1. The behaviour of fatty acids in dilute bile salt solution and in dispersions of triglyceride in bile salt solution was examined. The properties of fatty acids in bile salt solution were defined in terms of their saturation ratio, and of the critical micellar concentration of bile salt for each fatty acid as solute. The partition of fatty acids between the oil phase and the micellar phase of the dispersions was defined as the distribution coefficient K M/O. The phases were separated by ultracentrifugation.2. Of the fatty acids examined, palmitic and stearic acids behaved in bile salt solution as typical non-polar solutes. Lauric, oleic and linoleic acids had properties similar to typical amphiphiles. The effectiveness of these and other amphiphiles was expressed in terms of an amphiphilic index.3. The trans-fatty acids, vaccenic acid and linolelaidic acid possessed solubility properties similar to their cis-isomers. The properties of elaidic acid were intermediate between those of the non-polar and the amphiphilic solutes.4. The distribution coefficients of fatty acids differed less significantly than their solubilities in bile salt solution, but were influenced to some extent by the composition of the oil phase.5. The effect of pH on the partition of fatty acids was similar for all fatty acids examined. There was a marked increase in the partition of fatty acids in the micellar phase as pH increased above 6.2.6. In the presence of Ca2+ ions above a threshold concentration of I m-equiv./l. and at a pH higher than 6.4, the partition of stearic acid in the micellar phase of the bi-phasic system was restricted. No interaction of Ca2+ ion and pH was observed with oleic acid.7. The effect of some amphiphiles on the distribution coefficient of stearic acid was examined. The capacity of lysolecithin to solubilize stearic acid in the micellar phase of dispersions was about twice that of I-mono-olein. The significance of the results in relation to fat absorption in the pig and the sheep is discussed.
I. The true digestibilities of esterified lauric, palmitic, stearic and oleic acids in lard, coconut oil and soya-bean oil were determined by incorporating the [I-"C]-labelled acids into the fats by interestification. The true digestibilities of the labelled fats also were compared with the digestibilities of their unlabelled counterparts, corrected for endogenous fat by the conventional procedure.2. The true and corrected coefficients of digestibility of each of the fats were in good agreement, but there were significant differences between the true and corrected coefficients for individual fatty acids, particularly for palmitic and stearic acids.3. The amount of endogenous fat produced with each of the fats calculated from the values found with the labelled fatty acids was greatest when the fat supplement was lard, when a disproportionately high level of palmitic acid was excreted.
4.Evidence was obtained of the extensive hydrogenation of oleic acid in the digestive tract of the pig.
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