Since I had absolutely no evidence to back up my earlier statement that vegans are nutritionally challenged, I asked my friend Matt, who is in his first year of medical school at Yale and, therefore, an expert on everything. Here were his musings on the matter:
Good health is certainly achievable on a vegetarian and is just a bit more difficult on a vegan diet. The only thing that gives me reservations about the vegan diet, is the number of foods you're limiting yourself to.
As a staunch meat advocate (sustainability and ecological difficulties acknowledged!), I will be the first to say that a healthy diet should contain mostly plant-derived food. On the other hand, the best diet in the world is the most varied. Just as you should try to eat a rainbow of colored plant foods (orange, red, green, blue, etc.) to get the necessary array of nutrients and vitamins, there are several vitamins, such as B12, which are difficult to find outside of meat or animal-derived sources.
Luckily, today we have vitamin-fortified cereals and supplements - but still, the medium in which a vitamin occurs will alter how readily it is absorbed by the body, which has led many experts to question the nutritional benefits of a tablet multivitamin. As for complete protein (which you probably know, means it includes all essential amino acids which our bodies can't produce themselves), there are plenty of combinations of plant foods which will give sources of complete protein if eaten together (beans and rice, etc.).
(This email has already broken my favorite guideline of writing - never use 8 words to say something you can say with 4...so I'll briefly sum up my somewhat obscured point now...)
A vegan diet, when properly planned out, can be healthy for an adult (and only an adult). The problem is, this requires time and money that most people don't have when it comes to meal planning. As a result, many vegans end up eating a very limited set of foods, which is unhealthy.