Bioavailability and interaction of vitamin A and vitamin E in ruminants.

Recent interest in antioxidant vitamins and animal nutrition has resulted in the investigation of feeding levels of vitamin E which are considerably higher than NRC requirements. Relatively high levels of vitamin E are required to improve animal product quality such as extending beef color stability and minimizing off-flavors in milk due to lipid oxidation. 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 most animals studied. In the dairy cow, 675,000 IU of vitamin A acetate per head per day is required to significantly depress vitamin E utilization. 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. These various forms give rise to isomer differences, ester differences and formulation differences that can affect their absorption and subsequent utilization. The current research indicates that the commonly used standard activity values based on a value of 1.00 IU per mg of all-rac-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 bioavailability of vitamin E supplements.