oxidants and antioxidants in biology - Oxygen Club of California

Bioavailability of Vitamin E Forms
KLAUS KRAEMER AND UTE OBERMUELLER-JEVIC
Scientific Affairs Human Nutrition, BASF Aktiengesellschaft, Ludwigshafen,
Germany
In vitamin E research, ‘bioavailability’ and ‘biopotency’ have
often been used synonymously creating babbling terminology. For
vitamin E, the rat fetal resorption bioassay is the classical test that
measures biopotency in terms of preventing embryonic death.
However, in humans studies using functional endpoints that reflect
potency, such as hemolysis or lipid peroxidation, are limited for tocopherols
(natural or synthetic) and tocotrienols.
The Food and Nutrition Board (FNB, 2000) proposed changing
the potency ratio of RRR- : all-rac-α-tocopherol for humans
from 1.36 to 2.0. However, a critical re-evaluation of all published
data in humans that compared plasma responses (bioavailability) to
supplemental doses of unlabeled RRR- and all-rac-α-tocopherol
continues to support the presently accepted potency ratio of 1.36.
Recent studies using deuterated tocopherol reveal that the magnitude
of ratio changes with dose. While nutritional doses supported
the 2:1 ratio, supplemental doses resulted in a ratio close to 1.5.
This clearly indicates that no valid conclusion can be drawn from
bioavailability studies on the relative potency of all-rac- versus
RRR-α-tocopherol, even if they employ unlabeled or labeled
forms.
Apart from the proposed change in the α-tocopherol ratio, the
FNB has suggested that only α-tocopherol bears vitamin E activity.
Nevertheless, other vitamin E forms, particularly the most abundant
natural form; γ-tocopherol, exerts vitamin E activity too.
Studies applying more meaningful functional endpoints are urgently
needed to determine the relative potencies of the various vitamin
E forms for humans.
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