Here's a study from the USDA archives:
Vitamin B12 Studies In Total Vegetarians (Vegans), Journal of Nutritional Medicine, 1994
I can only see the abstract and it's difficult to follow the group(s) they studied, but they describe:
"In 10 who changed from a lacto-ovo-vegetarian (LOV) diet to a TVD,2 the mean serum B12 level dropped 35% from 415 +/- 187 to 268 +/- 75 pg ml-1 p<0.005) 2 months after starting the TVD. ... Serum B12 levels decreased noticeably within 2 months on a [vegan diet]."So, after 2 months, the group's average was hovering close to what the IOM considers a lower limit. After a year:
"61% of those on the [vegan diet] for 1 year or more had serum B12 levels below normal."Looks like they used 200 pg/ml as the cutoff for normal. All well and good but the IOM says, "a serum B12 value above the cutoff point does not necessarily indicate adequate B12 status," because "as deficiency develops, serum values may be maintained at the expense of B12 in the tissues."
They also looked at the prevalence of low B12 status in vegans. In 78 vegans, 47 were below normal. Even the remaining 31 had levels only reaching 293 +/- 85.
Everyone is different. Some people may be very efficient at reabsorbing any B12 they secrete into the small intestine via bile.3 But reabsorption is not 100%. And there is some loss of B12 after this point anyway, from cells that break off in the large intestine. Without some B12 input, there will be net loss over time. And anyone with absorption problems in the area of their distal small intestine where B12 is taken up (or if this part of the intestine is absent from, say, cancer or Crohn's surgery), and anyone with inflammatory issues in the stomach where intrinsic factor (IF) is secreted (IF assists in B12 absorption) will most definitely face B12 deficiency without some treatment.
So, how long will it take to fall below normal? From this data, somewhere between a few months to a few years depending on the health of the GI tract.
If I was eating a vegan diet, I would definitely supplement with vitamin B12, and/or eat B12 fortified food.
One more point from this study, about supplementing:
"The serum B12 level of seven of 16 adults with a low serum B12 who chewed a 100 microgram tablet of B12 once a week for 6 weeks increased by 150%, whereas the serum B12 level increased by only 12% in the nine who gulped down the tablets (with water)."
2 TVD: Total Vegetarian Diet, i.e. Vegan Diet
3 How much we lose in bile depends in part on how much fat we eat, since bile is secreted to emulsify dietary fat. Still, this is not a lot, maybe 0.5 to 1 mcg/day say the IOM.