What is HDL cholesterol and when is it too low?

Cholesterol does not dissolve in blood. To travel around anyway, it sits packed inside small spheres with a protein layer around them, and those spheres are called lipoproteins. HDL is the smallest and densest type. When your blood is drawn, nobody counts how many HDL spheres you have: what gets measured is how much cholesterol is inside them, in millimoles per litre (mmol/l). So you are reading the cargo and not the number of lorries.

That distinction immediately explains why the nicknames are so misleading. HDL is not a different, better kind of cholesterol than LDL. It is exactly the same molecule, just in a different type of packaging and heading in a different direction. LDL particles bring cholesterol out to your tissues and can get stuck in the artery wall. HDL particles collect it instead and take it to your liver, which disposes of it through the bile. Good and bad therefore refer to the direction of transport and not to the substance itself.

The limits on this page come from the NVKC, the Dutch society of clinical chemists, and they are risk limits rather than normal values: below 1.04 mmol/l goes with a higher cardiovascular risk, 1.04 to 1.55 with a moderate risk and above 1.55 with a low risk. They apply to men and women alike. Women do naturally sit on average 0.2 to 0.3 mmol/l higher because of the hormone oestrogen, so the same 1.1 mmol/l sits further below her average in a woman than in a man, even though it falls in the middle band for both. On your result itself the lab prints its own range, 0.9 to 1.7 mmol/l in our case, and that is a lab convention.

One thing you need to know alongside it, or you will misread your result. Your HDL moves inversely with your triglycerides, the fats in your blood. The more triglycerides floating around, the faster your HDL particles are cleared. A low HDL is therefore almost never a problem in itself: it is usually the trace of a triglyceride value that is too high. You do not need to fast for HDL, because eating barely changes this value.

Almost everyone knows one sentence about HDL: it is the good cholesterol and the higher the better. Both halves of that sentence are wrong, and explaining that is the most useful thing this page can do.

Why good is skewed: HDL is called good because it removes cholesterol instead of depositing it. But a high HDL does not prove that the removal is happening well. The test measures the cargo in your HDL particles and not how hard they work. So you can have a fine HDL number while the particles themselves function poorly, and nothing shows you that.

Why higher is better does not hold: researchers followed more than 116,000 Danes for years and looked at which HDL value went with the fewest deaths. That turned out to sit around 1.9 mmol/l in men and around 2.4 mmol/l in women. Above that, mortality went back up: in men from roughly 2.5 mmol/l and clearly above 3.0, in women from roughly 3.5. So the relationship is a U and not a straight line, and an HDL of 3.5 is not a better result than an HDL of 1.8.

And the strongest evidence: raising HDL does not work. If a high HDL really protected you, a drug that raises HDL ought to prevent heart attacks. That has been tried several times with agents that lifted HDL by tens of percent, and the number of heart attacks stayed the same; with the first agent in that series, more people actually died. People who have carried a gene since birth that raises their HDL do not have fewer heart attacks either. That is why the Dutch GP standard sets no target for HDL.

What does that mean for you? That your HDL is a thermometer and not a dial. A low HDL does not tell you that your HDL is the problem, but that something else is going on: abdominal fat, smoking, too little movement or triglycerides that are too high. Those things carry the risk, and those you can do something about. If you want to know how your arteries are doing, look at your LDL, your non-HDL cholesterol or your ApoB, because those are the numbers your GP bases a treatment on.

HDL is almost never drawn on its own. It belongs to a cholesterol test, together with total cholesterol, LDL and triglycerides. Those four together are what make your result readable, because separately they say little.

You do not need to fast. HDL barely changes after eating, usually by less than a tenth of an mmol/l. Your triglycerides are a different matter: they rise considerably after a meal, and because your LDL is calculated from the other values, a non-fasting result can distort your LDL. If you want the whole picture right in one go, get drawn fasting. Also wait a few weeks after flu, an operation or a hospital admission, because your whole cholesterol profile dips temporarily then and your result looks better than your usual situation.

A cholesterol test makes sense if you want to know where you stand, if cardiovascular disease runs in your family, if you smoke or carry excess weight, if your blood pressure or blood sugar is raised, or if you want to see whether a change in your lifestyle has had an effect. In that last case, wait at least three months before testing again, because any sooner too little changes to see anything.

In short, this is how your HDL behaves day to day. After a meal it barely changes, so you can be drawn at any time. Smoking makes it fall and it recovers after quitting. Excess weight around the middle makes it fall and is the most common cause of a low value. Regular walking, cycling or swimming makes it rise slowly and slightly, a real effect you should count in months. Alcohol makes it rise, but that is a nicer number and not better health. And high triglycerides press it down; tackle the triglycerides then and not the HDL. One result is a snapshot, so have an abnormal value repeated before drawing conclusions from it.