Health aspects of thermoxidized oils and fats

Early studies used extremely overheated fats which were toxic if fed to animals. Later fats and oils were heated in equipments for deep-fat frying under the conditions of good commercial practice. Such heated fats showed no detrimental effects even if fed in long-term feeding trials. The results are reported and critically evaluated. However, certain fractions of the heated fats, e.g. the so called “Total Polar Materials” caused clearly noticeable effects in the feeding studies: growth retardation, increased liver and kidney weights, and disorders of the enzyme system but only if fed in high doses. In the last years several research groups specialized in the use of cell cultures and enzymological methods and gained insight into resorption and metabolism. Model compounds or compound mixtures have been synthesized. Their structure and/or composition deviated more or less from the genuine compounds or fractions of the heated fats and the results of these studies must not be generalized. New analytical procedures allow to isolate well-defined fractions, and quantitative data about their contents in heated fats are obtained: (a) harmless mono- and diglycerides and free fatty acids, (b) dimeric and polymeric triglycerides and dimeric fatty acids, harmless as well because of very low resorption rates and (c) oxidized triglyceride monomers. Their saponification and degradation products are oxidized monomeric and dimeric acids, oxidized cyclic fatty acids, and other polar compounds. These low molecular oxidized compounds are nutritionally suspect. Since 60 years and with considerable effort more or less systematic investigations have been performed in many countries to answer the question, if heated fats are detrimental to health.