Fluorine is a highly reactive and highly electrochemical trace element, and its abundance in the earth’s crust results it in its near ubiquitous presence as fluorides in the environment. The main route of exposure for humans is through drinking water and food, but can also include cosmetics, drugs, air and toothpaste. The dissolves fluoride found naturally and added artificially to drinking water are the main route of exposure for fluoride and can account for around 60% of intake. Fluoride is present in a wide variety of minerals including cryolite (3NaF AlF2), apatite (3Ca2[PO4]2 CaF2) fluorspar (CaF2), mica and hornblende, and can also be found in sedimentary deposits and igneous rocks. Fluoride levels are high in soft, alkaline, calcium deficient water, and the high fluoride concentration of water reflect the highly soluble nature of the calcium and sodium fluoride found in rocks.
Fluoride absorption is on interest nutritionally because controversy surrounds its essential status in humans and evidence shows that it has toxic effects in humans, even at low doses. The mainstream healthcare viewpoint is that fluoride is essential for dental health and possibly for bone strength, but this is highly controversial and certainly not proven beyond reasonable doubt. The belief that fluoride is necessary for dental health in the young has lead to widespread fluoridation of water supplies, enforcing medication of large numbers of inhabitants of Western populations, with fluorosilicates. Water is the most difficult source to avoid and fluoride is ubiquitously found in all water at varying concentrations. The toxicity of fluoride relates to effects on skeletal health, the brain, as well as soft tissues and organs such as the pancreas, liver and kidneys. Fluoride toxicity is characterised by dental fluorosis and skeletal fluorosis.
Sodium fluoride used in toothpaste and fluorosilicates used to fluoridate water are almost completely absorbed from the gastrointestinal tract, even at high doses (>90%). The absorption from food however is lower at around 50 to 80%. Absorption of sodium fluoride is rapid and peak plasma levels occur 30 to 45min after consumption. Rats1 fed 1mL water with 50µg of sodium fluoride show that around 90% of fluoride is absorbed within 120 min with peak plasma levels occurring at 10min, while declining after 40min. The rapid absorption is accounted for by the fact that 25% of the fluoride was absorbed from the stomach, with the remaining 75% absorbed from the small intestine, at a rate equivalent to gastric emptying. Gastric absorption occurs because the fluoride exists primarily as hydrogen fluoride at the low pH of the stomach, rather than ionic fluoride (200:1) and can therefore passively diffuse across the stomach lumen.
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