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Processing and Bioavailability (WG2) page 55
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necessary for transfer to the micellar phase (Borel et al., 1996). Little information is currently
available as to the solubilisation capacity of mixed micelles for lycopene. The structures of
the micellar phases are generally closed packed and hence inclusion of a substrate is
geometrically constrained. Thus the solubility of hydroxy carotenoids will differ to those of
the hydrocarbon carotenoids. Initial results from in vitro digestive systems (Garrett et al.,
1999) suggest the ease of solubilisation of selected carotenoids can be ranked in the following
manner: lutein greater than β-carotene greater than lycopene. From this short discussion it can
be recognised that the digestion and absorption of the carotenoids is a multistage and
multiphase process. The relative ease of dissolution in each phase will be determined by the
structure and physicochemical properties of the carotenoid in question, the food matrix it is
incorporated within and the other components of the meal, principally the composition and
level of lipid. These differences in physical properties and their preferred lipid domains will
also control the possible transfer of carotenoids between lipid structures both in the gut lumen
and post absorption.
Carotenoids are passively absorbed from the micellar phase (Parker, 1997). However, it is
not known if all the carotenoids present in a mixed micelle is absorbed, or whether some is
left behind in association with unabsorbed bile salts and cholesterol, perhaps to be absorbed
more distally or lost to the large intestine. Factors that increase the thickness of the unstirred
layer on the surface of the gut, for example soluble dietary fibre, act as a barrier to the
absorption of dietary fats and may, therefore, also inhibit the absorption of carotenoids (Gee
et al., 1983, Rock and Swendseid, 1992). Disease states which impair lipid absorption, for
example cystic fibrosis and coeliac disease, also lead to low plasma carotenoid levels,
although in some cases persistent inflammation may be a significant factor in reducing plasma
levels of carotenoids (Homnick et al., 1993). The mass transfer of the carotenoids from
digesta to absorbable species is clearly a limiting step in the bioavailability of carotenoids on
the basis that free carotenoids given orally either as supplements, or as oil solution, or
suspension are much better absorbed (Faulks et al., 1997) than those from foods and the
evidence that homogenisation of the food and heat treatment enhance absorption.
Various types of dietary fibre were found to reduce the bioavailability of carotenoids in
foods (Erdman et al., 1986). Matrix effects were proposed as an explanation for the lack of
improvement in vitamin A status in Indonesian women fed green leaf vegetables compared
with a manufactured wafer containing a similar amount of carotene in oil solution (De Pee et
al., 1995). Rock and Swendseid (1992) tested the inhibitory effect of pectin, a typical dietary
fibre, and their results showed that this type of dietary fibre affected the absorption of dietary