There are two main types of harmful dietary fat, saturated fats and partially hydrogenated unsaturated trans fats.
Saturated fats enter the diet from animal sources and elevate the total blood cholesterol levels and low-density lipoprotein cholesterol levels. The increase in these blood lipids increases the risk of cardiovascular disease like atherosclerosis and neuroendocrine disease like diabetes mellitus type 2. The partially hydrogenated unsaturated trans fats occur naturally in some foods from animal products, but the vast majority are synthetically made during food processing.
Through partial hydrogenation of unsaturated fats, industrial foods are made less likely to spoil and easier to incorporate into cooking than their naturally occurring oils. The partially hydrogenated unsaturated trans fats increase the "bad," low-density lipoprotein (LDL) cholesterol and lower the "good," high-density lipoprotein (HDL) cholesterol, which increases risk of life-threatening cardiovascular disease like heart attacks or myocardial infarctions, coronary artery disease from atherosclerosis and limb ischemia from peripheral vascular disease.
These two dietary fats have a high percentage of either saturated fat or partially hydrogenated unsaturated trans fats or both saturated fat or partially hydrogenated unsaturated trans fats. The easiest way to identify the constituents of a diet that have saturated fats or partially hydrogenated unsaturated trans fats is to remember that these fats are solid at room temperature and include shortening, stick margarine, stick butter, beef fat, pork fat and bacon fat.
More Answers from Univ. of Nev. School of Medicine, Family Medicine