NEW YORK — LDL, or “bad” cholesterol, is a major risk factor for coronary heart disease. Now a new study suggests that, like smoking, it has a cumulative effect over a lifetime: The longer a person has high LDL, the greater their risk of suffering a heart attack or cardiac arrest.

Coronary heart disease, also known as “hardening of the arteries”, is the leading cause of death in the United States. It is caused by a buildup of plaque in the arteries that narrows the vessels and blocks the flow of oxygenated blood to the heart.

Often, people have no symptoms and remain unaware they have the disease for years until they develop chest pain or suffer a catastrophic event like a heart attack.

Using data from four large prospective health studies, researchers calculated LDL levels over time in 18,288 people who had multiple LDL tests taken at different ages. They calculated their cumulative exposure to LDL and followed their health for an average of 16 years. The study is in Jama Cardiology.

The researchers found that the longer a person had high levels of LDL — no matter what their LDL level is in young adulthood or middle age — the greater the risk for coronary heart disease. Compared with those in the lowest quarter for cumulative exposure, those in the highest had a 57 per cent increased risk.

They found no increased risk for stroke or heart failure associated with cumulative LDL exposure. The researchers suggest that many factors can contribute to heart failure, and their study had too few cases of stroke to achieve statistical significance.

The study controlled for race and ethnicity, sex, year of birth, body mass index, smoking, high-density lipoprotein (HDL, or “good” cholesterol), blood pressure, Type 2 diabetes, and the use of lipid-lowering and blood pressure medicines.

In people under 40, current guidelines recommend treatment with cholesterol-lowering statin drugs only with LDL readings higher than 190, but the researchers found that the increased risk for coronary heart disease may start at a much lower level. (LDL levels below 100 are generally considered normal.)

“Our figures suggest that the risk starts at LDL levels as low as 100,” said the lead author, Dr YiYi Zhang, an assistant professor of medical sciences at Columbia University in New York.

“That doesn’t necessarily mean that a person under 40 with an LDL of 100 should immediately start treatment. We need more evidence to determine the optimal combination of age and LDL level.”

Dr Tamara Horwich, a cardiologist and professor of medicine at UCLA who was not involved in the study, noted that medical guidelines on choosing who needs statin therapy are heavily weighted toward older people, since advancing age is a major risk factor for complications from heart disease.

Still, she said, “From autopsy studies, we have known for some time that atherosclerosis begins to develop in the arteries of young individuals, as early as the teens and 20s. I think this study may entice physicians to move the needle back on the age of starting, or at least thinking about starting, statin therapy.”

Young people have a low short-term risk, Dr Zhang said, but a high long-term risk. “The main message is to try to maintain low LDL through middle age. That will reduce your heart disease risk.”

This article originally appeared in The New York Times.