Actually, it does, and that's the perfect way to analyze this. Food is good for keeping humans alive, but it's a pretty foolish fuel to burn in a human engine to accomplish things. It's perishable, it's eaten by rodents, it's hard to transport, and more. I chuckle darkly when someone tells me that something is "green" because it's human powered. Growing food isn't free - it uses conventional energy sources to harvest, transport, etc., and it's a ton less efficient than just burning that fuel directly.
