People with vegetarian or vegan diets are generally healthier than the average omnivore. However, this is not to say that eating meat is bad for you in and of itself. In reality, the problem is that the average diet is disproportionate to the nutritional needs of the body, and vegans and vegetarians are more health conscious than the average person.
When you compare health conscious omnivores and vegans, there is less disparity. General mortality measures are similar in both groups, according to a study by Professor Tim Key. Generally, lacto-ovo vegetarians (those who eat eggs and milk) are the healthiest subset. Their diet has an appropriate balance of nutrients, and their increased vegetable consumption decreases cardiovascular and cancer risks.
There are a few areas where a vegan diet can lead to deficiency. Because of the lack of any meat and dairy, vitamins B-12 and D, calcium and long-chain n-3 (Omega-3) fatty acids may be lacking. For some of these, there are vegan options which can make up for the deficiency. However, for vitamin B-12, fortified foods are a must to obtain adequate levels. B-12 deficiency can lead to psychoses, disorientation, dementia, difficulty with concentration and other mood disturbances.
This analysis of health is most valid when considering western vegetarians and vegans. For example, when considering Indian vegetarians living in the UK, obesity is common. It’s important to realize that the result of vegan and vegetarian health is not so much from the exclusion of meat, but more from the proper inclusion of vegetables. As in all things, proper balance yields a healthier life.
Health Effects of Vegan Diets: http://www.ajcn.org/content/89/5/1627S.full.pdf
Health effects of vegetarian and vegan diets: http://www.martinfrost.ws/htmlfiles/july2008/epic_veg.pdf