Advancements in science sometimes begin with a simple observation. Around the turn of the century, brown stains were noticed discoloring the tooth enamel of people living in certain parts of the world. Some 30 years later, this "mottling" was attributed to naturally occurring fluoride in the water. It soon became apparent that people from these regions had a lower incidence of dental caries (cavities).

These observations led to studies to determine the relationship between fluoride and dental health. Following an examination of the dental records of 7,000 children from 21 cities across the United States, the definitive report was published in 1942. Fluoride in a concentration of one part per million (1 milligram per liter) in the water supply was found to provide significant protection from dental caries without causing mottling (which occurs between 2 and 8 parts per million) or other problems. This figure was later adjusted on a regional basis to correct for the effect of climate on water consumption.

Over the years, studies have shown that adding fluoride to the water supply is as effective as naturally occurring fluoride in reducing caries. In some low-fluoride areas, fluoridation decreased cavities by as much as 65 percent in children exposed to the treated water from birth. This result is impressive, considering the many factors involved in the formation of caries: diet, personal oral hygiene and professional dental care.

Fluoride is most valuable during childhood, when permanent teeth are being formed. How it works is not completely understood. Apparently the incorporation of fluoride into teeth makes the enamel more resistant to acid produced by bacteria.

Not surprisingly, fluoride supplements during pregnancy seem to increase the caries resistance of baby teeth developing in utero. But while there is no evidence that prenatal fluoride supplementation is harmful, the Food and Drug Administration has yet to decide on its efficacy and thus prohibits the addition of fluoride to prenatal vitamin preparations. Fluoride supplements for use during early infancy are available by prescription, and often are recommended by pediatricians, especially in low-fluoride areas.

The effects of fluoride on teeth during their formation result from the absorption of the mineral into the circulation. Already erupted baby and permanent teeth also can benefit from fluoride. As it washes over the teeth when you drink a beverage made with fluoride-containing water or use a fluoride toothpaste, it can be incorporated into the "hydroxyapatite" crystals that make up the structure of tooth enamel. Although less dramatic, the effects of this topical administration are important.

Fluoride may offer additional benefits. Because it is rapidly taken up by bone and is known to stimulate bone growth, scientists have wondered about its relationship to a bone disorder, osteoporosis. Almost 20 years ago, a study evaluated two towns in North Dakota with naturally high (5 parts per million) and low (0.15) fluoride levels in the water. Markedly decreased bone densities and an increased incidence of collapsed vertebrae -- both indicative of osteoporosis -- were found in the low-fluoride town. A later study suggests that fluoride levels may have to be substantially higher than those recommended for caries prevention to see this protective effect on skeletal bone.

Fluoride has been used successfully in experimental trials as part of a treatment program for those suffering from osteoporosis. Because undesirable side effects can occur at these high levels, therapeutic doses must be carefully monitored.

Now there's upsurge of interest in the possible link between fluoride and cardiovascular disease. Studies provide conflicting results. Some evidence suggests that fluoride might act in conjunction with magnesium to protect blood vessels from calcification. Much more research is required.

Although water is the primary source of fluoride in the diet, food can contribute as much as 25 percent of a day's intake, particularly if fluoridated water was used in processing. In some parts of the world, vehicles other than water have been used for fluoridation. In Hungary and Switzerland, fluoride is added to salt. It also has been tried in flour, milk, fruit juices and even sugar. But fluoridation of water remains the least expensive and most effective way to ensure fluoride intake.

Fluoridation of public water continues to cause controversy. Some worry about the potential side effects; others feel it infringes on their civil rights. However, a recent review in the American Journal of Clinical Nutrition points out that as of 1977, 35,000 papers had been published verifying the safety and usefulness of fluoridation. Considering its tremendous contribution to dental health and possibly to other health problems, and to savings in the cost of dental care, fluoridation must be seen as a highly desirable public-health measure.