Fluoride is of interest to nutritional scientists because of its toxic properties and because it is added to drinking water in many areas of the world. Fluoride is considered an essential element in man, based on its ability to accumulate in bone and teeth, but no beneficial physiological role for fluoride has been definitively established. Fluoride has a widespread distribution within food and water which ensures that deficiency is rare. In fact, addition of fluoride to drinking water by government authorities, its presence in many commonly consumed foods, as well as fluoride pollution in the environment (from pesticides and insecticides), are increasingly being understood to represent a cause for concern in terms of accumulated fluoride toxicity in humans. Understanding the absorption, distribution and excretion of fluoride is important in minimising this toxicity for the individual.
Fluoride is present in reasonable concentrations in many of the soils of the world, where it is incorporated into plants and then consumed. Tea (Camellia sinensis) is know to accumulate fluoride and is therefore an important source of dietary fluoride because it is so widely consumed. Fish bones also provide an important dietary source. Although fluoridated water can be considered to be a major contributor to intakes, food can provide an equally large source if the food if prepared or stored in fluoridated water. The fluoride added to drinking water is in the form of hexafluorosilicic acid (H2SiF6), which is a waste product of phosphate fertiliser production and the industrial production of hydrogen fluoride. However, fluoride is found in high concentrations in some drinking water as natural calcium fluoride, but this form is poorly absorbed.
Excluding the effects that fluoride is suggested to have in strengthening bone, the pharmacological actions of fluoride is overwhelmingly toxic. Fluoride inhibits several enzyme systems and decreases glycolysis and cellular respiration. Acute exposure to fluoride can lead to death, due to the inhibitory effect that fluoride has on calcium in the body, with death eventually coming from cardiac arrest. However, acute exposure is confined to a few rare cases of accidental pesticide exposure. In contrast, chronic overexposure in more common, and can lead to the development of osteosclerosis and fluorosis. Osteosclerosis results in increases in the bone density of the body as hydroxyapatite in the bone matrix is replaced with the denser fluoroapatite. Fluorosis is a condition whereby the enamel of the teeth becomes mottled and discoloured and is considered one of the first signs of fluoride toxicity.
Fluoride is absorbed from the skin and the lungs, but the main suite of absorption is the gastrointestinal tract. Fluoride compounds are absorbed at different rates and the absorption is related to the solubility of the fluoride compound. Sodium fluoride and hexafluorosilicic acid are both very soluble and almost completely absorbed, with almost 100 % absorbed from drinking water. In contrast, calcium fluoride, cryolite (Na3AlF6) and fluoroapatite (in bone meal) are poorly absorbed. Fluoride from food is only absorbed at 50 to 80 %. Humans absorb fluoride from the stomach, and so administration of even small amounts of fluoride in drinking water cause rapid plasma peaks at 30 to 45 minutes after ingestion. Fluoride is thought to be absorbed only minimally from the small intestine, with absorption occurring via passive diffusion.
Fluoride can be detected in all organs of the body, but it appears to accumulate only in the skeleton. The degree of storage in the bones and teeth is related to the age and daily intake of the individual. The main route of excretion of fluoride is via the kidney in the urine (> 90 %). However, some fluoride is excreted via the sweat gland to the skin. Dermal excretion suggests that activities that increase sweating, such as exercise and saunas, may be able to increase excretion rates of fluoride. High magnesium intakes appear to prevent the absorption of fluoride, at least in rats1. Magnesium is able to block the absorption of fluoride because magnesium and fluoride form an insoluble complex that is excreted in the faeces. Administration of magnesium to rats significantly reduced tissue accumulation of fluoride
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