Recent interest in antioxidant vitamins and animal nutrition has resul
ted in the investigation of feeding levels of vitamin E which are cons
iderably higher than NRC requirements. Relatively high levels of vitam
in E are required to improve animal product quality such as extending
beef color stability and minimizing off-flavors in milk due to lipid o
xidation. Concerns regarding a negative effect of vitamin A on vitamin
E utilization and the suitability of currently used standard activity
values for vitamin E supplements for ruminants have been raised. High
dietary levels of vitamin A have depressed vitamin E utilization in m
ost animals studied. In the dairy cow, 675,000 IU of vitamin A acetate
per head per day is required to significantly depress vitamin E utili
zation. This is approximately 10-fold greater than the highest levels
currently fed to dairy or beef cattle and therefore should not cause a
practical problem. Synthetic and naturally derived alpha-tocopherol,
and their ester forms, are commonly used as vitamin E supplements. The
se various forms give rise to isomer differences, ester differences an
d formulation differences that can affect their absorption and subsequ
ent utilization. The current research indicates that the commonly used
standard activity values based on a value of 1.00 IU per mg of all-ra
c-alpha-tocopheryl acetate are probably too low for the ruminant. The
roles of isomeric forms and gastrointestinal tract absorption have not
been completely resolved regarding their effects on the bioavailabili
ty of vitamin E supplements.