MeJennifer said:
As if people who do not eat meat are less healthy because of it.
Some are. But, no, it is not an essential part of the diet. The essential amino acids can also be obtained by combining vegetables and grains to balance those. Animal products are considered complete proteins, because you can obtain all of your essential amino acids from any single type of meat, and they are in the most digestible form for the human body. With a vegetarian diet, no single food item is a complete protein, and vegetable sources are not as digestible as meat sources, so one must be educated about what to combine to ensure they are getting adequate nutrients. Of course, proteins are not the only nutrients we require either, so we need more than just meats too.
Humans are omnivores and the structure and function of the digestive tract, not its length, is what tell us that humans are not efficient at digesting vegetables (that's why they are good sources of fiber, which means undigestible matter). We are also not obligate carnivores, so need more than just meat as well. The diet that our digestive tract has evolved to digest contains a mixture of meats, vegetables and grains.
So, one can be a healthy vegetarian if they are very careful about what they are doing, but there are also a lot of myths, such as the one the OP inquired about, that are spread among vegetarians (as well as myths spread among non-vegetarians about vegetarian diets). I would strongly suggest that anyone considering a vegetarian diet ask their physician to recommend a good nutritionist who can help them understand their dietary needs and approach their choice in a healthy and safe manner. Don't rely just on the advice of your physician...believe it or not, they do not receive much training on nutrition in medical school, so are not the best people to ask for help in formulating a diet. A registered dietician or nutritionist is a better resource.