I never really understood why vegetarianism is considered to be a good thing, so I thought I would look at some of the pros and cons of vegetarianism. In the end, this turned into an essay about why vegetarianism is almost always a bad idea; but I looked for some positives too.
My initial thought of why vegetarianism isn't a good idea is that humans are designed to eat meat. Meat is a powerful source of nutrients and energy, and the adaptation of meat into the diet of our ancestors directly created a boost to our strength and intellectual capability; humans may not exist as a species if we were not hunters and meat eaters. Our bodies are optimised to eat meat, fish, eggs and any other sort of nutrient, and this is an important factor which separates humans from many animal species.
A diet containing meat is neither more or less healthy than a vegetarian diet. Vegetarianism, or veganism, can be as equally healthy as eating meat, but this takes more work and health monitoring, perhaps including vitamin supplements which themselves have an environmental cost. Neither eating meat or abstaining from eating meat can be conclusively shown to be healthier than the other. Meat includes fats and cholesterols which are broadly less healthy than plant-based alternatives, and plant-based proteins are generally more healthy than meat based proteins, but most studies indicate that health factors other than diet far outweigh types of diet. This is logical, as all humans generally require the same calories and nutrients, and our bodies have adapted to extract that which we need efficiently. There may be risks associated with processed meat which may contain additives, chemicals, drugs; but there are also additives, pesticides/herbicides in non-meat foods. Based on this logic and evidence to date, becoming vegetarian on health grounds is not justified.
A third argument for vegetarianism is animal welfare, that if nobody ate meat, animals would generally be happier and/or treated better. This doesn't make sense as an argument; cows, for example, have been a domestic species for millennia and are now dependent on humans. Of course, we use cows for milk and dairy products, leather, and other products. Farmed cows generally live a happy and stress-free life until the moment of their death, which is, hopefully, performed with care and compassion. Wild animals, by comparison, find each day a struggle for survival, and will generally have a worse quality of life (and death) than farmed animals, so the general happiness of animals cannot be an issue. In the world there are almost no wild cows, few wild sheep or goats, and few wild chickens. These animals exist in large numbers only for human utilitarian purposes. If these animals were not eaten it is highly probable that they would become extinct as species, or considered pests, even if any wild variations of these domesticated species could be introduced.
When it comes to campaigning for animal welfare, meat eaters have more influence than non-mean eaters. Active participants in a system are better placed to reform it, and meat consumers, through their consumer power, are more able to affect positive change within the meat industry than non-meat eaters. Becoming a vegetarian to improve animal welfare standards is akin to becoming an atheist to help reform the Catholic church.
Another argument for vegetarianism is the impact of meat production on the environment, but it is likely that meat production has the same environmental impact as the production as any other food. All humans need the same amount of energy and same nutrients to remain healthy. The energy cost of these, whether from plant based, meat, fish, algae, or any artificial means, would be the same to create; so on a mathematical basis, no food type would be better or worse for the environment. Different animals and different crops have differing local or even global impacts. The methane produced by farmed cattle or pigs, for example, can contribute to greenhouse gasses. This should be taken into account, but all foodstuffs have different by-products and requirements, and it is difficult to prove that one is objectively worse than another for the global environment. Regarding the environmental cost of food, the quantity of people on earth who need feeding is a far greater problem than the type of food we consume. Becoming a vegetarian on environmental grounds is not justified.
There is also a moral argument about eating once-living things, or intelligent, or potentially sentient beings. Of course, plants are alive too, and perhaps it is insulting to carrots and wheat to consider their lives less important than cows or chickens, yet, I digress. Perhaps the intelligence of ones food is a moral point. Is it better to eat a less intelligent species like a sheep, compared to a more intelligent species like a dolphin or octopus? Is that too judgemental by us, to label the sheep as inferior and the octopus as superior? Would not the ability of a species to avoid capture determine its intelligence? Perhaps survival is the very nub of what intelligence is for. If cows are ensuring the survival of their species by serving humans by being eaten, are they not the intelligent ones?
Aside from digestive disorder, food intolerance, or allergy; which can apply to any type of food, the only argument against eating meat I can certainly determine is taste preference. It is somewhat ironic that lots of 'meat-substitute' products are created to emulate exactly this.