It isn't about what you eat, it's about getting up off your rear and doing something. You'll realize how out of shape you are if you try to run or jog just one-fourth of a mile, if you're not already in shape.
And even if a person has health problems that prevents them from doing intense or even moderate workouts, just doing some menial housework and/or stretching exercises can help.
There are fewer still who can't really do much no matter what, but those cases are usually rare. It is best to at least try, in any case.