A high level of so-called "bad cholesterol," or LDL-C, is the main reason why apparently healthy individuals suffer heart attacks or strokes in middle age, said the report in the Journal of the American College of Cardiology (JACC).

"Atherosclerotic plaques are present in 50 percent of middle-aged individuals (40-54 years old) with no classical cardiovascular risk factors," said the study.

The report was based on nearly 1,800 people who had no classical risk factors for heart disease or stroke. 

LDL cholesterol was the top predictor of arteries that were in the process of hardening, but had not yet caused a health crisis.

"Even in people with optimal blood pressure, blood sugar, and total cholesterol, we detected an independent association between the level of circulating LDL-C and the presence and extent of subclinical atherosclerosis," said study co-author Javier Sanz of Spain's Centro Nacional de Investigaciones Cardiovasculares Carlos III (CNIC).

Current guidelines from the US National Cholesterol Education Program describe LDL-C levels that are above 160 mg/dL as "high."

Those from 130 mg/dL to 159 mg/dL are considered "borderline high." 

Experts should consider lowering these thresholds to improve population health, the researchers argue.

"Although the absence of classical cardiovascular risk factors is linked to a low risk of cardiovascular events, people in this situation still have heart attacks and strokes," said lead author Leticia Fernandez-Friera. 

"We therefore need to define new markers of early atherosclerosis in these apparently healthy individuals."