The importance of bioavailability |This chart represents one of
the more optimistic views of vitamin C bioavailability. A
National Institutes of Health study suggests that only 200 mg of
a single dose, regardless of size, actually gets into the
bloodstream. Bioavailability, is only half of the picture,
however. The nutrient must also be able to pass into individual
cells.| When all of a nutrient is absorbed into the bloodstream,
as in a direct intravenous injection, the bioavailability is
100%. If only a quarter of an ingested nutrient is absorbed, the
bioavailability is 25%. Of course, bioavailability also depends
on and is limited by other factors such as declining health
status, advancing age, and the presence of other substances that
compete for absorption. The process of uptake from the digestive
system varies greatly from nutrient to nutrient. Vitamin C is
absorbed almost exclusively in the small intestine and requires
the presence of transport proteins. For vitamin C, these
transport proteins are calle