Oral Bioavailability - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 290

Bound fraction Solubilized fraction Absorbed fraction

Intestinal
epithelium

Soil particles

Distribution
Metabolism
Excretion
Toxicity

Systemic
circulation

Liver

Metabolites
Bile excretion

Bioaccessible fraction Metabolized Bioavailable fraction

fraction

Bioavailable fraction

Fig. 7.2 Steps involved in oral bioavailability

F Bacc × F Abs × Fn Met = F Bava (7.2)

where:

F Bava is the bioavailable fraction;
F Bacc is the fraction of ingested contaminant that is bioaccessible;
F Abs is the fraction of accessible contaminant that is absorbed;
Fn Met is the fraction of absorbed contaminant that is not metabolised in liver
(

=

1-fraction metabolised).

7.1.1.1 Accessibility

Contaminants that are released from soil particles within the gastrointestinal tract
are considered to be bioaccessible . For a contaminant in soil to be bioaccessible to
a human via ingestion, it must be released from the soil into solution in the gas-
trointestinal tract in a form that can be absorbed into the blood stream. Conditions
in the stomach, such as pH and residence time, may vary, depending on whether
the individual is in a fed or fasted state. It is generally considered, when simulating
human gastro-intestinal conditions, that a fasted state is likely to mobilise the high-
est amount of metals (Maddaloni et al. 1998 ; Van de Wiele et al. 2007 ). The fasted
state is, therefore, considered to be the most conservative for inorganics. Conversely,
for organic contaminants, the opposite is considered to be conservative, i.e. the fed
state, for the reasons discussed in Section 7.1.1.2 .