A new study shows it's never too soon to get fit: People who are in good shape as young adults — mostly in their 20s — are much less likely to die in middle age.
And it didn't matter too much if the people in the study were overweight. Those were who fit and fat were also less likely to die over the next quarter century, said Dr. Venkatesh Murthy of the University of Michigan, who worked on the study.
The effects were clear and easy to track. The longer a person could push on a treadmill test as a young adult, the less likely he or she was to die of anything, including cancer and heart disease, 25 years later, the researchers found.
"Every additional minute you could do added another 15 percent lower risk of dying," Murthy told NBC News.
That held true regardless of weight, cholesterol, blood pressure and blood sugar levels, Murthy's team reported in the Journal of the American Medical Association's JAMA Internal Medicine.
"No matter how we looked at it, we saw essentially the same findings, which is for every minute you could do more on the treadmill when you were a young, that was associated with a 15 percent lower risk of dying over the next 25 years."
The team looked at a group of 4,800 volunteers who have been taking part in a long-term heart health study. Back in 1985 and 1986, when they were aged 18 to 30, the volunteers were tested for cholesterol, high blood pressure and blood sugar, weighed, and took a treadmill test.
These are specialized treadmill tests, Murthy noted - not like just running or walking on a treadmill at home or on the gym. People are pushed to their maximum exertion in these tests and on average, the volunteers could go for about 10 minutes, flat-out. "It's kind of like running uphill," Murthy told NBC News.
The volunteers, from Birmingham, Alabama; Oakland, California; Chicago and Minneapolis were followed annually for 25 to 27 years.
"There's no single threshold (of fitness) where we can say that's good enough," Murthy said. "The more fit you are, the more you lower your risk of developing cardiovascular disease."
It's not an excuse to slack off once you hit 30, Murthy cautioned. Seven years later, 2,400 of the volunteers took a second treadmill test. If they had lost fitness over those seven years, they paid for it.
For every minute less someone could stay on the treadmill from their first test in the 1980s, his or her risk of dying over the next 25 years went up by 21 percent, and the risk of heart disease went up 20 percent, Murthy's team found.
They found something else surprising, too. One of the standard tests for heart disease looks for calcium in the arteries. Calcium is found in the artery-clogging plaques that slow blood flow and which can break off and cause a heart attack or a stroke.
The more calcium a doctor can detect in someone's arteries, the more likely he or she is to develop heart disease - or so the common medical wisdom goes.
But this team found no association between calcium in the coronary artery and heart disease or death in their volunteers.
"What is really surprising here is that we would have expected that fitter people would have had a lower chance of having these plaques in middle age," Murthy said.
But his team found that people were equally likely to have calcium, regardless of how fit they were.
"So something else must be important - maybe diet, maybe genetics," Murthy said.
They found something else less surprising. The less fit people were, the weaker their hearts were in contracting, and the more likely they were to have thickening of the heart muscle that's associated with heart disease.