The materialistic approach states that everything can be explained by the laws of physics, and all is caused by the interaction between the particles which compose all things. This approach is mostly accepted by the scientific community, and I believe most atheists accept it. Considering this approach, the suffering is nothing but chemical and physical reactions that occur in the central nervous system, not to mention that the animals themselves are just a bunch of particles that are not greatly different than the air itself. On the other hand, if you consider this approach, you can justify killing and torturing of humans, even on massive scales. By this approach, everything is meaningless. So how can we know what is right and what is wrong? Isn't mortality just some invention of our society to match our feelings and instincts? And if so, how can we tell what is moral and what's not? So, is suffering of animals bad? I don't think you can answer this question using rational thinking. For me, it is rational that animals are just particles that move somewhat together, and after they die the suffering they have gone through doesn't matter at all. Still, on the same basis I can conclude that it is rational to think that we too have no meaning, and I can torture someone, kill him or maybe kill myself if I feel like, and it wouldn't matter at all. I don't do it because I fell this is wrong. I don't want to be killed; I feel empathy when I see someone else is suffering and even animals suffering. When I eat meat, I don't feel any empathy towards the animal that it once was. I believe this feeling could be changed by the knowledge I acquire and the memories I have. For e.g. some people may acquire this empathy towards the animals that are eaten by them when seeing footage of how meat is produces. Others may need to go to the slaughter house itself and see it in their own eyes. Others may never be affected by those things. This is why I believe no one can determine if it is moral to eat meat or not, and therefore first issue of eating meat (1) is fully subjective, and can be change between people and even in different times in the same persons' life.
The health issue, as much as I can conclude from my own research, is highly controversial. it seems like there are health benefits in being a vegetarian or vegan, but it is also clear to me that the argument in the subject, and even scientific researches about is, are somewhat biased in accordance to the believes of the author. I did see a stronger evidence that support that there are in fact health benefits to being a vegetarian, and even more to being vegan, but those benefits are not very significant, and are probably very affected by the fact that vegetarians are much more aware of their diet and health than omnivores, and thus eat a well-balanced, healthier diet regardless to the presence of meat in their diet. There are some articles about the benefit of eating less meat, and we can also conclude it from the fact that animal products are the sole source of cholesterol in our diet, and a big source of fat, two things that are known to cause health problems such as cardiovascular diseases. In conclusion, I believe a self-aware non-vegetarian can make his diet even healthier than an average vegetarian diet (and of course by adding sport you can always be healthier). I do think lowering meat consumption is advised.
An article about vegetarian diet health implications: http://www.chiphealth.com/about_chip/documents/HealthImplicationsofVegetarianDiet.pdf
therefore, the moral issue is probably the sole issue that should be considered in becoming a complete vegetarian (I consider the economic issue less relevant, due to the controversy on its effectiveness, and the small probability it will be the main reason for someone to become vegetarian). Due to my conclusion that this issue is fully subjective and depends on the persons feelings and unproved believes, I conclude that the vegetarian issue as whole is subjective.
I myself decided to stop eating meat in Mondays for health reasons, as an opportunity to reducing meat consumption and raise vegetables and fruits consumption.
(Note: I wasn't quite sure whether this warranted a high level post or just a discussion. I haven't made a high level post yet, and wasn't entirely sure what the requirements are. For now I made it a discussion, but I'd like some feedback on that)
I've been somewhat surprised by the lack of many threads on Less Wrong dealing with vegetarianism, either for or against. Is there some near-universally accepted-but-unspoken philosophy here, or is it just not something people think of much? I was particularly taken aback by the Newtonmas invitation not even mentioning a vegetarian option. If a bunch of hyper-rationalists aren't even thinking about it, then either something is pretty wrong with my thinking or theirs.
I'm not going to go through all the arguments in detail here, but I'll list the basic ideas. If you've read "Diet for a Small Planet" or are otherwise aware of the specifics, and have counterarguments, feel free to object. If you haven't, I consider reading it (or something similar) a prerequisite for making a decision about whether you eat meat, just as reading the sequences is important to have meaningful discussion on this site.
The issues:
1. "It's cruel to animals." Factory farming is cruel on a massive scale, beyond what we find in nature. Even if animal suffering has only 1% the weight of a humans, there's enough multiplying going on that you can't just ignore it. I haven't precisely clarified my ethics in a way that avoids the Repugnant Conclusion (I've been vaguely describing myself as a "Preference Utilitarian" but I confess that I haven't fully explored the ramifications of it), but it seems to me that if you're not okay with breeding a subservient, less intelligent species of humans for slave labor and consumption, you shouldn't be okay with how we treat animals. I don't think intelligence gives humans any additional intrinsic value, and I don't think most humans use their intelligence to contribute to the universe on a scale meaningful enough to make a binary distinction between the instrumental value of the average human vs the average cow.
2. "It's bad for humans." The scale on which we eat meat is demonstrably unhealthy, wasteful and recent (arising in Western culture in the last hundred years). The way Westerners eat in general is unhealthy and meat is just a part of that, but it's a significant factor.
3. "It's bad for the environment (which is bad for both human and non-human animals)." Massive amounts of cows require massive amounts of grain, which require unsustainable agriculture which damages the soil. The cows themselves are a major pollution. (Edit: removed an attention grabbing fact that may or may not have been strictly true but I'm not currently prepared to defend)
Now, there are some legitimate counterarguments against strict vegetarianism. It's not necessary to be a pure vegetarian for health or environmental reasons. I do not object to free range farms that provide their animals with a decent life and painless death. I am fine with hunting. (In fact, until a super-AI somehow rewrites the rules of the ecosystem, hunting certain animals is necessary since humans have eliminated the natural predators). On top of all that, animal cruelty is only one of a million problems facing the world, factoring farming is only one of its causes, and dealing with it takes effort. You could be spending that effort dealing with one of the other 999,999 kinds of injustice that the world faces. And if that is your choice, after having given serious consideration to the issue, I understand.
I actually eat meat approximately once a month, for each of the above reasons. Western Society makes it difficult to live perfectly, and once-a-month turns out to be approximately how often I fail to live up to my ideals. My end goal for food consumption is to derive my meat, eggs and dairy products from ethical sources, after which I'll consider it "good enough" (i.e. diminishing returns of effort vs improving-the-world) and move on to another area of self improvement.