Quoted from Dianne

Now I'm feeling doubtful about the CALM. I called then two months ago myself and they told me it was corn and that they were hoping to find a new manufacturer and that they would then use the beet derived stuff. Hmmm!?

My same experience.... I called and asked about CALM too...was told it was corn derived.  I also
emailed NAP and asked the source of their magnesium citrate (which if you take a lot of the formulas, it's almost in every single bottle).  Never got a reply.   Just emailed them again.  Will update if and when
I get an answer.