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.

Inclusion Criteria:

• BMI between 20 and 38 kg/m2
• Age 25 to 65 years during the intervention
• Fasting glucose < 126 mg/dl
• Blood pressure < 160/100 mm Hg (controlled with certain medications)
• Total plasma cholesterol < 280 mg/dl
• Fasting triglycerides < 300 mg/dl

Exclusion Criteria:

• Use of prescription or over-the-counter medications or supplements that alter lipid metabolism.
• History or presence of kidney disease, liver disease, gout, certain cancers, thyroid disease, gastrointestinal, other metabolic diseases, or malabsorption syndromes.
• Type 2 diabetes requiring the use of oral antidiabetic agents or insulin.
• History of eating disorders or other dietary patterns which are not consistent with the dietary intervention (e.g., vegetarians, very low fat diets, high protein diets).
• Women who have given birth during the previous 12 months, pregnant women, women who plan to become pregnant or who become pregnant during the study or lactating women.
• Volunteers who routinely participate in heavy exercise or volunteers who initiate an exercise program during the study.
• Volunteers who have lost 10% of body weight within the last 12 months or who plan to initiate a weight loss program during the next 10 months.
• Use of prescription or over-the-counter antiobesity medications or supplements (e.g., phenylpropanolamine, ephedrine, caffeine, during and for at least 6 months prior to the start of the study) or history of a surgical intervention for obesity.
• Active cardiovascular disease (such as heart attack or procedure within the past three months or participation in a cardiac rehabilitation program within last three months, stroke or history/treatment for transient ischemic attacks in the past three months, or documented history of pulmonary embolus in past six months).
• Smokers or other tobacco users (during the 6 months prior to the start of the study).
• Unable or unwilling to give informed consent or communicate with study staff.
• Self-report of alcohol or substance abuse within the past twelve months and/or current acute treatment or rehabilitation program for these problems (long-term participation in Alcoholics Anonymous is not an exclusion).
• Other medical, psychiatric, or behavioral factors that in the judgment of the Principal Investigator may interfere with study participation or the ability to follow the intervention protocol.