Risk depends on characteristics of lipoproteins in the blood



Cholesterol is confusing, so stop eating those buttered egg yolks fried in lard and covered with cheese and put on your chicest thinking cap.

There‘‍s no quiz at the end.

Cholesterol and triglycerides are types of lipids -- fats, waxes, sterols and fat-soluble vitamins including A and D, among others. As the Mayo Clinic sums up, triglycerides store unused calories and give your body energy, while cholesterol is used to build cells and some hormones.

Because lipids don’‍t dissolve in our water-based blood, they must hitch a ride inside special protein taxis to be transported in the blood throughout the body. The proteins containing these lipids are known as lipoproteins.

But there are many different lipoproteins, which add to the confusion, so keep that cap on. The health impact of lipoproteins depends on their size, lipid content and density.

Density describes the ratio of the protein structure to the lipids it carries. In short, it refers to how big the protein is and how much room it provides to its lipid passenger. Low-density proteins might be compared with a lightweight plastic bottle carrying lots of liquid, or cholesterol. A high-density lipoprotein might be more like a golf ball -- or bowling ball -- with a small liquid center.

So high-density lipoproteins -- or HDL -- have been known as good cholesterol because they act like speedy sports cars that transport small amounts of cholesterol from the blood to the liver, where the cholesterol is sent to the gut and sent out of the body.

Low-density lipoproteins, with their big load of cholesterol, wobble around in the blood with a propensity to stick to the endothelial linings of arteries, where they contribute to plaque formation. Plaque buildup restricts blood flow and hardens the arteries. That process also induces inflammation which can lead to a heart attack. That’‍s the key reason why LDL has been generally described as bad cholesterol.

But those designations may no longer be valid because some LDL can be good and some HDL can be bad when all those characteristics -- size, density and lipid load -- are considered.

And that‘‍s where size of the lipoproteins becomes important. Much like T-shirts, lipoproteins can be very small, small, medium-small, medium, medium-large and large.

Indu Poornima, the Allegheny General Hospital director of nuclear cardiology and director of the hospital‘‍s Women’‍s Heart Center, who provided much of the information above, said science now is focusing on the characteristics of lipoproteins to determine the health risk of cholesterol and triglyceride levels in the blood.

“We’ve kind of moved on from the definition of good and bad cholesterol,” she said. “We now know more about individual particles, and most of that has come about in the last five to 10 years.”

It’‍s been known that small lipoproteins, especially LDL, can pose problems because they exist in greater numbers in the blood, Dr. Poornima said, with their size allowing them to penetrate the arterial wall, making it prone to produce plaque. The liver is the body’s largest fat-storage organ, but high cholesterol and triglyceride levels in the blood lead to arterial plaque. Once in place, plaque is not easy to remove, although statin drugs may halt the buildup of plaque.

People with diabetes and metabolic syndrome (prediabetes) typically have smaller LDL particles, Dr. Poornima said.

But what researchers now realize is that LDL and HDL each can have different sized particles. It helps explain why a person with high cholesterol and even high LDL levels may have no signs of heart disease while someone with cholesterol in the normal range can experience a heart attack.

The types of lipoproteins in the blood dictate risk. Blood tests that determine lipoprotein size and characteristics are available but expensive and rarely used. So they are used to assess high-risk patients or patients whose clinical risk is high but traditional cholesterol measurements for them don‘‍t suggest any abnormality, Dr. Poornima said.

Based on these factors, the American Heart Association has altered its guidelines for assessing risk and the need for treatment.

“We’‍ve moved away from looking at cholesterol levels independently,” Dr. Poornima said. “Now we look at the overall risk of the patient. The patient is deemed at high risk based on other risk factors -- age, race, diabetes, high blood pressure and smoking.

“New guidelines,” she said, “recommend treating a person with a high 10-year risk of having a cardiovascular event with a statin drug, regardless of their cholesterol levels, as opposed to what we did before.”


David Templeton: dtrempleton@post-gazette.com or 412-263-1578.

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