For many years, doctors have drawn a distinction between ‘good’ and ‘bad’ cholesterol. The advice has been to keep your levels of ‘bad’ LDL cholesterol low and to raise your levels of ‘good’ HDL cholesterol.
The concern is that LDL increases patients’ risk of heart disease and stroke. HDL, on the other hand, has been seen as the good guy, removing excess cholesterol and reducing the risk of heart disease.
Not surprsingly, drug companies have been ploughing money into treatments that raise HDL, thinking that high levels can reduce the incidence of strokes or heart attacks but, so far, efforts have been unsuccessful.
Now it seems that for some patients, high HDL levels, far from being helpful, actually confer no benefits and may even be detrimental.
New advice: For some, high levels of ‘good’ cholesterol HDL confers no benefits and may be detrimental
As Professor Eliano Navarese, an Italian cardiologist and director of SIRIO Medicine, a network of experts reviewing medical research, explains: ‘The thinking that increasing HDL, which is widely advised by clinicians, could provide health benefits has been denied by a growing body of evidence.’
Most of the cholesterol circulating in our blood is made by the liver, mainly from saturated fats. LDL transports cholesterol from the liver to cells where it is needed for such processes as strengthening cell walls and making hormones.
HDL does the opposite, taking surplus cholesterol from cells back to the liver, where it is recycled or removed from the body in bile.
The NHS recommends that total cholesterol should be less than 5mmol/l, with LDL less than 3 and HDL more than 1.
There is no recommendation for a maximum level of HDL, as the assumption is that it is ‘good’. However, HDL’s protective effects appear to reach their maximum when blood levels are roughly 1.5mmol/l, says Dr Dermot Neely, lead consultant for the Lipid and Metabolic Outpatient Clinic at Newcastle’s Royal Victoria Infirmary.
He says: ‘Beyond that, higher HDL might not provide additional protection, and research is now suggesting that very high HDL levels, that is in excess of 2.3mmol/l, may behave more like LDL, raising the risk of heart problems.’
For example, previously it was thought that high HDL levels protect women who are going through the menopause against furring of the arteries, but a recent study cast doubts on this.
Researchers from the University of Pittsburgh studied the build-up of fatty deposits (plaque) in the arteries of 225 healthy women in their 40s and concluded that as women go through menopause, ‘increases in HDL . . . were actually associated with greater plaque’, putting the women at risk of a stroke or heart attack.
Bad news? HDL takes surplus cholesterol from cells back to the liver, where it is recycled or removed from the body in bile – rather than strengthening cell walls and making hormones
One theory is that the fall in oestrogen with the menopause could change the quality of HDL and its function.
What is not clear is whether this effect on HDL is influenced by hormonal changes or the normal ageing process in general, which would be relevant to women and men, says Naveed Sattar, a professor of metabolic medicine at the University of Glasgow.
Professor Navarese adds: ‘The findings build on several lines of evidence showing that increased HDL levels are not only not protective, but also may increase cardiovascular risk.’
High HDL is not just associated with damaging the heart; a 2012 study in the Journal of the American Society of Nephrology, found high HDL levels were harmful to kidney dialysis patients, possibly by exacerbating inflammation and tissue damage. Other studies have suggested similar effects in those with such conditions as arthritis or diabetes.
‘In certain circumstances, such as acute infection, or in chronic diseases linked to inflammation, such as rheumatoid arthritis, research has suggested that the HDL particles change their make-up,’ says Professor Sattar, adding that more work is needed to understand the implications of these changes.
Concern: High HDL is not just associated with damaging the heart; a 2012 study in the Journal of the American Society of Nephrology, found high HDL levels were harmful to kidney dialysis patients
Professor Navarese says that the more HDL there is, the more chance there is that some of it may malfunction — meaning not only does HDL not do its normal job of clearing plaques, it actually makes the plaques more unstable by causing inflammation in the arteries.
Yet just having very high HDL is not in itself a risk, according to Gilbert Thompson, an emeritus professor of clinical lipidology at Imperial College London.
‘In any patient who has a high HDL level, their risk of heart disease will depend on various factors, including whether they have a family history of heart problems or faulty genes,’ he says.
Generally speaking, he says that having HDL above 1mmol/l is a good thing, but adds: ‘it is too simplistic to think that a high HDL reading is always beneficial to health because not all HDL will act in the same way.
‘What matters is whether the HDL is functioning normally, not how high it is.’
He suggests that patients who suspect they have an abnormally high HDL level may need to have additional tests, such as on their coronary calcium levels to know how much build-up there is and to identify their level of risk for furred-up arteries and heart disease.
Advice: Patients who suspect an abnormally high HDL level may need to have additional tests, such as on their coronary calcium levels to know to identify their level of risk for furred-up arteries and heart disease