I think that the best health indicator is longevity. Vegetarians benefit from greater longevity. Perhaps 1 1/2 - 2 years. See: http://www.ivu.org/oxveg/Talks/veglongevity.html.
They cite a study that includes thousands of cohorts and takes into account that vegetarians in industrialized countries are generally more affluent and have other healthier lifestyles.
Here is a list of vegetarians who hardly suffered from lethargy:
Carl Lewis, “Olympian of the Century,” Olympic medalist in track
Ruth Heidrich, Ironman triathlete, age-group record holder
Martina Navratilova, tennis champion
Desmond Howard, Heisman trophy winner
Stan Price, world-record holder in bench press
Bill Walton, NBA Hall of Famer
Phoebe Mills, Olympic medal-winning gymnast
Billie Jean King, tennis champion
Bill Manetti, powerlifting champion
Bill Pearl, four-time Mr. Universe and bodybuilder
Al Oerter, discus thrower and winner of four Olympic gold medals
Keith Holmes, WBC World Middleweight Champion
Robert Parish, one of the NBA’s “50 Greatest Players”
Jack LaLanne, fitness legend and media star
Edwin Moses, two-time Olympic Gold medalist in hurdles
Source: Vegetarian Times Magazine
In my spiritual organization, I have two friendly acquaintances, the ex US ping pong champion who represented the USA in a previous olympics, and a German who received the silver medal in the 1976 Olympics. These men were hardly victims of lethargy.
As to B12, dairy in the diet provides B12. Most multivitamines contain much more than the USDA requirement.
The Atkins diet works for some because taking cheap carbs and cheap sugars out of the diet will help you lose weight. A healthy vegetarian diet without white sugar, white rice, and white flour is very conducive to weight loss.
There is single lifestyle choice more powerful one can do to end world hunger and help the planet than becoming vegetarian. If you think of all the pollution caused by livestock farming (including farting cows), the energy used, the water consumed, the vegetable matter used, and the agrible land utilized, it is astounding. As I have stated, 7 times more land to feed an omnivore.
There are important ethical considerations as well that I do not want to go into.
Anyway, I would just like to underscore that going veggie is one lifestyle choice that is more healthy, more environmentally sound, and more ethical.
I know non-veggies that have virtues far beyond mine. I just want to emphasize that I think that this is a good choice.