A nutrient's bioavailability is the proportion of the nutrient that, when ingested, actually gets absorbed by the body. The remaining amount cannot be metabolized and is removed as waste. The ability to absorb nutrients varies by gender, disease state, and physiologic condition (e.g., pregnancy, aging). The bioavailability of a nutrient can also increase or decrease if other substances are present. For example, calcium and magnesium lose much of their effectiveness if taken with fatty foods. The intestines themselves may also regulate the amount of a mineral that enters the bloodstream. For these reasons, taking high-potency vitamin supplements does not guarantee that all of the included nutrients will enter one's system.
"Nutrients Are Team Players." University of California Wellness Letter 20(7):5.
British Nutrition Foundation. "Minerals." Available from <http://www.nutrition.org.uk>