I eat a relaxed primal diet that is very low-carb. I am allowed to break rules whenever I want but I don't want to.
On the "global sustainability" issue: I think that grain-based diets result in malnutrition, and I don't think that volunteering to accept malnutrition for my children helps anyone with food shortages elsewhere. I think that food production should be locally based. If you are talking global food, then you are talking unsustainable transportation and fuel (and fertilizer) issues to shift resources to other parts of the globe. Grain production takes a tremendous toll on the long-term ability of the earth to keep producing food. Agriculture is a lot more complex than diet for a small planet and vegetarian social positions suggest. Responsible land use would not increase field crops in preference to pasture, but would keep them in a balanced combination that generally favors pasture. Overpopulation simply does not justify me eating substandard foods if I have any choice at all. That sacrifice give only short terms gains at the expense of land and fuel losses anyway. I will eat "starvation rations" such as flour-based foods only if I have to, not to prove my ethics are superior.
I am so flicking sick of the cultural bias for vegetarian foods, the one that suggests responsible eating means plant foods and grains are better than animal products. Totally over it. I do not need to defend myself nor do I ever feel guilty about food whether I eat steak or bacon or chocolate cake. I have lost 20 unexpected pounds in the past 4 months after one year on my primal diet and feel awesome. One of the most important things I have gained from this diet is the disappearance of all munchie urges and the desire to have second helpings. I just don't overeat anymore. Overeating makes a far bigger impact that meat eating and American overeat constantly. I notice it is absolutely normal to eat meals 2-4 times larger than our bodies actually need for instance. And grains and sugars--guess what they do? They create blood sugar issues that make us want to snack and overeat. That's seriously bad for the 7 billion. Grain and sugar and avoiding fat are the reasons IMO that Americans are overweight and eat too much anyway. When I am in a public place more than 90% of adults are noticeably overweight everywhere I go. But then again grains are used to make cattle fatter fast with quick bulking so what do we expect? I on the other hand eat tiny high fat meals filled out with homegrown veggies and favor local pastured eggs as my #1 perfect food and have completely lost the habit of overeating... Proponents of "sustainability" need to really get over trying to make people like me feel guilty. The ethics of the veggie folks simply aren't one-size-fits-all. And less meat is NOT the first and foremost way to eat healthier or more responsibly and please stop trying to put that assumption on me. I'm just not interested.
I do hope some veggies learn a bit from what I wrote here though. That anti-fat anti-meat dogma is awfully strong and absolutely pervasive and I think it's time for you to question your assumptions enough to realize how casually so many toss around judgments and assumptions about this subject. Who needs to "prove" sustainability about whether they eat what nourishes their bodies or what makes their bodies go into foggy decline? Gee whiz. I don't even worry that much about the ancient biology of it, beyond getting the basic idea, since the plain fact is that my own present-day experiments are so darn convincing I don't even care about how precisely accurate each component is compared to my ancestors' diets.