News ID: 166684
Published: 0136 GMT August 12, 2016

HDL levels, premature death interlinked

HDL levels, premature death interlinked

Foods such as salmon, mackerel and sardines along with fruit and beans are commonly touted for helping reduce the risk of stroke and heart attack by boosting levels of 'good cholesterol' in our blood.

But a new study shows both high and low levels of high density lipoprotein (HDL) — known as 'good cholesterol' — may increase a person's risk of premature death, reported.

It is actually medium amounts that boost longevity, according to the research published in the Clinical Journal of the American Society of Nephrology.

Professor Ziyad Al-Aly, of the University of Washington in the US, said: "The findings surprised us. 

"Previously it was thought raised levels of the good cholesterol were beneficial.

"The relationship between increased levels of HDL cholesterol and early death is unexpected and not fully clear yet. This will require further study."

Cholesterol is a fatty substance found in blood that can narrow and block heart vessels, causing cardiovascular disease and stroke.

For years, HDL cholesterol has been credited with helping to remove plaque building 'bad cholesterol', or LDL (low density lipoprotein), from arteries.

So the researchers studied kidney function and HDL cholesterol levels in more than 1.7 million male veterans who were followed for about ten years from 2003 until 2013.

Patients with kidney disease frequently have lower levels of HDL cholesterol, which may explain their increased risk of early death. But the association between elevated levels and premature death has been unclear.

The study showed that both high and low HDL cholesterol levels were linked with an increased risk of dying among participants with all levels of kidney function.

 Al-Aly said: "The findings may explain why clinical trials aimed at increasing HDL cholesterol levels failed to show improved outcomes."

He said such findings regarding HDL cholesterol and premature death have not been reported in other large epidemiologic studies.

Al-Aly said: "However, the previous studies are limited in that the number of patients in those cohorts is relatively small compared with what a big data approach enabled us to see in our new research.

"Big data allow a more nuanced examination of the relationship between HDL cholesterol and risk of death across the full spectrum of HDL cholesterol levels."

Research data showed a relationship between HDL cholesterol levels and mortality as a U-shaped curve with the risk of death increased at both ends of the spectrum.

Al-Aly said: "Too low and too high are both associated with higher risk of death."

He said whether maintaining intermediate HDL cholesterol levels may increase longevity will need to be explored in future studies.

Al-Aly added: "A large database is critical in allowing us to research and challenge prior knowledge in our continuing efforts to create a better understanding of HDL cholesterol levels and risk of death."

The findings add to evidence showing increased HDL levels may increase cardiovascular risk.

A 2012 study also 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.

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 three and HDL more than one. 

There is no recommendation for a maximum level of HDL, as the assumption is that it is 'good'. 

But earlier this year Dr. Dermot Neely, lead consultant for the Lipid and Metabolic Outpatient Clinic at Newcastle's Royal Victoria Infirmary and trustee for the cholesterol charity Heart UK, said HDL's protective effects appear to reach their maximum when blood levels are roughly 1.5mmol/l.

He added: "Beyond that, higher HDL might not provide additional protection, and research is now suggesting that very high HDL levels, that are in excess of 2.3mmol/l, may behave more like LDL, raising the risk of heart problems."

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