Calcium solubility bioavailability

The in vitro estimate of potential availability was defined, somewhat arbitrarily, as calcium solubility (18,000 x g supernatant) after complete digestion. Potentially available calcium was expressed as a percentage of the total food calcium (Figure 1). With the exception of a low in vitro calcium solubility value for whole milk, our earlier data compared reasonably well with calcium bioavailability information in the literature (7.) ... 

The in vitro procedure was tested in "critical" experiments designed to make direct comparisons of in vivo and in vitro estimates of exchangeability and potential bioavailability and to test the use of in vitro exchangeability values in in vivo experiments. (8). Three foods which were expected to show different levels of calcium solubility and exchangeability, collards, soybeans and spinach, were intrinsically labeled with 45Ca in nutrient solution culture. They were used together with 47 Ca as an extrinsic label in both in vitro and in vivo experiments. 

Experiments were conducted to determine if varying the conditions in the in vitro digestion procedure would affect post-digestion calcium solubility and in some cases, exchangeability. This was done with two purposes to test the use of the in vitro digestion procedure for studying factors which might influence calcium bioavailability and to use the results to modify the standard procedure. 

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. 

Lead is toxic to all phyla of aquatic biota, but its toxic action is modified by species and physiological state, and by physical and chemical variables. Wong et al. (1978) stated that only soluble waterborne lead is toxic to aquatic biota, and that free cationic forms are more toxic than complexed forms. The biocidal properties of soluble lead are also modified significantly by water hardness as hardness increases, lead becomes less bioavailable because of precipitation increases (NRCC 1973). In salmonids, for example, the toxicity and fate of lead are influenced by the calcium status of the organism, and this relationship may account for the reduced effects of lead in hard or estuarine waters. In coho salmon (Oncorhynchus kisutch), an increase in waterborne or dietary calcium reduced uptake and retention of lead in skin and skeleton (Varanasi and Gmur 1978). 

Examples of the use of soluble salts to increase drag absorption include novobiocin, in which the bioavailability of the sodium salt of the drag is twice that of the calcium salt and 50 times that of the free acid. 

In soils, F can be found in four major fractions (1) dissolved in soil solution (2) sorbed to Al, Fe, and Mn oxides and hydroxides and carbonates (3) solid phases, such as fluorite and fluorophlogopite and (4) associated with organic compounds. The solubility of F in soil solution is variable and is affected by pH, speciation, adsorption and desorption reactions, and dissolution and precipitation reactions (Luther et al., 1996). Acidic conditions and low calcium carbonate content are favorable to F solubility and can therefore enhance both root uptake (Weinstein and Alscher-Herman, 1982) and migration to surface and ground water (Smith, 1983). These conditions can lead to human, plant, and animal health issues. Soils that do contain appreciable amounts of calcium carbonate and are neutral to slightly alkaline conditions can fix F as insoluble calcium fluoride (CaF2), and reduce its bioavailability and mobility (Kubota et al., 1982 Tracy et al., 1984 Reddy et al., 1993 Poulsen and Dudas, 1998). 

All other oxalates are sparingly soluble in water which would dramatically effect the bioavailability of the metal ions involved. Table III illustrates both the solubility and stability constants for a few selected metals (35.). It may be seen that with calcium it forms a practically insoluble salt at neutral or alkaline pH, being soluble to the extent of 0.67 mg per 100 ml of water at pH 7.0 sind 13°C. Zinc also has limited solubility (0.79 mg/100 ml, 18°) while Fe+2 and Fe+3 show solubilities of 22.0 ml/100 ml and "very soluble" respectively. These chemical facts and their effedt on iron absorption have recently been substantiated in a biological sense by Van Campen and Welch (36) who investigated the availability to rats of iron from two varieties of spinach. Also they compared the absorption of iron between FeClj and Fe-oxalate as well as the effects of adding 0.75% oxalate to the diet. They found that absorption of iron from both varieties of spinach was comparable to that from FeClj and that the iron was equally avail-... 

The solubility of monoferric phytate in water likely explains the high bioavailability of the iron from monoferric compared to iron from the relatively insoluble di or tetraferric phytates. Possibly the ferric phytates used by Moore et al. (4) for humans and by Bremner and Dalgarno (20) for calves were mixtures of all possible ferric phytates, but were predominately insoluble forms and the iron was poorly bioavailable. We found the iron of an insoluble calcium ferric phytate product to be of low bioavailability. Sharpe et al. (21) added sodium phytate and ferric chloride labeled with radioiron to milk and found that the radioiron was poorly absorbed by adolescent boys. Possibly insoluble calcium ferric phytate formed in that experiment. 

The oral bioavailability of crystalline nifedipine, a potent calcium-channel antagonist, is very low because of its poor solubility and its slow dissolution rate in water. Various hydrophilic macromolecules such as poly(vinylpyrrolidone)... 

In sulfate-dominated wetlands, production of sulfide (through biological reduction of sulfate) and formation of ferrous sulfides may preclude phosphorus retention by ferrous iron in regulating phosphorus bioavailability (Caraco et al., 1991). In iron- and calcium-dominated systems, Moore and Reddy (1994) observed that iron oxides likely control the behavior of inorganic phosphorus under aerobic conditions, whereas calcium phosphate mineral precipitation governs the solubility under anaerobic conditions. This difference is in part due to a decrease in pH under aerobic conditions as a result of oxidation of ferrous iron compounds, whereas an increase in pH occurs under anaerobic conditions as a result of reduction of ferric iron compounds. The juxtaposition of aerobic and anaerobic interfaces promotes oxidation-reduction of iron and its regulation of phosphorus solubility. 

Although the lipid solubility of sotalol is relatively low compared with other jS-blocking adrenoceptor drugs (28), oral bioavailability is deemed to be 1(X)%. Sotalol is absorbed somewhat slower than most other j8-blockers, with peak concentrations occuring within 2-3 hours (29). Although food may impair the absorption of sotalol (28), administration of either calcium carbonate or aluminum hydroxide antacids has little effect on absorption (30). After administration of a single 160 mg oral dose of sotalol, both enantiomers reached maximal plasma concentrations in approximately 3 hours (31) and, hence, did not exhibit stereoselective absorption. 

---