Chemical form of calcium in wheat, spinach and kale and absorption pathway of calcium oxalate
Abstract
Spinach, wheat and kale represent a broad range in bioavailability of calcium from foods (5-40%). Chemical properties of the calcium in these foods were investigated to explain differences in bioavailability. These plants were intrinsically labeled with $\sp{45}$Ca and examined for $\sp{45}$Ca solubility characteristics, including specific enzyme treatments, subcellular distribution, and in vitro bioavailability. Solubility was $<$2% for spinach, approximately 40% for wheat and 70% for kale, depending on the solvent. Solvents for solubility determination were ${\rm H\sb2O},$ 0.04M NaCl, 70% ethanol and 0.1M Acetic Acid. Solubility paralleled in vitro ${\rm\sp{45}Ca}$ bioavailability estimates of 0.3 $\pm$ 0.1%, 26.4 $\pm$ 5.2%, and 76.3 $\pm$ 1.2% for these three plant foods. Spinach calcium exists largely as calcium oxalate which is not easily dissociated. Phytase treatment of wheat indicated that a major portion of calcium was bound to phytate. Calcium in kale likely exists as a soluble organic salt such as calcium citrate or malate. The intestinal absorption pathway and plasma decay of calcium oxalate, the chemical form of calcium in spinach, was then elucidated in the whole body awake rat. Suspensions of ${\rm \sp{45}Ca}$-(${\rm\sp{14}C})$-oxalate and solutions of its respective labeled ions were given to rats by gavage to study their absorption mechanism, or intravenously to study their plasma decay. Following dosing, blood aliquots were drawn over time through a previously inserted intravenous catheter, and the serum assayed for radioactivity. The data was analyzed through kinetic modeling using the NIH program SAAM. Calcium oxalate was found to be absorbed intact via the paracellular route in the whole body rat. Addition of excess cold calcium to the test load confirmed absorption of the calcium as the intact salt. Knowledge of the chemical form of calcium in foods and the mode of its absorption is important in designing new processing procedures to improve calcium absorption, in food fortification, and in the development of calcium supplements and therapeutic agents.
Degree
Ph.D.
Advisors
Weaver, Purdue University.
Subject Area
Nutrition|Agricultural chemicals
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