Introduction:
Several studies have reported the antioxidant,
anti-inflammatory, anticancer, hypocholesterolemic and neuroprotective
properties of tocotrienols in different cell lines, animal models, and in
humans . However, question on the bioavailability of pure tocotrienols remained
unanswered. Therefore, it is important to understand the absorption and
bioavailability mechanism of tocotrienols before carrying out investigations
into the therapeutic efficacy in humans. The bioavailability of
naturally-occurring tocotrienols differ considerably in their absorption,
therfore therapeutic uses of tocotrienols remain controversial. It was reported
that after feeding rats mixed tocotrienols, the oral bioavailability of
α-tocotrienol was 28% compared to 9% of γ-tocotrienol and δ-tocotrienol.
Tocotrienols in humans were detected in postprandial plasma , and they were
found enriched in triacylglycerol-rich particles, HDL, and LDL after
administration of palm tocotrienol-rich fraction (mixture of 68% tocotrienol +
32% α-tocopherol). The key parameter of bioavailability determination, the
total area under the concentration-time curve (AUC0
- ∞, h) for plasma α-tocotrienol, was 60% larger than for γ-tocotrieno.
It was also reported that the bio-discrimination
of α-tocopherol (vitamin E) influences the rate of tocotrienol absorption, due
to high affinity of α-tocopherol with “α-tocopherol transfer protein” (α-TTP),
which mediates secretion of α-tocopherol (100%) from the liver into the
circulatory system, and is much higher than α-tocotrienol (12%) or other
tocotrienols. Moreover, α-tocopherol has been reported to attenuate the cholesterol-lowering
effect of tocotrienols through activation of the HMG-CoA reductase activity
(whereas tocotrienols have a desirable inhibiting effect on its activity). Also
α-tocopherol interferences with tocotrienol functions such as attenuation of
cancer inhibition , exacerbation of stroke injury, inhibition of absorption,
and induction of tocotrienol catabolism. 
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