Not All Vitamins Are Created Equal: Why Your Supplements Might Be Failing You

2. The Concept of Bioavailability
Bioavailability refers to how much of a nutrient actually enters your bloodstream and is utilized by your cells. For example, magnesium oxide is cheap but has very low absorption, while magnesium citrate or magnesium glycinate has much higher bioavailability. When buying, you should always look for forms that the body recognizes and absorbs more easily; otherwise, you are throwing your money away.