this is a question that i personally have wrestled with. for almost 3.5 years, i was eating mostly vegetarian, and at several points was or was nearly vegan. the thing is, i never chose that lifestyle for the moral/animal rights reasons. it was mainly for the purported health and ecological/societal benefits. i realized a while back however, that i had mainly rhetoric to back those reasons, and not too much science.
i did some reading and research (albeit mostly from the net), and i concede there remain many differing views (some obviously erroneous or suspect). but i have come to the conclusion that vegetarian, and esp. vegan, diets are simply not anywhere close to "optimal" or "natural" diets for human beings. it's probably even detrimental to your health in the long term.
the vegetarian arguments about the cruelty and unnaturalness of modern industrialized agriculture and also its environmental detriment are true of course, but that in itself should not be a rationale for vegetarianism, as many adherents will try to argue (as i myself once did). it should instead be a rationale for organic agriculture and natural, pasture-fed, free-range livestock.
remember, grains and legumes were introduced into the human diet only about 10,000 years ago with the neolithic agricultural revolution - a mere heartbeat in our evolutionary timeline. that's one reason why gluten, soy, and peanut allergies are so common. it also explains why we have only ONE hormone, insulin, to lower blood sugar, whereas there are numerous hormones (glucagon, epinephrine, GH) that convert other sources of energy (protein and fat) into blood glucose. this indicates evolutionary redundancy, ie, it was always more necessary in our evolution to utilize non-carbohydrate-dense foods.
one should also be mindful of the omega6
mega3 fatty acid ratio in ones diet. the ideal ratio, as seen in existing or documented hunter-gatherer societies, was usually no higher than 2:1. but because of modern grain/legume-heavy agriculture, the use of processed vegetable oils, and the feeding of grains to livestock (which should ideally be grazing on pasture), the american diet has a ratio of around 10:1 to 20:1 or worse.
the only way for a vegan to get enough O3 in the diet would be to consume foods like flax and walnuts, and also take a DHA supplement from algae. that last aspect would be the most important, as flax and walnuts only contain ALA, not the more vital long-chain O3's like EPA and DHA. the body has a difficult time converting enough ALA into the latter two, and this process is hampered further by an imbalance of high amounts of O6 in the diet.
anyway, after sifting through all the info i've had to sift through, i recommend one search for an article called "myths of vegetarianism" to explain more simply or summarizingly what i've tried to say. (it contains errors though, such as its information on vit b12 in hindu diets - it CAN come from topsoil bacterial sources although not the ones in our digestive tract as he correctly states).
also, there is
www.beyondveg.com - they're quite ardent and perhaps biased pushers of paleolithic diets, but the site has lots of info and links to studies and such. it's also specifically from the perspective of former vegetarians.
and definitely look into the omega-3 issue. that was one of the more convincing reasons for me to stop being vegetarian - not to mention i just wasn't *feeling* healthy from eating that way for all those years.
if you're trying to be vegetarian for moral reasons however, the above arguments won't help much. but MHO is that if you're doing it for moral reasons, vegan is the only way to go. lacto-ovo vegetarians do not fully free animals from cruelty or slaughter - even free-range animals. hormone-free milk requires the mother to give birth. what is one to do with the males? same with chickens.
ok... i've written enough. i just thought this was a good opportunity to share my recent revelations with people.