Answer:
   
   We don't know. Traditional Jews keep kosher because G-d demands it of
   us. However, we wouldn't be at all surprised if something which G-d
   demands would also be good for us.
   
   Note that vegetarian food is not always kosher: there are problems
   with cheese, vinegar, oils, grape jelly, insects, gentile cooking, and
   lots more. (No, we don't mean vegetarians eat insects. But strict
   kashrus requires careful inspection for insects.)
   
   Side note for meat eaters: kosher meat is healthier. USDA standards
   are disgustingly lenient regarding the animal's health. "Sixty
   Minutes" once did an expose on this--many kosher butchers reported a
   large increase in gentile customers. Cold-water plucking helps prevent
   the spread of salmonella bacteria, and meat from diseased animals
   cannot be considered kosher. Kosher slaughter is more humane than
   non-kosher slaughter, as it kills the animal in a painless fashion.
   Although kosher slaughter does not kill the animal instantly, the
   animal passes out from the sudden drop in cranial blood pressure and
   dies in a minute or so. There is no pain.
   
   Some Jews boycott particular foods or manufacturers as a political or
   human rights gesture. However, even if a rabbi declares a food
   prohibited in his community, although it is equally as forbidden as
   non-kosher food, this does not affect its kosher status. (For example,
   utensils coming in contact with morally-forbidden products do not
   become non-kosher.)