The amounts needed were often higher than those required to cause mottled teeth (dental fluorosis).
In addition, rodent studies showed that the effects of low fluoride were time-dependent.
When the same low dose of fluoride was consumed for extended periods of time, there was an INCREASE in caries at fluoride concentrations as used in Community Water Fluoridation programs, not a reduction (Wuthier & Phillips, 1959).
Overall, dental health got worse - not better - as time went on.
"No significant protective effect against dental caries was observed from any of these F~ levels studied. Earlier work demonstrated that 40 to 50 p.p.m. F- in the diet (Shaw et al., '45) or 20 p.p.m. F- in water (Hodge and Sognnaes, '53) were necessary to give a measurable reduction of caries in the rat." (Wuthier & Phillips, 1959)
Further, there was no real decrease in caries at all, it was merely a DELAY, at best. Long-term studies showed that caries was not arrested, only delayed (McClendon & Foster, 1941).
For Long Evans rats, even water with 25 ppm fluoride did not show a caries reduction (Shaw & Sognnaes, 1954). (---> see PFPC, 2020)
Hodge H, Sagnnaes, HR - “Experimental Caries and a Discussion of the Mechanism of Caries Inhibition by Fluorine,” in F. R. Moulton, ed., Dental Caries and Fluorine, Washington, DC: American Association for the Advancement of Science 53–73 (1946)
McClendon JF, Foster WC - "The Delay of Dental Caries by Fluorine" J Biol Chem 140:LXXV (1941)
McClendon JF, Gershon-Cohen J - "Experimental Cariostasis in Rats on a Tea Diet of High Fluorine Content" Journal of Dental Research 36(2):220–222 (1957) doi:10.1177/00220345570360020801
McClure FJ, Arnold FA - "Observations on Induced Dental Caries in Rats. I. Reduction by Fluorides and Iodoacetic Acid" J D Res 20:97(1941)
Miller BF - "Inhibition of Experimental Dental Caries in the Rat by Fluoride and Iodoacetic Acid" Experimental Biology and Medicine, 39(2):389–393 (1938) doi:10.3181/00379727-39-10215
PFPC - McPherson 2018 - Comments on the NTP Study (2020)
Russell AL - "Dental Effects of Exposure to Fluoride-Bearing Dakota Sandstone Waters at Various Ages and for Various Lengths of Time" Journal of Dental Research 28(6):600–612 (1949) doi:10.1177/00220345490280061201
Shaw JH, Sognnaes RP - "Experimental Rat Caries: V. Effect of Fluorine on the Caries-Conduciveness of a Purified Ration" Journal of Nutrition 53(2):207–214 (1954)
Shaw JH, Schweigert BS, Phillips PH, Elvehjem CA - "Dental Caries in the Cotton Rat. IV. Inhibitory Effect of Fluorine Additions to the Ration" Proceedings of the Society for Experimental Biology and Medicine 59(1):89-92 (1945) doi:10.3181/00379727-59-14992
Sweeney EA, Shaw JH, Rubin RP - "Effect of Alloxan Diabetes on Fluoride Retention and Caries Incidence in Rats" Journal of Dental Research 41(4):866 (1962)
https://journals.sagepub.com/doi/abs/10 ... 0410041901
Wuthier RE, Phillips PH - "The Effects of Long-Time Administration of Small Amounts of Fluoride in Food or Water on Caries-Susceptible Rats" Journal of Nutrition 67(4):581–588 (1959) doi:10.1093/jn/67.4.581