By BARNEY CALMAN FOR THE MAIL ON SUNDAY

• Doctors have been advising patients to take ‘good’ cholesterol when possible
• They warned against LDL, which is deemed bad and promoted HDL as good
• However, new research indicates that very high HDL levels could be bad
• Patients with too much ‘good’ cholesterol could be at higher risk of heart attack

For decades we’ve been told there’s ‘good’ cholesterol and there’s ‘bad’ cholesterol.

The bad type, known as LDL, is responsible for damaging blood vessel walls and contributes to the build-up of inflamed fatty deposits known as plaques, which raises the risk of a heart attack or stroke.

The good type, called HDL, does the opposite – clearing away cholesterol in plaques and taking it back to the liver where it is processed and removed from the body.

Early research suggested that having high HDL lowers the risk of a heart attack, while very low levels led to poor health.

But a growing body of data suggests that after a certain point, very high HDL levels are associated with an increased risk of heart attacks.

Such is the weight of evidence that guidelines are beginning to change to reflect that very high HDL doesn’t protect the heart.

Not that you’d know this from a quick internet search. You’ll find numerous articles, many on reputable medical websites, devoted to ways to boost HDL levels.

On the well-respected WebMD website is says. ‘HDL protects you from getting heart disease. So you want as much as possible.’

Other articles suggest you raise your levels by consuming lots of ‘healthy fats’ in your diet: olive oil, oily fish and nuts. But while this kind of diet is linked to a lower risk of heart attack, it might not simply be to do with HDL. The picture, as with so much when it comes to the heart, is much more complicated.

Consultant cardiologist Dr Laura Corr said: ‘It’s true that very low HDL isn’t a good thing, and the risk of a heart attack lowers as HDL rises. But we now know unusually high HDL in some people isn’t protective and is associated with an increased risk of heart attacks. It was a surprise when the research started to show this.

‘Of course, simply focusing on a single number – whether that’s overall cholesterol, or HDL, or anything else – won’t tell you your true risk. Heart health relies on a whole range of things, as does lowering heart attack risk.’

The initial thinking, that all HDL was ‘good’, comes from studies carried out in the 1990s. These revealed that patients with higher HDL had a lower heart attack risk. This led to a series of large trials that attempted to find drug treatments that would artificially raise HDL and harness this benefit. But they were unsuccessful.

Professor Peter Sever, an expert in drug treatments for the heart at Imperial College London, said: ‘The drugs failed to reduce numbers of heart attacks, and one, niacin, had nasty side effects, including severe skin flushing and itching, heart problems, nausea and other digestive discomfort, and liver damage.’

In 2018, a major analysis specifically examined the relationship between HDL and risk of heart attack and death. For four years, researchers from the Emory University School of Medicine, Atlanta, followed almost 6,000 patients, most with heart disease and with an average age of 63. During the study, 13 per cent had a heart attack or died from cardiovascular disease. But something quite unexpected was seen.

There were more heart attacks in those with very low HDL, of less than 1, which wasn’t a surprise. But a similarly high number were seen in those with very high HDL levels, of more than 1.4. Only those with HDL levels between this range saw a lower heart attack risk.

The pattern remained the same regardless of the patients’ ‘bad’ LDL cholesterol levels, whether or not they smoked or had diabetes.

Cardiologist Dr Marc Allard- Ratick, who led the study, said: ‘Traditionally, doctors have told their patients that the higher your “good” cholesterol, the better. However, the results from this study and others suggest this may no longer be the case.’

It’s unclear why, but women are more likely to have unusually high HDL. A second analysis, also published in 2018, which looked specifically at the link between HDL and heart attacks in postmenopausal women, came to a similar conclusion: very high HDL raised the risk.

Professor Kausik Ray, an expert in cholesterol at Imperial College London, said: ‘At present, all we can say is that very high HDL shouldn’t be a reassurance. We don’t know if it is a consequence of something else, or a cause of problems, but it is associated with a raised heart attack risk.’

So just what counts as very high HDL level?

When you have a cholesterol test, you’re given a few different numbers – including an overall cholesterol number and your HDL and LDL levels.

A total cholesterol level of five or more is considered a risk, but current guidelines do not set a limit for HDL. Based on the new evidence, however, once it rises beyond 1.4 it stops being protective. There are no treatments that specifically reduce HDL – cholesterol-lowering statins target LDL. However, lifestyle changes may help reduce other risk factors.

However, Dr Corr says: ‘An HDL of over 1.4 is uncommon, but if you do have a level that’s more than this, don’t panic – it’s not the only factor. It’s important to look at the bigger picture.

Could areas of your diet improve? Could you lose a bit more weight and drink a bit less alcohol?

‘If a patient isn’t on statins, then perhaps they might be needed.

‘Statins, while lowering LDL, may slightly raise HDL – but we know they lower the risk of heart attacks. So if this is the case, and you have raised HDL, you shouldn’t stop the statin. If you’re concerned, speak to your doctor.’

So why, given the early data showing that higher HDL levels were healthier, are we now seeing those with the highest levels are at increased risk?

The answer is not yet clear, and research is ongoing.

HDL and LDL are lipoproteins – compounds which contain cholesterol, which is a kind of fat, and proteins – made by the liver. Cholesterol is vital for a range of cell functions and lipoproteins transport it round the body. While we generally make just what we require, if there is an excess, it can be laid down in blood vessel walls, ultimately leading to a heart attack or stroke.

There are, in fact, different types of HDL, which vary in size and structure. Some may be beneficial while others are less so, or even harmful. There may also be different ages at which different types of HDL are more or less beneficial – such as during the menopause, with all the hormonal changes that occur.

And genetics also play a role. Our individual sensitivity and personal cholesterol threshold means that high HDL for one person might be totally normal for another.

Prof Ray says: ‘One thing is clear. We need to stop telling patients “the more the better” when it comes to HDL, because there is a limit.’