The objective of this work was to determine the apparent bioavailability of magnesium from four different types of calcined magnesites of Greek origin. These four types (A, B, C, D) were different in particle size fractions and production process. The solubility of the above four magnesium samples and their fractions were determined: (a) in the laboratory using ammonium nitrate solution after 24-h shaking and (b) in vivo using a nylon bag technique after 24-h incubation.
Solubility was correlated with apparent bioavailability as determined in a balance experiment with five wether sheep in a 5×5 Latin-square design.
Sheep were kept in metabolism cages and were given daily a basal diet (500 g hay + 700 g concentrate) plus 2 g magnesium supplements from types A to D. The magnesium bioavailability of the basal diet was used as a base for the determination of the bioavailability of magnesium in A, B, C and D supplements.
Magnesium apparent bioavailability of the basal diet was found to be 0·203 and, for the supplements A, B, C and D, 0·381, 0·339, 0·315 and 0·292 respectively. The solubility of supplements A, B, C and D in ammonium nitrate solution and in vivo (using the nylon-bag technique) was found to be 0·812, 0·805, 0·784 and 0·773 or 0·294, 0·152, 0·102 and 0·200 respectively.
The solubilities of the magnesium supplements in ammonium nitrate solution and in vivo both reflected dietary bioavailability differences up to a point, but neither was well enough correlated with the apparent bioavailability as determined by the balance experiment, this being judged to be the most precise method for bioavailability determination.