First, there isn't much logical thinking involved in veganism or vegetarianism. At their best, they're about ethics (which I certainly respect). At their worst, they make some very dubious claims about te economics of food and the environment.

But either way, note that people make significant and probably burdensome changes to their lives when it comes to upholding morals pertaining to food. I think that more (liberal) people would go a longer way making actual life changes when it comes to food than just about anything else. They might explain that the things you actually put into your body are the things you should care about most. Only there's little evidence to suggest that's the case. It is much more probable that national policies like increasing minimum wage or reforming education would have a bigger (though indirect) effect on your personal well being than optimizing your diet (as long as it's more or less "normal").

All religions, in this case the healthy diet religion, must not rely on facts in order to gain followers; they must rely on gut feelings, because gut feelings are much more powerful than proof when it comes to motivating life changes. And we just naturally feel that ingested food has to have a major effect on our health and general well being. It just makes sense, right? But, at least in the current state of scientific knowledge on the subject, that intuition is just superstition. After decades of research there's little consensus even on what makes us fat, let alone more subtle or indirect effects diet may have on our health. In fact, it's very probable that even if one day we do know exactly what's healthy to eat and what isn't, and at what quantities, the actual effect-size on our health will be quite small. It's quite possible that within some sane bounds, what food we eat simply does not determine our health to any large degree.

Here is an example comparing the amount of water needed to irrigate various crops compared to the amount needed to raise beef.

US Corn Production: 9 gallons water / lb corn

US Wheat Production: 14 gallons water / lb wheat

US Barley Production: 26 gallons water / lb barley

US Alfalfa Production: 31 gallons water / lb alfalfa

US Beef Production: 441 gallons water / lb beef

http://www.journalofanimalscience.org/content/71/4/818.full....

Global Spinach Production: 2 gallons water / lb spinach

Global Potato Production: 4 gallons water / lb potatos

Global Banana Production: 13 gallons water / lb bananas

Global Apple Production: 18 gallons water / lb apples

Global Lentil Production: 64 gallons water / lb lentils

Global Cashew Production: 121 gallons water / lb cashews

Global Almond Production: 252 gallons water / lb almonds

http://www.waterfootprint.org/Reports/Mekonnen-Hoekstra-2011...

I could look for more examples, but this took me about half an hour already, and I think it pains a rather compelling picture by itself. The difference would be less extreme when looking at pork and poultry, but there is still a stark contrast.

But the whole point of this thread seems to be that such an analysis is hard to do.

There are so many confounding variables here that it's really hard to tell. Everyone pretty much chooses to believe whatever data they like best. For example, cultivated land is probably less environmentally friendly than pasture land.

If x = environmental impact of cultivating crops

then R*x = environmental impact of raising livestock

where R = pounds of feed required to produce one pound of meat

Now if we raised our meat differently, such as only on grass-fed pastures, then things might look a lot different. But in the US at least, factory farming and animal feed is the norm.

Moreover, it's quite conceivable (again, no proof, but there's little proof af anything in this field) that herbivores would convert one pound of corn much more efficiently than we would. This might mean that humans are better off wasting energy by feeding vegetables to animals and then eating the animals, than eating the vegetables directly. Maybe we can't extract the nutrients from the vegetables as well as herbivores do, so that seemingly wasted energy is actually a net gain.

Consuming plants is not as efficient as photosynthesizing, and consuming animals is not as efficient as consuming plants.

In this case, the cows burn energy from the food they consume for as long as they're alive, meaning it's a physical impossibility for the meat produced from them to contain as much energy as the food they consumed.

Unless you're claiming that raising livestock violates the laws of physics, but that's an entirely different argument.

Giving an incredibly quick example, wheat and ground beef have roughly the same caloric density

Wheat: http://www.fatsecret.com/calories-nutrition/usda/whole-grain...

Beef: http://www.fatsecret.com/calories-nutrition/usda/ground-beef...

But since it requires numerous pounds of wheat to create one pound of beef, you are losing overall caloric density.

http://www.fatsecret.com/calories-nutrition/kroger/gummi-bea... https://www.google.ca/search?q=40g*11.3+to+lb https://www.google.ca/search?q=140*11.3

    First, there isn't much logical thinking involved in veganism or vegetarianism.

(There are few things as useless as discussions about nutrition at Hacker News.)

Of course it's ok, and in a lot of cases, preferable, to not behave logically.

I don't eat mashed potatoes, but it's not because of some reverence for the humble potato or concern about industrialization of farming, it's just because I don't like the taste.

It's absolutely possible that many vegans are vegans due to some psychological pressure towards purity or ascetism, and they just coincidentally chose a pressure vent that also makes sense in an economic and environmental sense. I'm sure many of them might even be aware of that dualism.