No offense to Leland, but he's very very skinny and he's one step off being a vegan from his vegetarianism. He may have a very high metabolism like I do, but I think he'd look slightly healthier (little bit more weight) if he ate meat... as an example.
Just a thought on that, James: Hardly. Le's 5'5", and his weight is well within healthy proportions for his height. He's slim, sure, but he's not "very very skinny" or sickly or anything. He also has a legitimate excuse for not using milk products: he's very lactose intolerant (a development that occured in early childhood, well before he was vegatarian).
I'm going to argue on a vegan diet, because I'm personally vegan, but most of this would apply to vegetarians too, less so to ovo-lacto vegetarians
The thing about being vegan (or vegetarian), is that you really have to do it properly, or it won't work. If you aren't completely conscious of the health risks and how to avoid them, anyone who attempts a veg diet is going to end up with iron definciency or very low levels of B12, or many other problems. It's so important to be completely aware of how to be a proper vegan (vegetarian).
The main health concerns that most people have about a vegan diet are: protein, calcium, iron, vitamin B12, and eating disorders (I'll explain that one in a minute).
For protein, there is a misconception that plants do not contain "complete" proteins. Most people believe that vegans would have to combine foods to get a full protein, but this is not true. Plants have the "complete" proteins needed in most common and easy-to-find vegetables (such as potatoes, rice, wheat and theoretically, corn, though the common corn most people eat is so different from actual corn that it contains hardly any nutritional benefits). There's a theory that animal proteins actually cause a higher chance of osteoporosis, because they contain sulfur (vegetable proteins, on the other hand, do not).
Calcium is very easily gained from leafy green vegetables (cabbage, spinach, kale, etc.), almonds, soy products, dried fruits, beans and many other things. Non-vegans should really be eating a lot of those things anyway.
Iron is one of the bigger ones that people point out in vegan diets. Iron deficiencies are actually often misdiagnosed by people themselves. Only doctors can diagnose an iron deficiency; it is a very serious disease. Vegans actually tend to have higher intakes of iron, it's just that plant iron is not quite as easily absorbed. Consuming a lot of vitamin C (which is something that most vegans consume a lot of) aids in iron absorption. Cooked spinach, soybeans, chickpeas, as well as dried apricots, raisons, fortified cereals, are all good sources of iron.
Vitamin B12 is very hard to get in a vegan diet naturally. There is no actual, nonfortified plant source of B12. However, many soy products are fortified with vitamin B12, and there are supplements available (Le and I take a sublingual B12 supplement called Vegan B12 by Veglife - more than 16000% over your daily necessary intake of B12! Although we do have to have it shipped to us from a friend in BC, it's worth it.)
Many people believe that veganism is linked to eating disorders, but it's actually the other way around. Many people with eating disorders (particularly anorexia) will claim to have a vegan diet to avoid eating. Most vegans will put effort into finding something to eat rather than refusing to eat altogether.
A lot of people are turned off of veganism because they perceive it as being too difficult, having to constantly check labels for things like casein and whatnot. It can be difficult if you're unwilling to learn which products are okay to eat and which are not. Things become easier after a while, once you begin to remember and don't have to check labels so much.
I'm vegan for mostly ethical reasons, which is probably the more common. The most common argument against this is the theory by Steven Davis which claims that more animals are killed by harvesting crops than in slaughterhouses. Gaverick Matheny offers a
better rubuttal than I can, but I'll give you the summary: "Davis’s argument fails on three counts: first, Davis makes a mathematical error in using total rather than per capita estimates of animals killed; second, he focuses on the number of animals killed in production and ignores the welfare of these animals; and third, he does not count the number of animals who may be prevented from existing."
Veganism is not an unhealthy diet by any means. It would be nice if the world were totally vegan, but I don't see that happening any time soon. You all can eat your meat, and drink your milk and eat your eggs, because that's your choice. Being vegan is mine.