http://dietary-supplements.info.nih.gov/factsheets/vitaminb12.asp Dietary Supplement Fact Sheet: Vitamin B12
Office of Dietary Supplements • NIH Clinical Center • National Institutes of Health |
http://www.vegsoc.org/info/b12.html Vegetarian Society Information Sheet: B12
http://www.mayoclinic.com/health/vitamin-B12/NS_patient-vitaminb12 Mayo Clinic Website
http://www.nlm.nih.gov/medlineplus/ency/article/002403.htm Medline PlusMedical Encyclopedia
Basically, if you are eating enough animal protein per day you most likely get enough B12, but yet as one study seemed to show in the NIH factsheet information, all age groups show similar prevalence of deficiencies, but younger people are simply less easily diagnosed by current methods. And there seems to be no harm in supplementing.
One interesting question, perhaps, is whether it would be wise to take extra B12 after illness involving diarrhea, since B12 is secreted and resorbed through the lower intestine. If one has a lot of diarrhea, on top of the poor diet that often accompanies illness, it may lead to low B12 levels in the bloodstream temporarily, and lower reserves overall. This is just my own musings, so someone who has the equipment to test blood B12 levels in sick people with and without diarrhea ought to test this sometime.
B6:
http://lpi.oregonstate.edu/infocenter/vitamins/vitaminB6/ The Linus Pauling Institute
http://en.wikipedia.org/wiki/Vitamin_B6 Wikipedia
http://dietary-supplements.info.nih.gov/factsheets/vitaminb6.asp Dietary Supplement Fact Sheet: Vitamin B6 from NIH, Office of Dietary Supplements
Vitamin B6, unlike B12, has an upper limit to safe dosages, so while it may also be a good idea to take extra B6, it should be taken with more care. The Linus Pauling Institute's recommendation of 2mg per day sounds reasonable, though up to 100mg per day seems fairly safe from the above sources' information.