An understanding of some terminology is helpful in becoming familiar with the role of fat in the diet. Saturated, polyunsaturated, and monounsaturated are all chemical terms. They refer to the structure and orientation of the molecules of carbon, hydrogen, and oxygen.

Saturated refers to a molecule which is filled with hydrogen atoms. It is saturated like a household sponge and will not accept any more hydrogen.

Below is a diagram of a saturated fat, note that there are no empty spaces:

When openings are created by a double bond between two carbons, the fat is not saturated. If one double bond (indicated by =) is present, the fat molecule is said to be monounsaturated. A monounsaturated fat is pictured below:

A fat with two or more double bonds is called polyunsaturated. This means there are four or more vacant spots that are not filled with hydrogen atoms.

A polyunsaturated fat looks like this: