Maybe a clue : some people are enduring trouble with the low carb transition, due to depletion of glycogen stores that induces temporary potassium & magnesium deficiency. In that case sodium is likely to cause more harm and should be avoided.

http://www.westonaprice.org/letters/1952-letters-summer-2010.html
See the comment "LOW-CARB PITFALL" (at mid-page)

Then magnesium deficiency could lead to tooth decay.