It appears that fluoride may also inhibit magnesium uptake and re-uptake, and that conversely magnesium may inhibit fluoride uptake. The first abstract points out the importance of magnesium, the rising fluoride intake and falling magnesium intake and discusses possible implications.
http://www.mgwater.com/fl2.shtml
FLUORIDE-MAGNESIUM INTERACTION (Guest Editorial)
by A Machoy-Mokrzynska (Institute of Pharmacology and Toxicology, Pomeranian Medical Academy, Szczecin, Poland)
Fluoride (J. of the International Society for Fluoride Research), Vol. 28 No. 4; November, 1995, pp 175-177
In summary, it can be stated that in intoxication with fluorine compounds, magnesium plays a protective role by countering and reducing the toxic effects of F-.
http://jn.nutrition.org/content/117/3/496.long
Influence of Dietary Magnesium on Fluoride
Bioavailability in the Rat1'2
FLORIAN L. CERKLEWSKl
Department of Foods and Nutrition, College of Home Economics, Oregon State University,
Corvallis, OR 97331
Enhancement of fluoride bioavailability
in rats fed diets containing low magnesium,
and depressed fluoride bioavailability in rats fed diets
containing high levels of magnesium, can be explained
by the ability of magnesium to form an insoluble com
plex with fluoride in the intestinal tract.