Chemistry Reference
In-Depth Information
group stabilizes free radicals. The presence of at least one methyl group on the
aromatic ring is also critical. Vitamin E biological activity is defined in terms
of
-tocopherol equivalents (
-TE) whenever possible. RRR-
-tocopherol
has an activity of 1 mg
-tocopherol equivalent (
-TE/mg compound). The
activities of RRR-
, RRR-
and RRR-
-tocopherol are 0.5, 0.1 and 0.03,
respectively. Synthetic all-rac-
-tocopheryl acetate has an activity of 0.74 mg
-TE/mg. Of the tocotrienols, only
-tocotrienol has significant biological
activity (0.3 mg
-TE/mg). Lengthening or shortening the side chain results in
a progressive loss of vitamin E activity (Ingold et al., 1990).
In the determination of vitamin E, solid-phase extraction (Luque-
Garcia and Luque de Castro, 2001) and supercritical fluid extraction (Turner
and Mathiasson, 2000; Turner et al., 2001) are now used extensively for
sample extraction and clean-up. Methods used for the determination and
quantitation of vitamin E include normal-phase high-performance liquid
chromatography (NP-HPLC), reversed-phase high-performance liquid chro-
matography (RP-HPLC), gas chromatography (GC) and supercritical fluid
chromatography (SFC) (Pyka and Sliwiok, 2001; Ruperez et al., 2001; Turner
et al., 2001). Detection of fat-soluble vitamins after HPLC resolution can be
accomplished by UV (using diode array detection), fluorescence (FLD),
electrochemical (ED) or evaporative light scattering (ELSD) detection meth-
ods (Ruperez et al., 2001). The most commonly used detector for vitamin E
analysis is FLD, which is considerably more sensitive and selective than UV,
but less sensitive than ED.
A number of analytical methods have been described for the determina-
tion of vitamin E in milk and milk-based infant formulae, either alone
(Rodrigo et al., 2002; Romeu-Nadal et al., 2006a) or simultaneously with
other fat-soluble vitamins (Turner and Mathiasson, 2000; Rodas Mendoza
et al., 2003; Heudi et al., 2004; Chavez-Servin et al., 2006).
12.4.2.
Absorption, Metabolism and Excretion
Following ingestion, fats are emulsified into smaller particles, first in
the stomach and then in the small intestine, where they are mixed with
pancreatic and bilary secretions. Pancreatic esterases convert triglycerides
to monoglycerides and free fatty acids, and together with bile acids, these
products form micelles into which vitamin E and other hydrophobic mole-
cules become solubilized (Traber and Sies, 1996). Vitamin E is absorbed in the
proximal part of the small intestine, where transport across the brush border
is thought to occur by passive diffusion (Traber and Sies, 1996). There are no
selective differences in absorption between
-and
-tocopherols (Jiang et al.,
2001), but
- and
-tocopherols are absorbed poorly and are excreted in the
faeces. Together with triglycerides, phospholipids and apolipoproteins, the
