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The folate trap involves the 'folate cycle'.
Here is the 'folate cycle' (which is typically drawn in a circle):
Tetrahydrofolate (TH4) -> 5,10 Methylene-TH4 -> Methyltetrahydrofolate -> Tetrahydrofolate (TH4)
This is a 'cycle' because TH4 (the starting point) is regenerated.
The problem that can lead to the 'folate trap' is that the last step of the cycle, the conversion of methytetrahydrolfolate to TH4 (the enzyme is methylene - tetrahydro - folate - reductase - MTHFR), requires Vitamin B12. In the absence of B12 (Vitamin B12 deficiency is quite common, especially in the elderly), the folate cycle gets 'trapped' at methyltetrahydrofolate and can't continue to regenerate TH4.
High doses of supplemental folate can 'mask' the effects of B12 deficiency by compensating for the trapped folate (as methyltetrahydrofolate). So, high doses of folate can mask the anemia that would result from a severe B12 deficiency. Chronic B12 deficiency has serious consequences (even without the anemia) such as damage to the nervous system and the creation of a peripheral neuropathy.
Hope this was helpful. Best wishes.