About one-fourth to one-third of blood cholesterol is carried by high density lipoprotien (HDL). Often called the "good" cholesterol, high levels of HDL seem to protect against heart attack because it carries cholesterol away from the arteries and back to the liver where it is passed from the body. Optimal HDL levels are 40 mg/dL and higher.
When too much low-density lipoprotein (LDL) cholesterol circulates in the blood, it can slowly build up in the artery walls that feed the heart and brain. Together with other substances, it can form artery-clogging plaque that can eventually cause a heart attack or stroke. LDL cholesterol of less than 130 mg/dL is the optimal level for most people.