I'm a vegetarian (as of September this year), because after reading one of Peter Singer's books, I couldn't deal with meat anymore (not even fish). While I am ethically okay with my choice, it is a pain in the ass socially; I do not want to impose my choices on others, and I hate being that person, the one who no-one wants to have over or go out for dinner with because they are annoying about food. I also worry that for some, my choice will imply judgment on their choices, and I don't want that; everyone needs to do what they need to do to stay happy and healthy, and (particularly that latter bit) for people with many medical conditions that implies a diet which includes meat as a source of protein.
Which brings me to vegan activism: It is a form of able-bodied privilege. If you read (for example) this page, it states that 'a properly planned vegan diet is healthier than the average American diet' and that 'it is very difficult [for a sensible vegan] not to get enough protein'. Ironically enough, that page also recommends B12 supplementation, and while B12 can now be artificially synthesized, I would lay money that the synthesized form was tested on animals).
Back to the claims on Vegan.org, though, which as vegan activist sites go is fairly mild: both the claims listed there (and a number of others I have chosen not to address) rely on the assumption of an able body to hold true. We don't normally think of able bodies in terms outside of motor functions, neurology, and the senses working adequately, but none of those systems is the one most affected by veganism. If you have (as I do) a digestive tract that does not work correctly, not only will a vegan diet not nourish you adequately, it may actually kill you. Patients with bowel diseases need more protein than the average bear, particularly when they are ill. This on its own would be no problem, but when you combine it with the type of protein readily available in the vegan diet, it becomes a problem. Those with active bowel disease often need to be on a low-residue diet, that is a diet that won't leave too much stuff in their poo to irritate them, or block up narrowed areas in their intestines. Fibre (as is foind in legumes) and nuts and seeds are all potentially harmful (and in very rare cases, fatal) for many bowel disease patients with active disease. This leaves only highly processed soy protein as a protein option to fulfil those extra protein needs which (in addition to being a very dull diet for patients with high protein needs and risking taste fatigue and thus further weight loss) may be a cancer risk (it is also an allergen, which would leave some people completely unable to use it).
Now, as I have said, I do not judge others for their choices about their diet. Like Jill, I believe that it is best to eat things that are humanely farmed, and I personally will only eat free range eggs, but I also recognise that veganism (and to a certain extent vegetarianism) rely on economic privilege as well. I also recognise that for some, their health is a barrier to a meat-free diet; this is likely to be true for me in my lifetime. Having said all that, I respect the choices and commitments that vegans make, but vegan proselytizing, particularly the kind where everyone should go vegan and consequences be damned, is a form of able privilege, and I am here to call it out.