HDL cholesterol

HDL is a transport particle (lipoprotein), often called "good" cholesterol – it carries cholesterol back to the liver (reverse cholesterol transport). How HDL is to be read for your sex is set out in your findings report.

What HDL does in the body

HDL starts as lipid-poor apolipoprotein A-I, which the liver and gut release into the blood. At the cells – including the scavenger cells in the vessel wall – it takes up excess cholesterol via the transporters ABCA1 and ABCG1. The enzyme LCAT esterifies this cholesterol and stores it in the core of the particle, creating spherical, mature HDL. The liver takes over the cholesterol directly via the receptor SR-B1, or cholesteryl ester transfer protein (CETP) passes it on to LDL and VLDL, which also bring it to the liver; there it is excreted into the bile as cholesterol or bile acid. HDL also carries enzymes such as paraoxonase-1, which break down oxidised fats, and dampens inflammatory signals at the vessel wall.

Low HDL – what happens in the body?

When little HDL is present, there is less transport capacity available to collect cholesterol from the tissues. HDL often falls together with rising triglycerides: when there is a lot of VLDL, CETP exchanges cholesterol esters from HDL for triglycerides, the triglyceride-rich HDL is shrunk by hepatic lipase and its apolipoprotein A-I is excreted more quickly via the kidneys. This pattern occurs with insulin resistance, a lot of abdominal fat, lack of exercise and smoking; HDL also falls temporarily during acute inflammation. Rare inherited disorders of ABCA1 or LCAT lead to very low values. Anabolic steroids also lower HDL markedly.

High HDL – what does it mean?

Higher HDL values are often due to inherited factors, oestrogens, regular endurance exercise or alcohol. How well HDL works, however, does not depend on its amount alone, but on how promptly it takes up and passes on cholesterol. With some inherited variants – for example reduced CETP activity or an altered SR-B1 receptor – HDL accumulates because it passes on its cholesterol less well; in studies, very high HDL values were therefore not consistently linked with favourable findings. In chronic inflammation, HDL can also lose its protective enzymes and be composed differently without the measured amount changing.