Rapid autoxidation of fish oil in diets without added antioxidants.

Feeding of purified diets containing fish oil without added antioxidant leads to rapid autoxidation of the oil and the possibility of artifactual results due to the feeding of autoxidation products. Purified diets containing menhaden oil without any added antioxidant deteriorate quickly. Peroxide value of the diet is elevated 5- to 6-fold within 24 h and 12-fold within 48 h when exposed to air at room temperature. Addition of 0.02% t-butylhydroquinone to the fish oil prevents this deterioration for at least 72 h. Determination of fatty acid composition is not a sensitive indicator of diet integrity. Supplementation of fish oil diets with vitamin E to help protect against in vivo peroxidation is discussed.