Effect of ratio of unsaturated to saturated fatty acids of the dietary lipid fraction on utilization and metabolizable energy of added fats in young chicks.

Relationships between the ratio of unsaturated to saturated fatty acids (U:S) in the diet and the utilization and AME(n) of added fat were studied using the results of a number of previously reported experiments. Mathematical equations relating fat AME(n), fat utilization, and palmitic, oleic, and linoleic acid utilization to the U:S were calculated for young broiler chicks. Best fit regression equations for added fat utilization and fat AME(n) were exponential, showing fat utilization increased very steeply in the U:S range of 0 to 2.5, reaching a near asymptotical maximum at a U:S of 4 or more. Synergism between added fats, due to blending vegetable oils with animal fats or using basal diets with unsaturated lipid fractions, led to increased animal fat utilization values. The utilization of saturated fatty acids was especially affected by synergism, whereas utilization of unsaturated fatty acids was not influenced by changing U:S. The origin of effects of factors influencing fat utilization such as level of fat inclusion and basal diet composition, appeared to be through variation in degree of saturation of the total dietary lipid fraction. For young broilers, about 75% of the variation in fat utilization and AME(n) was due to differences in the chemical composition of the fat fraction.