I think you might be missing the point too. Yes, the carbon in the food we eat is where the carbon in our breath comes from. But the carbon that we used to get the ingredients in that food didn't certainly come from the atmosphere (e.g. half of the nitrogen used in agriculture comes from fossil fuels). You can't be a perfectly optimal salad eating machine. One of your fellow humans will ruin the equation the moment they buy produce from the modern supply chain.