A balancing act The role of electrolytes

Your body maintains its fluid balance through the action of substances called electrolytes, which are mineral compounds that, when dissolved in water, become electrically charged particles called ions.

Many minerals, including calcium, phosphorus, and magnesium, form compounds that dissolve into charged particles. But nutritionists generally use the term electrolyte to describe sodium, potassium, and chlorine. The most familiar electrolyte is the one found on every dinner table: sodium chloride — plain old table salt. (In water, its molecules dissolve into two ions: one sodium ion and one chloride ion.)

Fluoridated water: The real Tooth Fairy

Except for the common cold, dental cavities are the most common human medical problem.

You get cavities from mutans streptococci, bacteria that live in dental plaque. The bacteria digest and ferment carbohydrate residue on your teeth (plain table sugar is the worst offender) leaving acid that eats away at the mineral surface of the tooth. This eating away is called decay. When the decay gets past the enamel to the softer pulp inside of the tooth, your tooth hurts. And you head for the dentist even though you hate it so much you'd almost rather put up with the pain. But almost doesn't count, so off you go.

Brushing and flossing help prevent cavities by cleaning your teeth so that bacteria have less to feast on. Another way to reduce your susceptibility to cavities is to drink fluoridated water— water containing the mineral fluorine.

Fluoride — the form of fluorine found in food and water — combines with other minerals in teeth and makes the minerals less soluble (harder to dissolve). You get the most benefit by drinking water containing 1 part fluoride to every 1 million parts water (1 ppm) from the day you're born until the day you get your last permanent tooth, usually around age 11 to 13.

Some drinking water, notably in the American Southwest, is fluoridated naturally when it flows through rocks containing fluorine. Sometimes so much fluoride is in this water that it causes a brownish spotting (or mottling) that occurs while teeth are developing and accumulating minerals. This effect doesn't occur with drinking water artificially supplemented with fluoride at the approved standard of one part fluoride to every million parts of water.

Because fluorides concentrate in bones, some people believe that drinking fluoridated water raises the risk of bone cancers, but no evidence to support this claim has ever been found in human beings. However, in 1990, a U.S. Public Health Service's National Toxicology Program (NTP) study of the long-term effects of high fluoride consumption on laboratory rats and mice added fuel to the fire: Four of the 1,044 laboratory rats and mice fed high doses of fluoride for two years developed osteosarcoma, a form of bone cancer.

The study sent an immediate frisson (shiver of fear) through the health community, but within a year, federal officials reviewing the study issued an opinion endorsing the safety and effectiveness of fluoridated water.

Here's why: First, the number of cancers among the laboratory animals was low enough to have occurred simply by chance. Second, the cancers occurred only in male rats; no cases were reported in female rats or mice of either sex. Finally, the amount of fluorides the animals ingested was 50 to 100 times higher than what you get in drinking water. To get as much fluoride as those rats did, human beings would have to drink more than 380 8-ounce glasses of fluoridated water a day.

Today, more than half the people living in the United States have access to adequately fluoridated public water supplies. The result is a lifelong 50 percent to 70 percent reduction in cavities among the residents of these communities.

Under normal circumstances, the fluid inside your cells has more potassium than sodium and chloride. The fluid outside is just the opposite: more sodium and chloride than potassium. The cell wall is a semipermeable membrane; some things pass through, but others don't. Water molecules and small mineral molecules flow through freely, unlike larger molecules such as proteins.

The process by which sodium flows out and potassium flows in to keep things on an even keel is called the sodium pump. If this process were to cease, sodium ions would build up inside your cells. Sodium attracts water; the more sodium there is inside the cell, the more water flows in. Eventually, the cell would burst and die. The sodium pump, regular as a clock, prevents this imbalance from happening so you can move along, blissfully unaware of those efficient, electric ions.

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