Sweet, salty, sour, bitter ... calcium?

Yesteryear's researchers wrote of four basic tastes -- sweet, salty, bitter, and sour -- that register on the human tongue. Since then, scientists have acknowledged a fifth taste, umami, a brothy soy sauce flavor emphasized in Eastern cuisine.

But why stop at five? Now, some taste researchers believe evolution may have equipped our tongues with an even more refined system able to detect six, seven, or even more tastes.

The latest proposal comes from geneticist Michael Tordoff of Philadelphia's Monell Chemical Senses Center. At a recent national meeting of the American Chemical Society, he announced that his team has identified special calcium taste receptors in mice, and he believes humans likely can taste calcium as well.

"Calcium can now be considered a basic taste,'' Tordoff said in an interview. Other researchers have found evidence that our tongues may taste additional compounds including fat, water, and starch.

Such assertions are not widely accepted -- "There is no consensus about the existence of other tastes such as the taste of fat,'' says University of Michigan taste biologist Jianzhi Zhang -- but they do raise questions about our evolving palate.

Why wouldn't evolution give us more than five tastes? In theory, the more flavors we can discern, the smarter hunting and gathering choices we'll make. On the other hand, natural selection is a thrifty boss; it wouldn't provide an array of taste buds unless we really needed them.

Each of the five established tastes has an evolutionary purpose. For instance, sweet and umami tastes signal that a food is nutritious, loaded with sugars or proteins we need to survive. In contrast, a bitter taste is like a gustatory warning label -- a sign that a food may be bad for us, perhaps even poisonous.

Salty and sour tastes are more complex, as is Tordoff's proposed calcium taste. While bitter flavors always taste foul, salt tastes good in certain amounts. Likewise, a slight sour flavor can be appealing, but not if it's strong.

For calcium, too, palatability may also depend on how much you eat, according to Tordoff.

Calcium tastes good "at very low concentrations but once you get above that, it gets increasingly unpleasant.''

Tordoff can explain why calcium taste buds might be advantageous. Our aquatic ancestors had access to the sea's calcium, he says, but "when we got onto land, then we needed to go out and find calcium.'' But there may be a problem with this explanation: Calcium taste buds can't help us eat calcium if the stuff tastes chalky and bitter to us.

One possibility is that calcium tastes bad for an evolutionary reason. According to medical anthropologist Susan Brown, author of "Better Bones, Better Body,'' the health benefits of calcium have "been overplayed.'' She says that's because previous studies tended to look at people deprived of vitamin D, which our bodies needs to help us absorb the calcium we do eat. Too much calcium may increase risk of kidney stones and heart problems.

So perhaps early humans had all the calcium they needed, especially since they spent more time in the sun, where our bodies make vitamin D. "It wasn't that nature said, 'You've got to have a lot of calcium. You need to save every little bit of it,' " Brown says.

The other explanation is that to our ancestors the taste of calcium might have hit the spot. Human taste preferences aren't exactly objective. In "Food: The History of Taste'' University of Exeter archaeologist Alan Outram writes that prehistoric people "ate semi-rotten food.'' Maybe they liked it, along with calcium. Today, the flavor of calcium may prevent us from consuming all that we need for healthy bones -- 1000 milligrams daily for adults, according to the US Department of Agriculture's "adequate intake'' level.. That's why Tordoff, with support from the National Institutes of Health and Monell's corporate sponsors , want to solve this dietary dilemma by "providing tastier high-calcium foods,'' he says.

Designing a calcium taste-blocking molecule that could be sprinkled on food and cap the tongue's calcium receptors, he says, may be the answer for people who want the benefits of calcium but not the taste.

We probably can't taste all the calcium we consume, since the receptor molecules Tordoff studies only detect free-floating calcium, not calcium bound tightly to the rest of your food. "In milk, calcium is bound so strongly that we're protected from the taste of calcium,'' he says, while in vegetables and some other foods, the calcium flavor may be stronger.

Still, taste isn't the only barrier keeping us from getting enough calcium. To consume 1000 milligrams, you would need to eat a few ounces of cheese, chased by a cup of milk. Then swallow another two glasses of milk. For many, that's enough to cause intestinal distress.

With that in mind, calcium is being added to non-dairy foods and drinks, such as orange juice. For many, that's a preferable route. But if you want your calcium from non-dairy sources, you can also expect to eat a lot of sardines, kale, and soy, not exactly our national dishes.

Jessica Tanenbaum can be reached at jessica.tanenbaum@gmail.com