Its main role is to be synthesized in the liver, forming an anti-atherosclerotic lipoprotein that transports cholesterol from extrahepatic tissues to the liver for metabolism and has bile excreted from the body, allowing the body to avoid various diseases caused by having too much cholesterol. For example, coronary artery disease, through other atherosclerosis, etc., it can play a positive role in limiting the rate of cholesterol accumulation in the arterial wall and promote the removal of cholesterol yet, common physiological reduction, there are obesity, smoking, severe malnutrition, and intravenous high nutrition therapy or after stress. Other diseases such as severe hepatitis, diabetes, nephrotic syndrome or myocardial infarction, thyroid dysfunction, uremia can also lower it.