At equal diet, the bicycle always win. You don't eat significantly more meat because you are using a bicycle to move vs a total couch potato that would use a Prius and eat the same.
The energy does have to come from somewhere. If you're only cycling 5km in a day the reason you don't notice the difference in food quantity is because the amount of energy used for that small amount of cycling does not really exceed the amount of energy your body uses in a day for everything else. When you're cycling 100km, it's a different story.
It's an interesting question, and in fact the conversion of food to mechanical energy isn't actually very carbon-efficient compared to electricity generation or even gasoline.
Cars are actually very efficient at what they do, it's just that what they do (hauling around a 1000kg metal box) is an inefficient way to transport a human, and that's where the inefficiency comes from.
If you fill up a large car with full occupancy and go on a long road trip, I'd venture to say it's carbon-wise likely to be more efficient than all of the occupants cycling, regardless of diet.
- 2000kcal from food = how much CO2 including all the energy needed to farm it?
If it's 2000kcal from beef, you're looking at about 72.88 kg [1]
If it's 2000kcal from fish, you're looking at about 15.21 kg [1]
If it's 2000kcal from brassicas, you're looking at about 6 kg [1]
- 35000kcal from gasoline emits how much CO2?
35000 kcal is 140440 kJ, which would consume about 4.36 liters of gasoline, which would be about 10 kg of CO2 emissions [2].
So the CO2 efficiency of a car isn't that much different, and falls somewhere in-between a biker on a fully meat and biker on a fully vegetable diet. Biking isn't vastly more efficient than a car, CO2-wise.
That said, a motorized bike is hellishly efficient, CO2-wise, and trumps almost everything else.
[1] https://ourworldindata.org/grapher/ghg-kcal-poore
[2] https://natural-resources.canada.ca/sites/www.nrcan.gc.ca/fi...
If you're going to do that, what about the CO2 (gasoline) emitted to collect the gasoline? Given you need oil to extract oil, it's still far, far, far less efficient.
You could have also Googled that number instead of trying to make a comeback for the sake of it.
I used to be an elite racing cyclist, I know what it is to need fuel in a 200km bike race
Besides, riding at conversational slower pace only need a fraction of that energy. When I was commuting 75km a day myy food intake may be at worst marginally higher than a day off.