This pathway is a little biochemically-intense, so I encourage you to watch the video first and then read the text below for clarification.
Vitamin B12 converts homocysteine to methionine by transferring methyl groups from 5-methyl tetrahydrofolate to homocysteine.
In a vitamin B12 deficiency, we often see elevated homocysteine levels, because vitamin B12 is not available to transform homocysteine into methionine. So, homocysteine levels increase.
In a vitamin B12 deficiency we can also see large red blood cells, referred to as macrocytic anemia, because vitamin B12 is not available to convert 5-methyl tetrahydrofolate back into folate. This is called the "methyl folate trap", where folate is "trapped" in the form of 5-methyl tetrahydrofolate. Folate is used to create red blood cells and DNA, and when there isn't enough folate available, red blood cells do not fully mature and stay large. On a lower folate diet, such as with many versions of the standard western diet, we tend to see this situation.
However, when someone with a vitamin B12 deficiency eats a higher folate diet, such as on a plant-based diet, there is a constant supply of folate to create red blood cells and DNA, so red blood cells may appear normal in size. This is how folate can mask a vitamin B12 deficiency. In other words, one needs look beyond the size of red blood cells when testing for vitamin B12 deficiency, and employ other reliable vitamin B12 tests. Accurate vitamin B12 testing by a knowledgeable, licensed clinician is essential for determining one's vitamin B12 status.
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