Summary and ConclusionsA male pig was reared for three months on a diet containing about 1% fat and 0.2% dienoic acid. The back fat was fractionated into glyceride types by crystallization methods, and the fatty acid composition of each was determined by the spectrophotometric procedure. The relative amounts of the glyceride types almost perfectly fit the hypothesis that there is an oleic acid in the 1 position of each glyceride molecule, that the residual oleic acids are in the 3 position, that the linoleic acid is in the 2 position of the resultant dioleins, and that the saturated acids complete the structure.The GS2U and GSU2 fractions were hydrolyzed by pancreatic lipase. The iodine values of the resultant monoglycerides and free fatty acids indicated that the unsaturated acids are predominantly in the 1 and 3 positions and the saturated in the 2 position. From these data and from previous knowledge of the structure of liver lecithin and of the mechanism of lecithin and triglyceride synthesis, which are discussed, it is postulated that the structure of endogenous animal fat is a resultant of the specificity of the acylating enzyme and of the nature and relative proportions of available fatty acids. The possible influence on depot fat structure of exogenous fat, of the dynamic state of the glycerides, and of the homeostatic mechanism are discussed.Linoleic acid, when fed at a low level, was found to be preferentially deposited and not utilized for energy purposes.