10 - Magnesium bioavailability

Abstract

Publisher Summary Magnesium has been recognized as an essential dietary element for about 70 years, and the element's role in hypomagnesemic tetany, also known as grass tetany, has been known since the early 1930s. The solubility of magnesium compounds in various solutions has been used by several researchers to predict bioavailability of supplemental sources. The solubility of magnesium from forages has been examined by feeding ruminally fistulated animals various forages and collecting ruminal fluid for magnesium analysis or by measuring the disappearance of magnesium from forages incubated in dacron bags in the rumen. Generally, urinary magnesium concentrations are considered to provide more accurate information than serum magnesium concerning the element's status in animals. Several factors have been found that influence the absorption and, consequently, bioavailability, of magnesium. It is generally accepted that absorption of magnesium from the intestinal tract and tubular reabsorption of magnesium in the kidney are lower in older animals than in younger animals. Availability of magnesium from tall fescue and fescue-red clover herbage was greater in mature wethers than dry cows, with weaned calves being intermediate. Standard magnesium sources used in comparative assay studies have generally been either magnesium sulfate or magnesium oxide with some use of magnesium phosphate. Magnesium sulfate is a highly soluble, uniform product and is considered the standard choice. Magnesium oxides, especially feed grade products, are considerably more variable in parent material, processing, solubility, and in their absorption by animals. Only limited testing has been conducted with magnesium phosphate.