LDL cholesterol

The cholesterol carried by low density lipoproteins. As it rises, fatty plaque can build up in the arteries, which is why it is called bad cholesterol.

Cholesterol does not dissolve in blood, so it travels through the bloodstream wrapped in particles called lipoproteins. A cholesterol test reports how much is being carried by each kind, which is why the result arrives as several numbers rather than one.

LDL is low density lipoprotein, and it carries the fraction the National Heart, Lung, and Blood Institute calls bad cholesterol. High levels of it may create a buildup of plaque, meaning fatty deposits, in the arteries, and that buildup can lead to a heart attack, a stroke or other problems.

HDL, high density lipoprotein, works in the other direction. The same institute describes it as carrying cholesterol and plaque to the liver to be flushed out of the body, and says high levels of it may lower the risk of those problems.

The distinction matters when reading a study. A result showing no change in HDL says nothing about what happened to LDL, and a small rise in LDL is not cancelled out by a steady HDL. Units differ between journals as well: some report these figures in millimoles per litre, written mmol/L, and others in milligrams per decilitre, so the size of any change has to be read against the unit the study used.

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