Samples of " used " fats were obtained from restaurants, bakeries, potato chip or doughnut fryers and similar sources; with several of the 34 samples, samples of unused fat were taken also. The fats were included at 20% in a diet which allowed good growth of weanling rats when unused fats were given. No differences in growth, food intake and post mortem appearance of liver and kidneys were found when the " used " fats were given. In contrast, fats heated at 182°C for 120 h in the laboratory consistently caused initial depression of growth, reduced feed efficiency and caused significant enlargement of liver and kidneys although the rats appeared healthy. The availability of the energy in commercially-used fats was unimpaired, but on average fats heated in the laboratory retained only 73% of their original energy value. The extent of heating of the fats in the laboratory appeared to be similar to that of some of the commercial samples, but the latter differed in the introduction of foods, stirring and occasional or continual addition of fresh fat. D. Duncan.
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