MINERAL METABOLISM 377 



by feeding special diets are reduced or prevented when fluoride is in- 

 cluded. 



In certain areas the water supplies contain fluoride just sufficient for 

 the protection of teeth. Many cities in the United States simulate the 

 natural case by adding fluoride ion to the water to make the total con- 

 centration 1.0 to 1.5 p.p.m. The exact amount depends on the climatic 

 conditions, which in turn control the quantity of water drunk. Topi- 

 cal application in the form of tooth paste containing sodium or stan- 

 nous fluoride is recommended in areas without the anion in the water 

 supply. Tea is a fairly rich source of fluoride, providing tea drinkers 

 with an adequate supply. Unfortunately children do not profit from 

 this source in most families. Sea foods are also relatively high in 

 fluoride. 



The mechanism of tooth protection by fluoride is unknown. Of 

 two popular theories, one suggests that fluoride inhibits the acid-form- 

 ing enzymes of oral bacteria; the other, that fluoride alters the struc- 

 ture of the enamel, making it more resistant to attack. Tooth decay 

 is the only known deficiency symptom whatever the mechanism of 

 action. 



Insoluble fluorides are not absorbed and play no part in the diet. 

 Soluble forms are readily absorbed, and the normal animal maintains 

 a more or less constant body level, excreting the excess in the urine. 

 Small amoimts of fluoride occur in perspiration. 



Water supplies containing more than 2 p.p.m. of fluoride produce 

 mottled and stained teeth in human beings. At 5 p.p.m. the stains be- 

 come prominent, and pitting and erosion are serious. Thus the opti- 

 mum and toxic levels are quite close together, and the use of fluorides in 

 foods and feeds requires careful control and supervision. Generalized 

 poisoning is observed in adults in certain areas in Madras (an Indian 

 province) where the water supplies may be very high in fluoride. The 

 teeth are affected first, then the spine stiffens until the vertebrae fuse, 

 and the rib connections become rigid. Ligaments and tendons calcify 

 and emaciation proceeds until death results. 



Farm animals are subject to the same problem of toxicity as are 

 human beings. In both cases the natural foods contain fluoride, but 

 the amounts present are invariably below dangerous levels. Fluorine 

 in water is a serious factor for livestock as well as man. However, 

 poisoning seldom becomes as drastic in animals, probably because their 

 shorter life spans do not allow time for such prolonged ingestion and 

 the consequent development of such severe symptoms. 



On the other hand, one special aspect of toxicity occurs only with 



