The objectives of this study are to : 1) determine if vaccenic acid (trans-11-octadecenoic, a naturally occurring trans fatty acid isomer from ruminant animal products) raises LDL cholesterol in a quantitatively comparable manner as mixed trans fatty acid isomers from partially hydrogenated vegetable oils at 3% of energy intake, 2) determine if cis-9, trans-11 conjugated linoleic acid (a naturally occurring polyunsaturated fatty acid from ruminant animal products) raises LDL cholesterol compared to a control diet, and 3) demonstrate that mixed trans fatty acid isomers from partially hydrogenated vegetable oil raises LDL cholesterol compared to a low trans fatty acid diet.