From what I've read, Bioavailability is the degree to which food
nutrients are available for absorption and utilization in the
body. How would you explain this with an example?
Bioavailability is a concept which applies to nutrients and
drugs which pass through first-pass metabolism, i.e. orally (and
to some extent nasally) consumed substances. Anything absorbed
in the gut first passes through the liver before reaching the
rest of the circulation, and both the gut and liver may
metabolise it to some extent. The liver in specific has the
powerful Cytochrome P450 system, a huge variety of enzymes to
break down all sorts of substances, although in some cases it
can actually produce more active or even toxic forms instead of
breaking them down. This can lead to drastic reductions in the
amount available in the systemic circulation after oral
administration. E.g. propanolol (a beta blocker) needs to be
given in 100mg doses orally while intravenously (avoiding first-
pass metabolism) only 5mg are needed.