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Processing and Bioavailability (WG2) page 57
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Absorption: methods of measurement
Whilst the emphasis in the concerted action ‘Role and control of anti-oxidants in the
tomato processing industry, FAIR CT 97-3233’ is on the ‘tomato’ carotenoid lycopene, data
on the bioavailability of this carotenoid are very limited. This chapter is therefore dependant
on examples related to beta-carotene, the most widely studied carotenoid. The points raised
with regard to the importance of understanding factors affecting bioavailability, and methods
for estimating carotenoid absorption are, however, applicable to the study of lycopene.
Various approaches have been used to assess carotenoid absorption and in each case
assumptions are made in order to estimate values, either absolute or comparative. The sections
below describe the most widely used approaches for assessing carotenoid absorption. It
should be remembered, however, that whilst absorption is a major element of carotenoid
bioavailability, it is only part of the story; it take no account of their metabolic fate.
Metabolic balance techniques
Metabolic balance studies are commonly used to measure the absorption of minerals
because they are still measurable as minerals even if they do undergo changes in oxidation
state, or are associated with other molecular species during processing or passage through the
GI tract. Such studies are normally carried out over a period (5-8 days) known as the balance
period.
Balance studies measure the difference between intake of a compound and its excretion.
This may involve several meals given over several days (chronic dosing) or a single meal
given after an overnight fast (acute dose).
The ‘chronic’ method is dependent on establishing an equilibrium between intake and
excretion and requires faecal markers at the beginning and end of the balance period to ensure
that all relevant faecal samples are collected. Because absorption is the difference between
intake and excretion, both values need to be determined with great accuracy. The use of food
tables is insufficient and foods and or supplements need to be assayed accurately.
If the method is applied to measuring absorption of a carotenoid from a single acute dose,
the diet needs to be carotenoid free for 5 days before and during the test period. Faecal
collections need to be continued until no further carotenoid from the test dose is lost in the
faeces. The collection period is usually 3-5 days giving a total study period of 6-10 days