While saturated fats raise blood cholesterol levels, monounsaturated fats lower blood cholesterol levels. Trans fatty acids were virtually unknown until the mid-1980's, when the food industry started to use less saturated fats, primarily from animal and tropical oils, in processed foods. Because saturated fats cause increased levels of blood cholesterol, consumers pressured the food industry to replace harmful saturated fats with the more beneficial unsaturated fats. Trans fatty acids are formed when liquid fats are hydrogenated, or partially hydrogenated, to become more solid at room temperature.
Saturated fats have a higher melting point and less potential to become rancid.
These properties are beneficial in many crackers, baked goods, and fried foods. To replace saturated fats, the food industry chose to use hydrogenated oils, creating trans fatty acids in the process. Although less harmful to our cardiovascular system than saturated fats, trans fatty acids raise blood cholesterol levels, therefore contributing to heart disease. Effects of trans fatty acids on HDL and LDL cholesterol levels are still unknown. The American Heart Association recommends that daily total fat intake be limited to no more than 30 percent of total calorie intake, and that trans fatty acids be limited as much as possible.