CALCIUM ABSORPTION: MECHANISMS ANDCONSIDERATIONS

Four major physicochemical factors need to be considered in evaluating calciumabsorption efficiency for potential deposition in body calcium tissue pools. Theseinclude:

• Solubility of calcium salt in the acid medium of the stomach contents

• Reprecipitation of calcium salts following neutralization by bicarbonateions from pancreatic secretion

• Calcium complexation with anions while in the neutral environment ofthe small intestine

• Rate-limiting reduction of calcium salt exposure to the absorptive site ofthe small intestine due to gastric emptying rate or trapping in fecal material

Because the mechanisms underlying the cellular pathway are potentially ratelimiting, an upper limit to the capacity of calcium transport via the cells explainsthe saturable component that represents the transcellular pathway. In contrast, thenonsaturable component represents transport via a paracellular pathway (Pansu et al.1981). The rate of calcium transport via the saturable route attains a constant valueat different intralumenal calcium concentrations, whereas the rate of calcium transportfrom the nonsaturable route increases as a linear function of the intralumenalcalcium concentration.Intestinal calcium transport from the transcellular pathway is primarily dependenton vitamin D (DeLuca and Schnoes 1983; Wasserman et al. 1992) and occursat the greatest extent in the duodenum. The finding that 1,25-(OH)2D3receptoroccupancy is correlated with the expression of the cytosolic calbindin production,which in turn is correlated with the rate of intestinal calcium transport, is evidencethat the role of vitamin D in regulating intestinal calcium transport involves biosynthesisof calcium-binding proteins (Hunzikar et al. 1982). Other researchers havealso indicated a role of vitamin D action on intestinal calcium transport that goesbeyond the induction of calbindin biosynthesis (Wasserman et al. 1982).In contrast, the paracellular transport mechanism of calcium occurs primarilythroughout the distal segment of the small intestine, and possibly the large intestine,and is independent of vitamin D. Instead, mucosal sodium and lactose concentrationscan greatly enhance calcium absorption. This pathway represents a solvent drageffect that facilitates calcium absorption by an active extrusion of calcium in thebasolateral membrane. This results in a convective transfer of lumenal calcium,which increases calcium concentration in the extracellular space otherwise occupiedby the lamina propria (Karbach 1992). This nonsaturable, energy-independent (andtherefore passive) absorption of calcium (Bronner 1992) occurs by an osmoticgradient that attracts lumenal water along with solutes and is regulated by thepermeability of tight junctions.The duodenum and ileum have the potential to absorb calcium against anelectrochemical gradient, even though a greater rate has been observed in theduodenum (Behar and Kerstein 1976). Notwithstanding this finding, dietary calciumis considered to be absorbed mainly in the distal part of the lower intestine whena normal level of available dietary calcium is present in this intestinal section. Earlystudies by Marcus and Lengemann (1962) demonstrated an equivalent proportionalabsorption for strontium and calcium in various segments of the intestine thatrepresented 62 and 86%, respectively, for liquid and solid forms of calcium. Relativeefficiencies of calcium absorption from stomach, duodenum, jejunum and ileumwere estimated to be 0, 15, 23 and 62%, respectively, from a liquid diet.Cramer and Copp (1959) also reported regional intestinal intralumenal calciumconcentrations in rats fed a solid diet to be 2.4, 2.0 and 8.1 mMfor duodenum, distaljejunum and distal ileum, respectively. Moreover, the level of calcium intake willpotentially influence the preferential route of absorption as evidenced by the resultsof Pansu et al. (1981), which showed increasing calcium intake could result in anincrease in the transcellular and a decrease in the paracellular absorption calcium.Therefore, part of the redistribution of calcium absorption is explained by the downregulationof intestinal calcium-binding protein when calcium intake is high or whenabsorption is enhanced by a co-nutrient such as lactose (Pansu et al. 1979). Whetherthis is the case for bioactive caseinophosphopeptides derived from milk protein hasnot been determined.Finally, it is well recognized that mineral solubility is a primary prerequisite foroptimal bioavailability (Ait-Oukhatar et al. 1997; Duflos et al. 1995). For calciumto be absorbed efficiently it must be present in a free and soluble form in the luminalcontents of the gastrointestinal tract. Physicochemical factors influencing the degreeof absorbability of dietary calcium or calcium supplements include the solubility ofcalcium in the gastric acid and the intestinal luminal contents, as well as the chelationor complexing of calcium by reactive anions in the distal part of the small intestine.Thus, foods that enhance gastrin and gastric acid secretion, such as high-qualityprotein sources, could contribute in part to a modification in calcium absorption. Incontrast, foods that contain reactive substrates, such as oxalate in spinach or phytatesin bran products, may reduce calcium bioavailability. The studies of Heaney and coworkers(1989) provide clear evidence of a meal effect on calcium bioavailability.This concept was based on the hypothesis that the co-ingestion of a meal withcalcium from milk or from calcium supplements in the form of pills will reduce therate of gastric emptying and increase gastric acid secretion. The result is a slowerrelease of calcium into the intestinal tract in a form that is optimal for absorptionand exposure to absorption sites located on the intestinal mucosa (Duflos et al. 1995).In relative terms, the large intestine has a minor role in calcium absorption inthe rat (Urban et al. 1980) and humans (Sandstrom et al. 1986). Certain evidenceshows that colonic fermentation of soluble fibers producing short-chain fatty acids(SCFA), namely, acetate and propionate, can facilitate a nonsaturable diffusion ofcalcium ions. This occurs from the release of hydrogen ions derived from the uptakeof SCFA, with low pKa values, that enhance the production of hydrogen ion requiredfor calcium absorption (Lutz and Scharrer 1991). It is doubtful that products of milkproteins would have a direct role in facilitating calcium absorption at this site of theintestine.Numerous experimental approaches have been used to assess or compare thecalcium absorption efficiencies of different dairy and food products. These methodsvary fromin situmethods that involve infusion of radiotracers into isolated segmentsof the intestine to whole-animal calcium balance studies involving longer-termstudies conducted under steady-state conditions. Various researchers have alsoattempted to relate the relative bioavailability of different calcium preparations infoods with bone mass and bone mechanical activity (Yamaguchi et al. 1998; Yuanand Kitts 1994). It is important to recognize the advantages and limitations ofdifferent methods used to assess calcium bioavailability before drawing specificconclusions or comparisons.

Date: 2015-12-11; view: 1136