Good cholesterol and bad cholesterol. For years that’s how we’ve been told to think of the waxy, fat-like substance in blood playing a key role in determining whether we get heart disease.

On one side, there is HDL (or high-density lipoprotein), the ‘good’ version that scoops up excess cholesterol particles in our blood vessels and escorts them to the liver, where they are broken down and removed as waste.

On the other side is LDL – or low-density lipoprotein. It sticks to the inside of our arteries forming deposits, called plaques, which can then break apart and form blockages that can cause a heart attack or stroke.

But recent research shows there is at least one other type of cholesterol that most of us have never heard of, yet may be just as important in terms of our heart health.

Called remnant cholesterol, it represents most of the cholesterol that is left in the blood once HDL and LDL are subtracted from the total.

Now it emerges that even if you have low LDL, too much of this remnant cholesterol can be harmful – with new research showing that high levels dramatically raise the risk of heart attack and stroke.

While it’s not routinely measured in cholesterol tests, research suggests this risk starts to rise significantly once remnant levels reach 1mmol per litre of blood.

(By comparison we’re advised to keep our LDL below 4mmol/l, and HDL above 1mmol/l for men and 1.2mmol/l for women; low HDL is a bigger danger in women as levels of LDL rise dramatically after the menopause. And total cholesterol should not exceed 5mmol/l.)

Crucially, studies also suggest that statins – drugs taken by more than eight million people in the UK to reduce LDL cholesterol – are less effective at reducing remnant cholesterol, since they mainly target the liver to produce less LDL.

‘Where statins normally lower LDL levels by 50 per cent, they only reduce remnant cholesterol by around 30 per cent,’ says Kausik Ray, a professor of public health at Imperial College London.

But what makes remnant cholesterol different – and what can we do to keep it in check if statins have a limited effect?

For years, remnant cholesterol was dismissed as a bit-part player in the game of heart disease.

But several recent studies have found it’s much more significant than anybody realised.

It forms when another type of fat in the bloodstream – triglycerides – is broken down to be used as fuel for the body or stored in fat cells.

Triglycerides are transported in particles called lipoproteins.

A small proportion – less than 5 per cent – of each of these lipoproteins is made up of cholesterol, which gets discarded into the bloodstream once the main component (triglycerides) has been stripped out. This discarded part is remnant cholesterol.

The problem is that, although the cholesterol proportion in each triglyceride molecule is low, the triglyceride lipoprotein molecules are at least four times bigger than LDL or HDL particles.

So the amount of ‘remnant’ cholesterol left over can be sizeable. A landmark study in 2021 by Johns Hopkins University in the US, involving 17,000 adults, found high remnant cholesterol increased the risk of heart attacks and strokes by up to 50 per cent, even in those with low LDL, according to results in the European Heart Journal.

And a recent study by the Chinese Academy of Medical Science, in Beijing, involving more than 40,000 people, found that those with raised remnant cholesterol had three times the risk of a heart attack or stroke over a ten-year period than participants who had much lower readings.

It appears to trigger harm in a similar way to LDL – by causing deposits and inflammation in the arteries. What’s worrying some experts is that while standard cholesterol checks might give patients the all-clear, they may still be at hidden risk because of raised remnant cholesterol levels.

A 2024 study by Changi General Hospital, in Singapore, published in Frontiers in Cardiovascular Medicine, found 11 per cent of those with ‘healthy’ LDL scores had potentially dangerous levels of remnant cholesterol that hadn’t been picked up during checks.

‘And there is some evidence that once remnant cholesterol particles get into the blood vessel wall they are more nasty and more virulent [than LDL], leading to more inflammation,’ says Professor Ray.

So how would you know if your remnant cholesterol level is high?

Although standard cholesterol tests do not measure it, they do record triglyceride levels.

As triglycerides are the ‘carriers’ of remnant cholesterol, this can give some indication of whether it’s likely to be a problem.

The British Heart Foundation says a healthy triglyceride level is below 1.7mmol/l for a fasting test (i.e. no food or drink for at least ten hours) or below 2.3mmol/l for a non-fasting test.

Professor Ray says: ‘Patients should be paying more attention to their triglyceride levels and GPs should be saying that if their triglyceride levels are high, it may mean they need to be treated more aggressively – either with higher doses of statins, or an additional second medication to lower their readings as much as possible.’

Dietary changes can help reduce triglycerides, he notes, but involve limiting intake of carbohydrates and saturated fat to no more than ten to 15 per cent of overall food consumption.