Several factors influence the bioavailability of a nutrient. These include:
Distribution of the nutrient by the circulating blood, and
Entry of the nutrient into the specific body tissues and fluids in which it may be physiologically effective.
Bioavailability may be quantified by measuring
The amounts of the nutrient in various body tissues and fluids or
The growth or enzyme activity that depends on the particular nutrient.
A nutrient is rarely stored in a single body tissue, and therefore determining the nutrient levels in single tissues may not accurately reflect the true bioavailability. For example, levels of nutrients in blood, which is an accessible tissue for measurement purposes, may not reflect the levels in other tissues that are the major stores, such as liver.