MAG365 Magnesium supplement is a powder with a highly bioavailable active ingredient magnesium citrate. All natural ingredients and no fillers. Magnesium is foundational to good health and over 600 body processes.absorbable, effective, fast acting magnesium available anywhere. It is 100% natural magnesium and helps to dissolve excess calcium in the body. Mag365 is a powder made up of two ingredients; magnesium carbonate and citric acid. When you add hot water, the two combine and create liquid magnesium citrate, a highly bioavailable magnesium supplement.