Someone who doesn’t eat meat products. Vegans don’t eat any animal products
One issue is vitamin B-12 deficiency, which is a necessary nutrient found only in animal foods. Vitamin B-12 pills are poorly absorbed. No 100% vegetarian population exists, except for India which eats lots of dairy.
Protein is okay in vegetarian diets, but Zinc is low. The recommended dietary allowance for Zinc is 11 mg daily, but due to anti-nutrients in seeds, the vegetarian requirement is 15 mg. Vegetarians usually do not consume enough zinc.
But it has health benefits over the standard diet because we eat less fruits and vegetables. Anthropological/nutritional evidence shows that we should eat mostly plants with some animal products. Our hominid species evolved to eat red meats, but only during successful hunts and probably not daily.
Vegetarians choose this diet to avoid causing pain to animals. However, the U.S. Humane Slaughter Act requires animals to be painlessly knocked unconscious before slaughter. Rubbing the belly of a crab knocks it unconscious before cooking. Oysters arguably feel no pain. Fish poisons such as Jamaican Dogwood are non-toxic to humans but painlessly sedate and kill fish.
Vegetarians also believe that it takes more energy to grow meat than to grow plants, but this isn’t true for grass fed meat. Fish farming can be more sustainable than extra plants for vegetarians if managed to avoid overfishing. Oyster farms are good for ecosystems.
I choose to be a vegetarian because it's morally, nutritionally, and environmentally superior to a meat-based diet.