Biochemistry is the chemistry of living things. From a biochemical point of view when we swallow fluoride we are on a potential “hiding to nothing.” On the one hand, there are no known biochemical processes that need the presence of the fluoride ion to function properly. On the other hand, there are many processes that are harmed by fluoride – given a sufficient tissue concentration.
Ions – particularly metal ions – play many important roles in biochemistry. Sodium and potassium ions are involved in neurotransmission and calcium ions are involved in messaging systems. Copper, zinc, iron, manganese, and magnesium ions often find themselves at the active sites of enzymes and other proteins. Calcium and magnesium ions often help to give the shape to macromolecules, which is critical because biochemical function is often exquisitely tied to the shape of molecules. Sometimes magnesium ions help to align molecules prior to their interaction with other molecules (particularly nucleic acids).
Non-metal ions also play important roles. Phosphate ions are critically important in both energy storage and utilization in much of the mechanical and other work carried out in the muscles and other tissues. The iodide ion is critical for thyroid function and the chloride ion is critical for maintaining osmotic balance. However, the fluoride ion is not known to play any biochemical role at all. This is probably why the level of fluoride in mothers’ milk is kept very low (0.004 ppm, NRC, 2006, p.40). It is simply not needed by the growing baby.
So what about the supposed benefit of fluoride as far as fighting tooth decay is concerned? This benefit is not derived from any interaction with living tissue but rather from an “inorganic” interaction with the mineral of the tooth enamel after the teeth have erupted into the oral cavity. The fluoride ion is thought to replace the hydroxide ion of the calcium hydroxy apatite (the mineral that makes up the enamel). The calcium fluorapatite formed is more resistant to acid attack, the first step in tooth decay.
At higher concentrations fluoride is also thought to help fight tooth decay by “poisoning” the bacteria (e.g., streptococcus mutans) in the mouth – which convert sugars to mineral-attacking acids. Fluoride poisons these bacteria by interfering with some of their enzymes. So on the one hand, fluoride’s benefits have nothing to do with biochemical function (i.e., reacting with the inorganic material of enamel) and on the other hand, they derive from a poisonous rather than beneficial property (i.e., poisoning the enzymes of bacteria in the mouth).
Some of the earliest opponents of water fluoridation were biochemists precisely because of fluoride’s known poisonous interactions with enzymes, the proteins which act as the catalysts that steer so smoothly practically all of the 10,000 or so chemical reactions in the body. Dr. James Sumner, for example, who won a Nobel Prize for his work with enzymes, said in the 1950s,
“We ought to go slowly. Everybody knows fluorine and fluorides are very poisonous substances and we use them in enzyme chemistry to poison enzymes, those vital agents in the body. That is the reason things are poisoned, because the enzymes are poisoned and that is why animals and plants die.”
Today, we know that fluoride interferes with many other biochemical molecules and processes in addition to interfering with enzymes. At the heart of this biochemical interference is the fact that the fluoride ion is small and negatively charged. It has a strong attraction for centers of positive charge. Thus it seeks out the metal ions at the active site of some enzymes; it surrounds and combines with other positive ions like aluminum, forming stable complexes which can mimic and interfere with the biochemistry of phosphate ions (e.g., aluminum tetrafluoride can switch on G-proteins which are involved in the transmission of messages across membranes). Fluoride can also interfere with hydrogen bonds which are critically important for both the structure and function of many important molecules, like proteins and nucleic acids.
All of fluoride’s interactions in biochemistry are concentration-dependent and the places where it is most likely to strike are in the regions where calcified tissues like the teeth and the bone (where fluoride concentrates) interface with adjacent tissues like connective tissue. For those who want more details of the very extensive negative biochemistry of fluoride, an excellent review has recently been published, titled “Molecular mechanisms of fluoride toxicity.” (Barbier et al, 2010).