Huh? Biking is the most energy efficient way to get around (more efficient than walking).
Which is why electric bicycles are a very efficient way to get around. But we're not talking about electric bikes; we're talking about human powered bikes. And unfortunately, the CO2 footprint per unit energy out of growing food, harvesting it, shipping it, cooking it, digesting it, and turning it back to kinetic energy via the muscles, is often ridiculously high compared to far more efficient ways of harvesting chemical energy (such as directly burning it in an ICE or gas turbine)
If a cyclist's energy comes overwhelmingly from efficient, locally grown starchy / fatty plant sources, the efficiency of a bicycle can overwhelm the inefficiency of using food as an energy source, and they can get a better CO2 footprint per kilometer than a Prius. On the other hand, that's not a typical diet. If half their calories are from beef, for example, they might as well be driving alone in an SUV.
One thing to keep in mind is the carbon cycle. Burning gas / oil / coal unlocks carbon that has been locked away for a loooooooooooooooooooong time
Are you under the impression that the CO2 footprints from food production don't?
And note that right now I'm only talking about CO2 footprints. Should we also go into the vast amounts of habitat destruction and water consumption used to produce food? Take a look at a satellite image of how much of our planet we've turned into a food-producing machine, and all of the rivers that no longer reach the ocean, or are so full of fertilizers that they make dead zones. Let's not pretend that the act of voluntarily consuming more calories (aka, exercise) is unrelated.
And exercise is good for you.
Note that my post wasn't about health. :) This is absolutely true, most people would benefit from more exercise, health-wise (although too much is also bad for you). Although cycling does put you at much greater risk of injury than driving.
Also, see this post.
It's perfectly reasonable to look at all aspects - health, injury, CO2, etc. But I find that all too many people are not only willing to ignore the negative effects of cycling or walking as a mode of transportation, but even get shocked and indignant when someone points them out (see the responses to my post for examples, including the speechless "What? No. Seriously." response).
There are good health effects for people who need more exercise. But there also are negative effects (injury, CO2, land and water use, etc), and let's not pretend that they don't exist.