HDL and LDL

Blood cholesterol is complex. Your blood test results will include measurements of some of the individual lipoproteins, HDL and LDL. But there are further levels of complexity that should make us wary of dismissing the risks of elevated blood cholesterol on the basis of “good cholesterol” levels and ratios.

Cholesterol and other fats do not dissolve in blood and are transported in the blood as lipoprotein particles. These are small globs of cholesterol, fat, and special proteins that give each type of particle their characteristics.

Your blood cholesterol report will usually start with “total cholesterol”, which is the total amount of cholesterol in all of the lipoprotein particles in the serum. (Serum is the liquid between the blood cells).

LDL particles (low density lipoprotein) transport cholesterol from the liver to other parts of the body. LDL is often called “bad” cholesterol because abnormally high levels cause artery disease. There is nothing “bad” about LDL when it’s not elevated.

HDL particles (high density lipoproteins) transport cholesterol back to the liver for recycling. HDL is often called “good cholesterol” because high levels are considered to protect arteries from cholesterol build up.

Triglyceride is a measure of the ordinary fat being transported in the blood. Several lipoprotein particles are involved and the fat comes both directly from food and from fat synthesized by the liver. High triglyceride levels are associated with low HDL levels and qualitative changes to LDL that increase heart risk.

Cholesterol test results also include a ratio of HDL relative to total cholesterol (or sometimes LDL). A lot of emphasis is placed on this ratio in Australia. Your doctor may tell you that it doesn’t matter how high your cholesterol level is, as long as the ratio is good. We disagree.

Total cholesterol, HDL and LDL are useful measurements but are far from the whole story. Both types of particles come in different sizes and composition: some LDLs are worse than others and not all HDLs are beneficial.

LDL particles vary in size. Large lower density particles are more likely to predominate with healthy lifestyles and are less damaging to arteries. Small dense particles are associated with high cholesterol diets, diabetes and obesity and cause cholesterol plaque build up in arteries. The most damaging form of LDLs are the small dense oxidised LDL particles. The oxidative stress of a fatty high cholesterol meal increases oxidised LDL for several hours.

The protective, cholesterol recovering and recycling properties of HDL are of more importance in populations with high cholesterol levels, such as Australia. In populations with very low total cholesterol levels, due to plant strong low fat diets, the average HDL levels are also low, yet the incidence of heart disease is very low. You don’t need high HDL levels to remove cholesterol from your arteries when there is not much there to start with.

HDL particles also vary in quality. The HDL measurement in your pathology result is the total volume of HDLs but it may be that the number of HDL particles that is more important. It is the efflux capacity of HDL that really matters – that is its capacity to pick up and carry cholesterol. This capacity may be reduced by the same dietary patterns that oxidise LDL particles.

HDL might not be the anti-heart disease entity we once thought it was. That there is a strong inverse association between HDL levels and heart disease is not in dispute. Low HDL levels in people with high cholesterol levels are a strong heart risk predictor. But this low HDL may be a marker of a more general adverse profile of cholesterol particles associated obesity and insulin resistance. But very high HDL levels may not offer extra protection. A recent genetic study by Dr Sekar Kathiresan (The Lancet 2012), suggests that inherited high HDL levels does not protect against heart disease.  Furthermore, pharmaceutical trials of drugs that artificially elevate HDL levels have so far failed to reduce heart risk.

Some foods, such as vegetable oils and alcohol, improve LDL and HDL levels but do not lower heart risk.

The limits of only measuring fasting cholesterol levels and ignoring what happens after meals is discussed in the endothelium and atherosclerosis page.

The documentary, “The Last Heart Attack” includes discussion about the differing types of LDLs and HDLs.