It is important to know that once vitamin B12 is taken up by the cells in the body, it is converted into either A.) methylcobalamin, after receiving a methyl or single carbon (-CH3) group from active folate or converted into B.) adenosylcobalamin, which is an essential cofactor in converting methylmalonyl-CoA into succinyl-CoA which is needed for energy production of the cell. Methylcobalamin transfers its methyl group to homocysteine (Hcy) and converts it into methionine, which is critical for reading genes and methylation reactions. It is not important for the purposes of this X Fact to understand the chemistry, but what is important it that if there is not enough vitamin B12 inside the cells to participate in these two important functions, then the substance that is not being converted begins to accumulate and spill over into the blood and urine. Elevated homocysteine (Hcy) can be detected when there is a deficiency of either Vitamin B12, Folate or B6, whereas elevated methylmalonic acid (MMA) is a very sensitive test of vitamin B12 deficiency at a cellular level, and can often identify vitamin B12 deficiency long before clinical symptoms are recognized.