How long does it take for Vitamin B Twelve deficiency to cause permanent damage?


Q: How long does it take for vitamin B12 deficiency to cause enough damage that it could not be replaced by adding vitamin B12 to the diet?

For instance, if someone was a vegan for three years, and they did not supplement their diet with any form of vitamin B12, and their body's stores of vitamin B12 were completely depleted at the start of this three year deficiency, could the damage by this deficiency be reversed by supplementing vitamin B12 into the diet?

Assume the person in question was 14 and male when they became deficient.


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