Exercise is good for you, and the more the better. Right? Maybe not, says a study led by researchers at the University of Illinois at Chicago and Kaiser Permanente. The study found that white men who exercise at high levels raise their risk of a buildup of dangerous plaques in their heart by the time they reach middle age.

Researchers looked at the activity levels of 3,175 black and white participants, and gauged the amount of coronary artery calcification, or CAC, among them.

Heart disease is the No. 1 cause of death in the United States. CAC is a clinical measure of the accumulation of calcium and plaque in the arteries of the heart, which is an indicator of the risk of heart disease.

Researchers categorized participants into three groups based on their physical activity: exercising below the national guidelines (less than 150 minutes a week), meeting the national guidelines for exercise (150 minutes a week), and exercising three-times above the national guidelines (more than 450 minutes a week).

They discovered that white men who exercised the most were 86 percent more likely than people who exercise at low levels to experience a buildup of plaque in their arteries.  

The findings surprised the researchers.

"We expected to see that higher levels of physical activity over time would be associated with lower levels of CAC," said Deepika Laddu, assistant professor of physical therapy in the UIC College of Applied Health Sciences.

Instead, she found that those who exercised the most were 27 percent more likely than those who exercised the least to develop CAC — measured by a CT scan — by middle age.

When the study's findings were divided by race and gender, the findings were even more surprising. The results showed that white men were at the highest risk — they were 86 percent more likely to have CAC. Odds did not increase for black participants who exercised at the highest level.

Researchers aren't sure exactly how dangerous the higher CAC levels actually are. "High levels of exercise over time may cause stress on the arteries leading to higher CAC," said study co-author Dr. Jamal Rana. "However, this plaque buildup may well be of the more stable kind, and thus less likely to rupture and cause heart attack, which was not evaluated in this study."

Even though the study suggests that white men who exercise at high levels may have more CAC, "it does not suggest that anyone should stop exercising," Laddu said.

The study was published in Mayo Clinic Proceedings.

According to the American Heart Association, cardiovascular diseases accounts for one out of every three deaths in the U.S.