Long-term Rat Feeding Study with Used Frying Fats

A study was made to learn whether fats which had been exposed to the heat and aeration of actual frying differ significantly from fresh fats in their nutritional properties. Partially hydrogenated soybean oils, cottonseed oil, and lard were used for frying under practical restaurant-type frying conditions until they became unfit for further use owing to excessive foaming during frying. The used fats were fed to groups of 50 male and 50 female rats as 15% of the diet for 2 years. The used fats were slightly less absorbable than unheated control fats, and gave correspondingly slower growth rates. Other than this there were no differences in clinical, metabolic, or pathological criteria to suggest that the used fats adversely affected the rats consuming them. Mortality among the heated-fat groups was no higher than among the control groups. Distillable non-urea-adductable fractions concentrated from the used fats proved somewhat toxic when large doses of them were administered by stomach tube to weanling rats. The results indicate that, although heating of fats under actual frying conditions does cause the formation of substances which can be shown to be toxic, the level of such substances and the degree of their toxicity are so low as to have no practical dietary significance.