Being young, fit, eating right—and even being a fitness guru—isn't a guarantee against having a heart attack.

Biggest Loser host Bob Harper, 51, shared this week that he is recovering from a heart attack, despite his commitment to a healthy lifestyle.

Being in shape and eating right lower the risk of a heart attack, but "physical fitness and a heart healthy diet do not confer immortality," Dr. Prediman K. Shah, a cardiologist at the Cedars-Sinai Heart Institute, told USA Today.

About 3% of US men and 2% of US women have heart attacks between 40 and 59, according to the American Heart Association. Shah says many young heart attack patients had no warning signs and weren't screened for the most common cause of heart attacks—coronary artery disease. That's when fat, cholesterol, and other substances build up in arteries. You can't tell who has it just by looking at them, either.

"A lot of people think if they exercise rigorously and they appear fit, that they don't have to worry," Shah says. "What matters are the insides, and they don't always match the outsides."

• Family history, especially if you have a father who had heart disease before age 50 or a mother who had it before age 60
• High blood pressure
• High cholesterol levels
• High blood sugar or diabetes

A healthy lifestyle doesn't make a heart attack impossible, but it still makes a big difference: 50-60% of heart attacks can be prevented through optimal diet, exercise, and available medications—so staying on top of those risk factors is important.

So is understanding your individual risks and numbers (blood pressure, cholesterol) and talking to your doctor about screening.

The Cedars-Sinai SHAPE (Society for Heart Attack Prevention and Eradication) program recommends a coronary calcium scan (CCS) or carotid artery intima-media thickness (CIMT) test for men 45 and older and women 55 and older if they have one or more of the risk factors. People with multiple risk factors should talk with their doctor about having a scan even sooner.