No idea how this compares to the scale of taking a bus or a solo car, but there is an increased cost. Growing food (often with fertilizers, harvest machinery, etc) in order to bring it to the market for you to consume and convert into motion doesn't seem like it's particularly energy efficient.

I bike a lot, it's better for me physically and mentally, but I do wonder how the carbon impact is affected. (Of course, this ignores the fact that petroleum is a limited supply.)