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A model system for assessing physicochemical factors affecting calcium absorbability from the intestinal tract
C Y Pak1, J Poindexter, B Finlayson
1Center in Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical School, Dallas.
Summary
Calcium salt solubility and absorption in the jejunum depend on stomach acid levels. Calcium citrate shows higher solubility and less reprecipitation than calcium carbonate or phosphate, influencing absorbable calcium.
Area of Science:
- Gastroenterology and Nutritional Science
- In Vitro Digestion Models
- Calcium Metabolism
Background:
- The proximal jejunum is a key site for calcium absorption.
- Physicochemical factors, including stomach acid and pancreatic secretions, influence calcium salt solubility and complexation.
- Different calcium salts (tricalcium dicitrate, calcium carbonate, tricalcium diphosphate) exhibit varying dissolution and absorption characteristics.
Purpose of the Study:
- To investigate the in vitro solubility of three calcium salts under varying simulated gastric acid conditions.
- To assess the impact of simulated pancreatic bicarbonate secretion on calcium salt reprecipitation.
- To determine the extent of ionic versus complexed calcium in the proximal jejunum model.
Main Methods:
- An in vitro model simulating proximal jejunum conditions was used.
- Solubility of calcium salts (500 mg calcium) was tested in varying hydrochloric acid concentrations (0–24.2 mEq).
- Filtrates were titrated to pH 5, 6, and 7 to simulate pancreatic bicarbonate secretion, followed by reprecipitation analysis and calcium complexation calculations.
Main Results:
- Calcium citrate demonstrated significantly higher solubility than calcium carbonate and calcium phosphate, especially in low-acid conditions.
- Substantial reprecipitation of calcium phosphate occurred at higher pH levels, particularly from solutions with high initial hydrochloric acid content.
- Calcium citrate formed considerable soluble complexes (60-65%, mainly CaCit-), while calcium carbonate and phosphate showed negligible complexation.
Conclusions:
- Calcium citrate exhibits superior solubility and reduced reprecipitation compared to calcium carbonate and phosphate in simulated jejunal conditions.
- Stomach acid levels critically influence the solubility of calcium salts.
- The formation of soluble calcium complexes, particularly with citrate, affects the amount of ionic, potentially absorbable calcium.