Letter writer Chris Knapp (“Fluoride linked to lower IQ in kids,” Oct. 7) referred to a recently released health study of modest size done in three hospitals in Mexico City — please note that the public water supply in Mexico City, like the rest of Mexico, is not fluoridated. It’s a study comparing fluoride levels found in archived (previously collected and preserved) urine samples from mothers, and from their sons/daughters over a period of years — at the age of 4 and several times between the ages of 6 through 12 — hoping to discover any link that may exist between fluoride ingestion and the developing human brain.
Dietary intake of fluoride in Mexico is primarily from fluoridated (and iodized) salt, provided by the government in lieu of fluoridating public water supplies, and, as in this country, also from widely varying amounts of naturally occurring fluoride in the available drinking water, which is sometimes admittedly more than any recommended amount. The actual fluoride intake for this study was not recorded, relying instead on urinalysis, which the World Health Organization warns can vary from time to time for any individual and even varies greatly throughout the day.
According to WHO, “urinary fluoride excretion is not suitable for predicting fluoride intake for individuals” (WHO manual for Assessment of Renal Fluoride Excretion — revised manual published in 2014) [see full excerpt below], so we know not what level of fluoride was actually ingested during the period of this study.
Ongoing and more definitive studies in this area are a fine idea (preferably in our own country), but if Knapp has a disagreement with WHO, he should explain his reasoning to the rest of us.
ALAN T. BROWN
(Pages 8-9): A proportion of ingested fluoride is excreted in urine. Plots of daily urinary fluoride excretion against total daily fluoride intake suggest that daily urinary fluoride excretion is suitable for predicting fluoride intake for groups of people, but not individuals. While fluoride concentrations in plasma, saliva and urine have some ability to predict fluoride exposure, present data are insufficient to recommend utilizing fluoride concentrations in these body fluids as biomarkers of contemporary fluoride exposure for individuals. Daily fluoride excretion in urine can be considered a useful biomarker of contemporary fluoride exposure for groups of people, and normal values have been published (14).
Reference 14: Rugg-Gunn A.J., Villa A.E., Buzalaf M.R. Contemporary biological markers of exposure to fluoride. Monogr Oral Sci, 2011, 22 : 37 – 51.