How fast can you run a mile?

For people in midlife, this simple measure of fitness may help predict their risk of heart problems as they age.

In two separate studies, researchers from the University of Texas Southwestern Medical School and the Cooper Institute in Dallas analyzed fitness levels for more than 66,000 people. Overall, the research showed that a person's fitness level at midlife is a strong predictor of long-term heart health, proving just as reliable as traditional risk factors like cholesterol level or high blood pressure. The two reports were published last month in Circulation and the Journal of the American College of Cardiology.

In the studies, fitness was measured using carefully monitored treadmill testing to gauge cardiovascular endurance and muscle fatigue. But in analyzing the data, the researchers suggested that the treadmill results could be translated to average mile times, offering a simple formula for doctors and individuals to rate their fitness level at midlife and predict long-term heart risk.

"When you try to boil down fitness, what does fitness mean?" said Jarett D. Berry, assistant professor of internal medicine and cardiology at Southwestern Medical School and a co-author of both papers. "In both these studies, how fast you can run in midlife is very strongly associated with heart disease risk when you're old. The exercise you do in your 40s is highly relevant to your heart disease risk in your 80s."

Berry cautioned that more study is needed before mile times could be used as an accepted benchmark of cardiovascular risk. Still, he noted that the pace at which a person runs is a measure of fitness to which people can easily relate, and a good starting point for measuring overall fitness.

From the study data, Berry calculated that a man in his 50s who can run a mile in 8 minutes or less, or a woman who can do it in 9 minutes or less, shows a high level of fitness. A 9-minute mile for a man and 10:30 for a woman are signs of moderate fitness; men who can't run better than a 10-minute mile and women slower than 12 minutes fall into the low-fitness category.

The categories make a big difference in risk for heart problems, the study found: Subjects in the high-fitness group had a 10 percent lifetime risk, compared with 30 percent for those in the low-fitness group.

Berry notes that fitness varies greatly with age and gender and that mile-time estimates are just easy benchmarks for patients and doctors to begin a discussion about fitness. Overall, he said, a 10-minute mile for a middle-aged man and a 12-minute mile for a woman suggest a good level of fitness.

"The principal finding of these studies is that your fitness level when you're young is highly predictive of heart disease risk 30 to 40 years later," he said. "If we're trying to boil this down into practical implications, it's the speed at which you can run. Heart disease risk increases markedly for every minute longer it takes you to run a mile."

Timothy Church, a professor at the Pennington Biomedical Research Center in Baton Rouge, La., said more research was needed to validate the notion that a person's mile time correlates with the risk categories in the original study. But he agreed that exercise experts needed to come up with a better way to communicate exactly what fitness represents.

"You can't look at someone and judge whether or not they are fit," Church said. "What is fitness? From a risk-factor standpoint, it's about avoiding low fitness."

And he sounded another note of caution about the mile-time benchmarks.

''I'm nervous about people testing fitness on their own," he said. "I don't want a 45-year-old sedentary male to go out and run a mile as fast as he can."

Even so, Church noted that most of the health benefits of exercise come with moving from low fitness to moderate fitness, and the challenge is finding a way to communicate with and motivate people in the low-fitness category.

"You know whether you're in the unfit category," he said. "If you're physically inactive, if you sit 18 hours a day, if you get exhausted walking up a flight of stairs. If you've got a choice between walking two blocks or taking a taxi and you wait 20 minutes to take a taxi, you're unfit."