A reaction cycle is considered and subjected to analysis, based on three main enzyme states: E, in the absence of Ca2+; E', in the presence of Ca2+; and *E, subsequent to phosphorylation.

Adenosine Triphosphatases↗
PubMed2005

Calcium absorption from corn tortilla is relatively high and is dependent upon calcium content and liming in Mexican women.

Corn tortillas are the staple food of Mexico. During their preparation, calcium is added to the tortillas; therefore, tortillas are the main source of calcium for a large proportion of the population. The bioavailability of calcium from lime-treated tortillas in humans is not known. The objectives of the present study were to determine calcium absorption from corn tortilla, to determine the effect of lime treatment on calcium absorption from corn tortilla, and to compare calcium absorption from tortilla prepared with a commercial corn flour and tortillas prepared with the traditional lime treatment at home. Nonpregnant, nonlactating women (n = 9) were administered 3 different treatments: 1) 180 g of corn tortilla prepared from corn flour with no lime treatment (CF), 2) 180 g of corn tortilla prepared from lime-treated commercial corn flour (LTCCF), or 3) 180 g of corn tortillas prepared from lime-treated home-prepared corn flour (LTHCF). Calcium absorption was measured using an established dual-tracer stable isotope technique. Calcium absorption of CF, LTCCF, and LTHCF was (mean +/- SD): 44 +/- 3.2, 32 +/- 4.4, and 30 +/- 2.4%, respectively; the fractional calcium absorption from CF differed from that of either LTCCF or LTHCF (P < 0.01). The total amount of calcium absorbed per treatment was higher in LTHCF (98.10 mg +/- 21.7) than in LTCCF (59.9 mg +/- 23.7, P < 0.001) and CF (3.78 mg +/- 0.9, P < 0.0.001). In conclusion, calcium absorption from corn tortillas is high and dependent on calcium concentration. The addition of calcium during lime treatment increases calcium concentration and total calcium absorption.

Absorption↗
PubMed1996

Increased efficiency of calcium absorption during short periods of inadequate calcium intake in girls.

Adequate calcium intake is essential for skeletal integrity, particularly during the period of peak bone mass acquisition from 9 to 17 y of age. Currently, the calcium intake of many adolescent girls is below the recommended dietary allowance. The purpose of this study was to evaluate the ability of girls to respond to acute periods of inadequate dietary calcium intake. Calcium absorption was evaluated in 11 girls aged 11.6 +/- 2.4 y after 10 d on both a low-calcium (7.05 +/- 2.03 mmol/d) and a high-calcium (35.30 +/- 2.28 mmol/d) diet. Fractional calcium absorption was determined by using oral (46Ca) and intravenous (42Ca) stable isotopes of calcium. During a low calcium intake, fractional calcium absorption was significantly greater (0.582 +/- 0.087 compared with 0.260 +/- 0.068, P < 0.0001) and urinary calcium excretion was significantly lower (1.30 +/- 0.83 compared with 3.08 +/- 1.98 mmol/d, P < 0.004) than values obtained during a high calcium intake. Concentrations of 1,25-dihydroxyvitamin D (combination of cholecalciferol and ergocalciferol) were greater during the low calcium intake, although the difference was not significant (108.7 +/- 30.6 compared with 90.0 +/- 25.1 pmol/L, P < 0.1; n = 9). Excretion of N-telopeptide was significantly greater during the low calcium intake (761 +/- 508 compared with 413 +/- 341 nmol bone collagen equivalent (BCM)/mmol creatinine, P < 0.02; n = 9), indicating that bone resorption was increased. These results suggest that during short periods of inadequate calcium intake, girls are able to significantly increase the efficiency of calcium absorption and decrease urinary calcium losses to conserve calcium required for bone mineral acquisition.

Absorption↗