I've been looking for an easy to understand explanation of the folate trap and came across a couple of great articles. There's still quite a bit that is beyond me, but the first line says it all (and explains why you should take B12 and folate together):
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B12 and folate deficiency 
AM I FOLATE DEFICIENT 
Folate deficiency
Diagnosed with folate deficiency... and lost a lot of weight 
