Magnesium bioavailability from human milk, cow milk, and infant formula in suckling rat pups

Little is known about the absorption of magnesium from infant diets. Magnesium bioavailability was evaluated from infant diets that varied by protein and carbohydrate source; magnesium, calcium, and phosphorus contents; and the form of magnesium fortification. Diets were separated into soluble, insoluble, and fat fractions to determine magnesium distribution. Most of the magnesium (> 62%) was found in the soluble fraction. Gel filtration of the soluble fraction from all diets studied showed that > 95% of magnesium is free or associated with low-molecular-weight compounds. Distribution of 28Mg and native magnesium in fractions of the diets was similar, thus validating the use of an extrinsic label. In vitro digestion decreased the percent insoluble magnesium from as high as 35% to 2-8%. Whole-body retention of 28Mg-labeled diets in suckling rat pups 4 h after oral intubation ranged from 51% to 92%. No significant differences were found between human milk, cow milk, and infant formula. In conclusion, magnesium from the infant diets studied has high bioavailability, and moderate differences in their composition do not affect bioavailability significantly.