A pyrex jug with liquid fat from cooking bacon

Is fat bad?

Separate food fibs from food facts

We all need some fat, but not too much. Cholesterol, for example, is needed to make up cell membranes, hormones and other molecules.

Along with other lipids (including triglycerides and phospholipids), cholesterol is transported in the blood in complexes called lipoproteins, which include high-density (HDL) and low-density (LDL) forms. The cholesterol carried by these is known as HDL (‘good’) cholesterol and LDL (‘bad’) cholesterol, respectively. HDL lipoproteins carry cholesterol from parts of the body to the liver, where it is cleared from the body. High levels of LDL cholesterol lead to cholesterol building up into brittle plaques that block arteries – heart disease. See our animation on atheroma in the artery.

Some cholesterol is made in the body (including in the liver), and some we get from our diet. Eating lots of saturated fats, in which all the carbon atoms are linked to as many hydrogen atoms as they can take, tends to increase LDL cholesterol. This effect on blood cholesterol level is much greater than that caused by eating cholesterol.

Unsaturated fats have a double chemical bond between one or more carbon atoms. However, not all unsaturated fats are good. A double bond is more rigid than a single bond, and the way it makes other chemical groups stick out affects its chemical and physical properties.

Trans fats are made when unsaturated fats have more hydrogen added but not enough to make them completely saturated. They are relatively rare in nature but are often included in processed foods. Trans fats increase LDL cholesterol and raise the risk of heart disease.

Lead image:

Joy/Flickr CC BY


Further reading

About this resource

This resource was first published in ‘Food and Diet’ in June 2011 and reviewed and updated in August 2016.

Physiology, Health, infection and disease
Food and Diet
Education levels:
16–19, Continuing professional development