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Cholesterol levels are represented by two measurements, the high-density lipoprotein (HDL) and the low-density lipoprotein (LDL). For decades, doctors and experts have identified HDL to be the "good" cholesterol. Good HDL levels help with clearing arteries and improving heart health. In a new study conducted by a team from Cleveland Clinic, researchers found that HDL cholesterol can become abnormal and start to harm blood vessels.

For this study, the researchers ran tests on 627 participants. The researchers knew that the primary protein found in HDL is called apolipoprotein (apoA1). apoA1 helps build the molecule that transfers cholesterol from the artery wall to the liver where it can be excreted. LDL cholesterol has been dubbed the bad cholesterol because it stays within the artery walls and could cause hard plaque build-up, potentially leading to blockages, heart attacks and strokes.

The researchers also reported that apoA1 is responsible for HDL's cardio-protective qualities. From these tests, the researchers discovered that during atherosclerosis, a large amount of apoA1 in the artery wall becomes oxidized. When this occurs, HDL starts to negatively affect blood vessels.

"Identifying the structure of dysfunctional apoA1 and the process by which it becomes disease-promoting instead of disease-preventing is the first step in creating new tests and treatments for cardiovascular disease," said Dr. Stanley Hazen, Vice Chair of Translational Research for the Lerner Research Institute and study researcher reported by the Financial Express. "In the artery walls it is acting very differently to in the circulation. It can become dysfunctional, and contributes to the development of heart disease. This data does not change the message of eat healthily."

Dr Shannon Amoils, a senior research adviser for the British Heart Foundation, added according to BBC News, "Although traditionally we think of HDL as 'good' cholesterol, the reality is much more complex. We now know that under certain conditions HDL can become dysfunctional, potentially helping to clog blocked arteries. This interesting research pins down the exact chemical change that causes the 'good' HDL cholesterol to become bad. This knowledge could allow scientists to monitor coronary artery disease more closely or even target the 'bad' HDL with drugs."

The findings were published in the journal, Nature Medicine.