I used to be vegetarian and nearly vegan. I did it for mostly ethical reasons and secondarily for health reasons. That was one of the biggest mistakes of my life! My health and my husband's health deteriorated so rapidly that it made me question whether vegetarian and vegan diets are appropriate for humans. It got particularly bad when I cut cheese out of my diet for 6 months, due to concerns about animal rennet being used in cheese making. I ended up doing a ton of research and concluded that I was deficient in a bunch of different nutrients. I didn't want to start eating meat again, so I tried to determine if there was a way to get the nutrients I needed by including different foods in my diet. Long story short, it became obvious pretty quickly that I would not be able to correct the problem on a vegetarian diet. I did not have access to quality supplements at the time (I was in Argentina), and I did not want to have to rely on supplements anyway. Our ancestors did not need supplements to maintain their health, so a balanced diet should supply everything we need.
In addition to the obvious nutrients like iron and B12, there are more complex relationships between nutrients that can be a big problem in vegetarian diets. For instance, zinc and copper need to be maintained in a specific ratio in the body. Many foods that vegetarians rely on are very high in copper and low in zinc, and once you're deficient in zinc in relation to copper it is essentially impossible to correct using only vegetarian foods. Red meat is one of the best sources of zinc, and it also supplies easily absorbed iron and B12. I had an issue with B12 as well - my hair started turning grey at the age of 22! After adding meat and other animal products back into my diet, my hair stopped turning grey. Vitamin A is another big issue for vegetarian and especially vegan diets, as preformed vitamin A is only found in animal products. Relying on carotenes for vitamin A can be dangerous, as some people cannot make the conversion from carotenes to vitamin A, or make the conversion poorly. And few people understand enough about carotene absorption to ensure that they are actually absorbing the carotenes they do eat. After I began taking cod liver oil, which is rich in vitamin A which is important for vision and eye health, my vision, which had been rapidly deteriorating since childhood, stabilized. I have had the same glasses for almost 7 years now.
The biggest health issues I experienced were digestive problems and infertility. My digestion had never been optimal but on my vegetarian diet my digestion basically stopped happening! I frequently experienced vomiting during this time, which always began with a splitting migraine that would not go away even with high doses of pain medication. The headaches were also accompanied by extremely painful intestinal cramps. The only thing that would stop the headache was vomiting. And hopefully this is not too much information, but during these vomiting episodes it became clear that the food I was eating was not being digested. There was the food, still sitting completely undigested in my stomach as long as 12 hours after I ate it. It was very disturbing to me.
The infertility thing was also really disturbing. I was tracking my monthly cycles as part of fertility awareness for birth control. But it quickly became apparent that I was not ovulating. I tracked 3 cycles and did not ovulate once, which meant I could not get pregnant. My hormones were that screwed up. Turns out your body uses cholesterol to manufacture sex hormones, and my diet was practically devoid of cholesterol. After changing my diet to include meat and other animal products, my cycles normalized and I began ovulating.
I also experienced debilitating depression. I had been depressed before going vegetarian but it got a lot worse. I had daily brain fog that made it really hard to think. After adding animal products back into my diet, my depression and brain fog went away. Noticing a pattern?
All of that said, simply adding meat and animal products back probably wouldn't have been enough. It was the specific kinds of animal products I included that made a lot of the difference. I began taking cod liver oil to replace fat soluble vitamins, and including homemade bone broth in my diet regularly. The bone broth was probably the thing that helped my digestion the most. It is very soothing to the lining of the digestive tract and helps digestion in a number of ways. I began eating a lot of eggs, yogurt, cheese, and butter, and including organic and grass-fed meats in my diet pretty much daily. I made these changes slowly and gradually so my body could adjust to the changes. If you aren't used to eating a lot of protein and fat, you will have a hard time digesting them. Bone broth helps a lot with that.
I do think there are some people who can be healthy on a very well-planned vegetarian diet, but my idea of well-planned is probably a lot different than the average vegetarian's idea. And there are many people who simply will not be healthy on any vegetarian diet. I personally believe, after studying nutrition extensively, that vegan diets are unsuitable for human health and I do not recommend them for anyone. Everyone is of course free to choose whatever diet they want. It's their life, so they will bear the consequences of their choices.
As far as what Thy Unveiling said about eating meat being like eating death, I have a pretty different view on that. First of all, life and death are interconnected. You cannot have one without the other, and life is dependent on death. When something dies, whether an animal or plant, it decomposes and becomes incorporated into the soil, supplying nutrients for new life to flourish. Life literally grows out of death, and death flows out of life. It's a beautiful thing.
Please note that when you eat organic vegetables, fruit, grains, or beans, those foods are grown on soil fertilized with animal manure and oftentimes with bone, blood, and feather meal. The nutrients in these products are needed to replenish the soil. I am an organic farmer so I can assure you that these products are regularly used in organic agriculture. Non-organic farming is horrible for the environment and directly and indirectly harms a wide variety of animal life, although technically the fertilizers used are made from petroleum. Please note that petroleum is actually an animal product itself. Animal products are more nutrient-dense than vegetable matter, which is why they are needed to replenish the soil. In nature, animals die and their bodies become incorporated into the soil. The organic farmer simply mimics this process using animal products made from the bodies of animals.
Furthermore, farmers (organic or not) use a variety of methods to control pests. Most organic farmers still spray their crops with pesticides and herbicides, just ones that are approved for organic. Many of these organic sprays are very harmful both to the humans that apply them and to insect life, including beneficial insects like bees. Some sprays are harmful to small mammals as well. Organic famers cannot use sprays that are as strong as the synthetic ones, so they often rely on traps for pests like gophers. These traps generally kill the pests, sometimes in particularly brutal ways. There would be no point in trapping a pest and then releasing it because it would just come back and eat your crops. Tilling a field to plant crops often kills many small animals such as rabbits, gophers, moles, etc. I have seen this firsthand. My point is that by the time the crop is harvested and you buy it at the store, quite a few animals have lost their lives to produce that food, even if the food itself is vegan. Contrast that with the production of beef. When a cow is slaughtered, only a single animal loses its life, and that animal can feed many people. Many animals lose their lives in the production of vegetarian crops, its just that those deaths are hidden from the consumer who buys the food. It's a matter of perspective.