Blood Lipids

Blood lipids are fats transported in the blood by lipoproteins. Obesity is a risk factor for dyslipidemia (lipid abnormalities). Dyslipidemia is a risk factor for atherosclerosis, which is the main cause of cardiovascular disease and stroke.

Free fatty acids and cholesterol are absorbed by the gut and packaged into chylomicrons as triglycerides and cholesterol esters, respectively. Chylomicrons are acted upon by lipoprotein lipase in the blood to become lipoproteins.

Low-density lipoprotein (LDL) transports cholesterol from the liver to tissue. Cholesterol is used to synthesize cell membranes, steroid hormones, and bile. When there are high levels of LDL, the artery wall takes up more LDL, forming atheromas on the wall. Excess LDL is also taken up by white blood cells, forming foam cells. Foam cells and atheromas cause wall damage, leading to plaques and thrombi, which can produce low blood flow and high blood pressure. If thrombi break off as emboli, they can cause stroke or death. LDL taken up by the liver is used in synthesizing bile. Levels of LDL can increase due to saturated fat intake or hereditary conditions.

High-density lipoprotein (HDL) returns excess cholesterol to the liver, reducing the incidence of atheromas and resultant cardiovascular disease.

[Page 85]Triglycerides transfer energy to cells. While it is unclear that increased triglycerides independently increase the risk of cardiovascular disease, high triglyceride levels decrease HDL levels.

Screening blood lipids is recommended for men older than 35 years and women older than 45 years. If other risk factors such as diabetes, high blood pressure, smoking, or a family history of premature cardiovascular disease, exist, screening should begin after 20 years of age.

Fasting measurement of total cholesterol, triglycerides, and HDL allows LDL and very-low-density lipoprotein (VLDL) calculation. Optimally, total cholesterol should be under 200 mg/dL, LDL should be under 100 mg/dL, HDL should be over 41 mg/dL, triglycerides should be under 150 mg/dL, and the total cholesterol to HDL ratio should be under 4. The Framingham score considers total cholesterol, HDL, smoking history, blood pressure, and age to predict the risk of developing coronary heart disease.

Treatment of dyslipidemia involves lifestyle modifications including diet and exercise. Several classes of medication, including statins, can be used if lifestyle modifications are unsuccessful.