I found a post by "C.R.MacDonald" (C.R. for Calorie Restriction? is s/he a member on this forum as well?) entitled "The potato diet" which reads, in part:
"While food variety is likely recommended for long-term adherence, I am wondering if a person theoretically could [eat only potatoes] long-term (longer than a year) with sustained excellent health. ... is it possible one can thrive on "just" potatoes long-term (with B12 supplementation)? Would a person be deficient in any vital nutrients? And is there a concern of eating "too many" potatoes resulting in too much potassium (hyperkalemia)?"
The renowned Registered Dietician to whom the question was addressed refused to entertain it. He doesn't see it "as valuable to addressing the real problems at hand," believing it is a "distraction."
(See entire message https://www.drmcdougall.com/forums/viewtopic.php?f=22&t=52437&p=538020#p536807 )