The recommended daily allowance (RDA) for Vitamin B12 ranges from 0.4 to 2.8mcg per day. The daily value for vitamin B12 is 2.4mcg per day, which has been recently reduced from 6mcg per the USDA food labeling standards.

For the values above the amounts for children less than 12 months old is actually the adequate intake (AI) not RDA.

Cyanocobalamin is the most common synthetic form of vitamin B12. However, methylcobalamin may also be used. Current evidence does not suggest one form is better than the other, so any form of added B12 can be considered to be equally effective. (2)

Intrinsic factor can be thought of as "absorption chemicals" in your gut needed for absorption of vitamin B12. Your body only creates and secretes a limited amount of intrinsic factor. So despite consuming supplements or fortified foods, only limited amounts of vitamin B12 will be absorbed by your body. As such you might only absorb 10mcg of a 500mcg supplement. (2)

Due to this factor, sometimes vitamin B12 is injected directly into patients, as opposed to supplements taken orally. These vitamin B12 injections can be a faster way to boost vitamin B12 levels and alleviate symptoms from anemia.

Nori is the thin dried seaweed wrap typically used to make sushi rolls. According to this article published in the journal Nutrients, dried nori seaweed contains up to 51.7mcg of vitamin B12 in 100 grams. A sheet of nori typically weights 1.25 grams so a single sheet would provide about 0.5mcg of vitamin B12 or 8% DV.

While this sounds great, it is unlikely that nori can be relied upon to be a consistent B12 source, so while it will make a good addition to a balanced vegan diet, it should not be solely relied upon. Further, nori seaweed is very high in iodine, and should be avoided by people with thyroid issues.

• Protection Against Heart Disease - Adequate levels of vitamins B12, B6, and B9 have been shown to lower levels of a protein in the blood called homocysteine. Lower levels of homocysteine have been shown to improve endothelial function, which in turn may boost cardiovascular health and decrease risk of heart attacks. (2)
• Protect and Repair DNA to Reduce Cancer Risk and Slow Aging - Absorption of vitamin B12 and Folate (B9) is essential for DNA metabolism and maintenance which helps to prevent cancer and slow aging. (4)
• Protect Against Dementia and Cognitive Decline - Lack of vitamin B12 increases homocysteine levels, which in turn decreases the body's ability to metabolize neurotransmitters. Due to limitations with creating long-term controlled studies in human populations, no definite link between increased vitamin B12 levels and cognitive function has been found, however, several observational studies suggest increased homocysteine levels increase the incidence of Alzheimer's disease and dementia, and low levels of vitamin B12 have been associated with cognitive decline. (2)
• Alzheimer's Protection - A study has shown that a deficiency in Vitamin B12 and Folate (B9) can double the risk of Alzheimer's Disease. (2)
• Energy and Endurance - A lack of vitamin B12 will lead to anemia and weakness. Adequate levels of vitamin B12 are necessary to maintain normal energy levels. Claims of vitamin B12 as an energy or athletic enhancer remain unproven. (2)

Cows are not given any form of synthetic vitamin B12. Vitamin B12 is naturally synthesized by bacteria that flourish in certain animals (most animals actually, just not humans). Cows, in particular, ferment their food in 4 different stomachs as they digest it. This fermentation lets the bacteria in cows create vitamin B12, and allows cows to store any excess vitamin B12.