FITNESS MATTERS, FATNESS DOESN'T:

So, what's it worth to lace up those sneakers and break a sweat for about 30 minutes a day? About 3.5 extra years of life, on average -- and about 4.2 additional years for those willing to step up the intensity or put in closer to an hour a day of brisk walking or its equivalent, according to a new study.

Even for the severely obese -- those with a body mass index above 35 -- exercising for about 2.5 hours a week at moderate intensity or for 75 minutes at vigorous levels puts average life expectancy a notch above that of a normal-weight person who is sedentary, the research shows.

It's no surprise that exercise is good for you and will help you live longer. But the study published Tuesday by the journal PLoS Medicine sounds a loud wake-up call to "healthy weight" couch potatoes who believe their good BMIs will ensure them a long life.

Even for people with a BMI between 20 and 25, those who told researchers they were physically inactive were far more likely to die in the next decade or so than were overweight or obese exercisers. Among the 431,479 study participants over the age of 40, the sedentary were almost twice as likely to die during the course of the study than were participants who were highly active.

"This finding may convince currently inactive persons that a modest level of physical activity is 'worth it' for health benefits, even if it may not result in weight control," the study authors wrote.

Follow @brothersjudd