When people started becoming more aware that everything has protein, the meat, dairy, and egg industry started the B12 scare. It really is a myth. Most studies show no higher rates of deficiency in vegans than in meat eaters. The problem is that too many doctors and scientists use the word deficiency inappropriately. Deficiency means that someone has developed symptoms due to a lack of a nutrient. It does NOT mean a lower level. The issue is that because the "normal" range for blood values is based on an average of the population, it means that the "normal" range is based on an overweight, unhealthy meat eater. Therefore, we don't really know what the normal range for a healthy individual should be.
"Vitamin B12 deficiency or hypocobalaminemia typically features a low blood level of vitamin B12 however functional B12 deficiency can occur at any serum level; with or without anaemia and/or macrocytosis."
"The results of the Framingham Offspring Study indicate that B12 deficiency may be more common than was previously believed. Deficiency is most significantly linked to improper absorption rather than low consumption, as many who consume high amounts of B12 may still experience deficiency."
Absorption is a greater issue than consumption. It is often claimed that humans get most of their B12 from diet which is entirely untrue. We actually get plenty from our gut bacteria as long as we have no absorption problem. If this wasn't true, then anti-biotics should have zero effect on our B12 levels, but they do. This is because the anti-biotics kill the flora in our stomachs which create B12.
"long-term use of antibiotics can lower vitamin B levels in the body, particularly B2, B9, B12, and vitamin H (biotin), which is considered part of the B complex vitamins."
Source: Vitamin B12 (cobalamin) | University of Maryland Medical Center http://umm.edu/health/medical/altmed/su ... z325PVpLKu
Most animal products contain high amounts of anti-biotics that do effect humans and their gut flora. They can also lower your natural immune system. With fish, you likely either have anti-biotics and growth hormones from fish farms, or mercury and possibly radiation from wild fish.
For the vast majority of people who develop a deficiency, a supplement will not help them because they still can't absorb it properly. The exception is properly used sublinguals can raise levels, but they usually require consistent use. The other option for these people is either B12 patches, or injections.
If you are very worried, get your family checked. But just remember this. Eating meat is the leading case of cancer and heart disease. You have a 1 in 3 chance of dying from either cancer or heart disease, and yet your chances of developing symptomatic B12 deficiency is so low that they don't even reference a number.