Fractional calcium absorption

Calcium carbonate is the salt of choice because it contains the highest concentration of elemental calcium (40%) and is the least expensive (Table 3-3). It should be ingested with meals to enhance absorption from increased acid secretion. Calcium citrate absorption is acid independent and need not be taken with meals. Because the fraction of calcium absorbed decreases with increasing dose, maximum single doses of600 mg or less of elemental calcium are recommended. 

Calcium Adequate intake (Table 3-1) in divided doses Absorption-predominantly active transport with some passive diffusion, fractional absorption 10-60%, fecal elimination for the unabsorbed and renal elimination for the absorbed calcium... 

Hicks, P. D., and Abrams, S. A. (2006). Formula-fed full term infants have lower fractional absorption but greater total calcium absorption than human milk-fed infants. FASEB ]. 20, A991. 

Both the active and passive modes of calcium transport are increased during pregnancy and lactation. This is probably due to the increase in calbindin and serum PTH and 1,25-dihydroxyvitamin D concentrations that occur during normal pregnancy. Intestinal calcium absorption is also dependent on age, with a 0.2% per year decline in absorption efficiency starting in midlife. The fractional absorption of calcium depends on the form and dietary source. Absorption rates are 29% for the calcium in cow s milk, 35% for calcium citrate, 27% for calcium carbonate, and 25% for tricalcium phosphate. Other factors that limit the bioavailability of calcium in the intestine are oxalates and phy-tates, which are found in high quantities in vegetarian diets and which chelate calcium. 

Calcium absorption can be measured by the double-isotope technique. In this technique a meal containing caldum-45 is consumed and the radioactivity in the urine measured. The measurement of urinary Ca alone cannot provide the fractional absorption, because some of the Ca absorbed is taken up by cells, deposited in bone, or excreted in the bile. A second isotope of calcium, Ca, is used to correct for the fates of absorbed calcium, other than excretion in the urine. The use of the Ca is intended to eliminate cell uptake, bone deposit, and biliary losses as variables in the study of the absorption of the dose of cdcium-45. 

It is a well-known fact that fractional absorption of calcium in man ranges from 20 to 70% (Heaney etal. 1990). A number of studies have also shown there to be a decrease in the rate of fractional absorption of calcium with increasing age (for a review, see Armbrecht 1990). Adaptation of intestinal calcium absorption to changes in calcium intake via the vitamin D endocrine system has also been demonstrated in human subjects (Sheikh etal. 1990). A low serum Ca level stimulates the renal synthesis of 1,25(OH)2Dj (Fraser 1980), which in turn leads to increased absorption of calcium from the intestinal lumen. It is through this mechanism that the fractional absorption of calcium from the intestine... 

Yergey, A.L., Vieira, N.E., and Coveil, D.G. (1987) Direct measurement of dietary fractional absorption using calcium isotopic tracers. Biomed. Environ. Mass Spectrom., 14, 603-607. 

Fig. 4. Dependence on filler concentration (

Elution volume calibrations were performed using radioactive tracers of the rare earth elements and 133Ba, with atomic-absorption or flame-emission analysis of iron, sodium, potassium, calcium, and magnesium. As shown in Fig. 5.14, any barium added to the second columns is eluted at the start of the light rare earth element fraction . To ensure barium removal the sample can be put through the first column again. 

Choice of Potential Bioavailability Criterion. It is usually assumed that calcium must be soluble and probably ionized in order to be available for absorption ( ). For the in vitro procedure, as a first approximation we chose calcium solubility after centrifugation at 18,000 x g as the measure of potential bioavailability (Figure 1). We assumed that this would probably overestimate the available calcium and later work based on fractionation might define the bioavailable calcium more precisely. The data in Table IV illustrate how the choice of criterion for "solubility" could affect the in vitro estimate of potential availability, even if in vitro conditions closely resembled in vivo conditions. Since our in vitro criterion unexpectedly underestimated calcium bioavailability for two of the three foods in the direct in vivo - in vitro comparison (8), it was necessary to determine the in vitro digestion conditions which might be limiting solubility before addressing the choice of appropriate criterion. 

Our studies do not resolve the question of phytate vs fiber for the effect of wheat bran on dietary calcium bioavailability. Phytate level clearly affected apparent absorption of calcium in HS-II in the presence of an amount of the water insoluble fraction of dephytinized bran equivalent to 12 g of untreated bran and the phytate supplied as sodium phytate. An additional trial using untreated bran and the same amount of fiber as the water insoluble fraction with sodium phytate could resolve the question of fiber vs phytate. In HS-I, the balances were positive when a relatively large amount of bran, 36 g/day, was consumed. Calcium intakes were possibly higher than most men consume, but under the dietary conditions imposed for 15 days, the phytate and fiber of 36 g of bran did not express an adverse effect on calcium balance. 

CalcitrioPs action primary function is in regulating plasma calcium concentration. In health, the plasma total calcium concentration is tightly controlled at 2.35-2.55 mmol/1. Only the ionized or free fraction, amounting to about 50% of the total, is physiologically active in for example, maintenance of membrane electrical potential and bone formation. The hormone causes increased bone resorption via activation of osteoclasts (see Section 9.4) and increased intestinal absorption of calcium following the synthesis of a specific binding protein in mucosal cells. As described in Section 4.7, some... 

Brink, E. J., van den Heuvel, E. G. H. M., and Muijs, T. (2003). Comparison of six different calcium sources and meal type on true fractional calcium absorption in postmenopausal women. Curr. Top. Nutraceut Res. 1,161-168. 

Heaney, R. P., Weaver, C. M., and Fitzsimmons, M. L. (1990b). Influence of calcium load on absorption fraction. JBMR 5,1135-1138. 

It is necessary to fractionate as rapidly as possible in order to prevent oxidation to the disulfide, which occurs almost completely even in the presence of nitrogen if the column is too long or if the distillation is carried out too slowly. Oil-pumped nitrogen is dried through an absorption tower containing soda lime and calcium chloride before passing to the distillation apparatus. The column should be vacuum jacketed or provided with a heated jacket. 

Some of the metallic constituents were somewhat soluble in the propane-oil portion, but all tended to be concentrated in the asphaltic portion. Although vanadium (0.02% by weight in the original crude petroleum) was present in all fractions, the greater part was found in the cyclohexane and benzene fractions. From the similarity of the absorption spectra of the vanadium concentrates from petroleum and those of synthetic vanadium porphyrin complexes, Skinner arrived at the conclusion that vanadium compounds from the petroleum of the Santa Maria Valley Field in California exist as porphyrin complexes. Additional metallic constituents were detected by Skinner as these became concentrated in the various solvent fractions, including aluminum, titanium, calcium, and molybdenum. 

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