Which of the following has been associated with INCREASED fluoride retention and a higher prevalence of dental fluorosis at any given water fluoride concentration?

Correct answer: D. Residence at high altitude

• A. High dietary CALCIUM does the opposite — calcium (and magnesium and aluminium) binds fluoride in the gut and REDUCES its bioavailability and retention.
• B. Thiazides affect calcium handling, not fluoride retention, and are not a recognised fluorosis risk factor.
• C. Dietary protein is not an established determinant of fluoride retention.
• D. CORRECT. Populations living at HIGH ALTITUDE show more fluorosis than lowland populations drinking water of the same fluoride concentration. The proposed mechanisms are hypobaric hypoxia altering ameloblast function and renal handling of fluoride, together with the mild metabolic changes and increased fluid intake seen at altitude. Altitude is therefore treated as an independent risk modifier when setting community fluoride levels.

Note: Fluorosis is a DEVELOPMENTAL defect: it requires chronic systemic ingestion during amelogenesis (roughly birth to 6–8 years for the permanent anterior teeth). Once teeth have erupted, no amount of ingested fluoride can produce fluorosis in them.