I'm often fascinated by how people obsess about diet and follow all the latest fads without question. I consider it a sign of stupidity. For a while, there was the Atkins diet, for example. But Atkins died. I'm pretty sure it's because of his eating plan. Now again, recently, the high-protein diet fad has re-emerged (it went to ground for a while), and again, people are shovelling in the meat and being told by doctors, to not take starch. I consider this horribly irresponsible and stupid for a number of reasons.
(1) Ethical. Meat is murder, especially the more conscious or intelligent the animal. If you think about it, Westerners are leery about eating dogs and dolphins. Why? Because they're intelligent. But they don't think much of cows, chickens or fish - because they're not intelligent. This is my intuitive understanding of our superstitions about which animals we eat. On the other hand, pigs are as intelligent as dogs, so we technically should be reluctant to eat pigs, too. But we're not. So I think, really, that it's got to do with tradition. Outside of Judaism, in the West, eating pigs is traditionally acceptable, so we do it. If we look at various cultures, they have different types of animals that they consider acceptable to eat. Indeed, some cultures consider eating humans to not be 'disgusting'. So I think there actually is no dietary truth here. The recommendation I'd like to suggest is that we base our meat-eating practice not on tradition, but that we reject our existing traditions, and ask just how sentient the animal is. I'd like to suggest that sentience is an increasing curve, and that within any particular order of animals (mammalia, reptilia, gastropoda, insecta, arachnida, crustacea, aves, etc.) that there is a grade of intelligence, and that we should avoid eating creatures on the higher-end of the intelligence scale PER ORDER, simply because they are more sentient, and therefore capable of greater suffering. This means, for example, that squid and octopus are forbidden, because they're actually very intelligent, whereas, for example, bugs might not be forbidden, because they're almost automata. In mammalia, no chimps, monkeys, dogs, etc., but maybe buck or anything dumber than that. What makes mammalia tricky is that smaller animals like mice and rats seem to be very intelligent compared to much larger things like sheep, so it is hard to tell whether they really are stupider or not. I'm contemplating giving up eating mammalia altogether because of this problem. Broadly speaking, I'd say that the orders of intelligence are probably something like this: insecta/arthropoda/arachnida/crustacea (dumbest), gastropoda, pisces, amphibia, reptilia, aves, mammalia. IE evolutionary age gives a rough indication of intelligence, with more recent phyla being more intelligent.
(2) Environmental. The more you encourage people to eat meat, the more rainforest etc., has to be levelled to make cattle farms. Then their flatulence attacks the ozone layer and contributes further to global warming. So that, at least, is a scientific reason why not to eat meat.
(3) Dietary arguments. Some writers claim that eating meat promotes a variety of ailments or dangers to health, e.g. claiming that it might promote cancer or whatever. These arguments often also take the form of claiming that our dentition or intestines are not like those of carnivores, and therefore, that we are not "meant" to eat meat. I have two responses to this. Firstly, it's a naturalist fallacy: that whatever nature 'designed' is how things are "meant" to be. Moreover, it's teleological, which is unscientific. That's why I've put quotes around "meant" and "Designed".
I can give a few examples of phenomena that seem to be natural functions, and point out how humans violate them. Consider excretion. Consider how that is a natural function. Now consider how some subcultures, e.g. homosexuals or certain fetishists, make use of those functions. Are we to prohibit such persons from practicing their lifestyles because it's putatively 'unnatural'? I do not think so. Piercing your ears is unnatural. Driving a car is unnatural, and causes lots of deaths.
Another example of this kind of thing follows. Chimps are genetically very close to us (98-99%). Yet they've been known to hunt with spears, and are omnivorous, as are baboons. So we could argue that since in nature, chimps are omnivorous, so should we be. Moreover, ancient palaeontological evidence shows that humans practiced carnivory probably before they could speak, so it follows, if we accept the argument from 'nature is right', that we ought to eat meat. But this is just another naturalist fallacy.
Comparitive anatomy is a bad argument. The 'dentition' argument, for example, strikes me as particularly fallacious since gorrillas are vegan and have the same dentition as us (but massive canines), which suggests, if canines are for eating meat, that they ought to be meat eaters. Likewise, ruminants lack top front teeth except molars and have four stomachs, so if we were meant to be plant eaters, I could argue, we should have four stomachs and no top front teeth. So the gut structure/dentition argument is fallacious or tenuous at best. One might as well say, well, our legs are very different to a leopard's, and leopards climb trees, therefore we ought to not climb trees. That kind of comparitive argument is nonsense. Chimps also don't fly planes, so we ought to not fly planes, too, then. The problem with the naturalist fallacy is it assumes the truth of the design hypothesis - that we are designed. We are not designed. We are the accidental products of a process of evolution in which our bodily features either helped us reproduce, or did not prevent us reproducing. In cases where a bodily feature prevented a creature from reproducing, that creature went extinct. Therefore, since humans have been omnivorous since the start of recorded time, it follows that omnivory is irrelevant to our survival as a species, and that our bodies are “designed" to cope with omnivory. Gut length, dentition, etc., is an accident of nature and genetics, which doesn't dictate what we "should" or "shouldn't" do; our dentition and gut dictates merely what is more or less efficient for our bodies to process as a food source. Some food sources are better than others. For example, we can't eat paper, but termites and cows, technically speaking, can, because they have bacteria in their guts which can digest cellulose, which is an isomer of sugar. We don't. So paper isn't a good food source for us. That's the only kind of relevance our gut and dentition structure has. I mean, my molars are good at crushing sweets; does that mean I must eat only sweets? Our wrists are "designed" for brachiation. That seems to imply that we should swing through the trees rather than walk. That is obvious nonsense.
Humans are a mish-mash of features that have resulted from evolution. The fact that we still exist, six million years after splitting from the chimps, shows that omnivory is not harmful to us as a species, or unnatural. It shows, in fact, that omnivory is correct for us, since we still exist. Only if omnivory drove us towards extinction (i.e. the world population started drastically decreasing), should we start to reconsider the scientific merits of the practice.
I think the only kind of argument from dietary appropriateness would follow from a scientific study of the effects of percentages of meat consumption, and I'm sure such studies have been done. Here's my requirement for a legitimate study. Put 1000 people or more on four different diets (Western - the control -and Atkins, Vegetarian, and Vegan - the experimental diets). Then see what their health is like after a year. So, if, for example, it was found that a mostly-meat diet correlated highly with bowel cancer, then we could conclude that a lower quantity should be consumed. If we noticed that only people on the Western diet got obese, we would know that that correlated with obesity. What needs to actually be established, scientifically, is what the actual required daily amounts are for all forms of foods (and note: protein does not mean 'meat', it means nitrated organic compounds). For example, most people take vitamin supplements, on the assumption that it's necessary. It is not. Unless you have scurvy or another vitamin-deficiency related disease, you have no reason whatsoever to take extra vitamins: you're just making, as I heard a doctor put it, "expensive pee".
It is for these considerations above that I consider the ethical and environmental arguments to be much stronger than dietary arguments for reducing meat consumption.
(4) In my view, I am convinced of only one thing relating to meat consumption and health. Some varieties of arthritis and kidney stones are caused by buildups of uric acid crystals, which are byproducts of protein digestion. It seems to me that adults should therefore simply reduce their protein intake from ALL sources. There are also plenty of plant proteins that do not suffer from the above problems (1-3). Lentils, gluten and soya being obvious examples, but there's no proof either way whether they would also cause buildups of uric acid if eaten in excess.