Some people abhor broccoli, complaining about its intensely bitter taste. Others (myself included) find broccoli's flavor interesting and pleasing—decidedly, not bitter. What leads to our differing culinary opinions is the possession of, or lack of, (in my case, evidently) genes conferring a super sensitivity to bitter taste. Science has recognized such genetic differences for at least a decade.
A new study conducted with twins now demonstrates that people can similarly inherit a high or low sensitivity to sour tastes. However, the data also show that the same doesn't hold for sensing saltiness. Although people have a broad range of salt sensitivity, much of the difference between individuals traces not to genetics but to environmental influences—both to whether mom ate a lot of salt during pregnancy and to what cuisine a person was subsequently raised on (see Born to Love Salt).
More than a century ago, scientists identified taste buds as the centers of cellular-scale taste reception, but there's "still a tremendous amount to learn about taste," notes Paul A.S. Breslin of the Monell Chemical Senses Center in Philadelphia. In particular, he notes, there are huge gaps in scientists' understanding of how cells in taste buds distinguish flavors: sweet, salty, bitter, sour, and savory (also known as umami). Among these sensations, "the two we know the least about are sourness and saltiness," Breslin says.
To probe potential genetic influences on individuals' thresholds for detecting sourness and saltiness, Breslin's team recruited 109 pairs of twins attending either the 2003 or 2004 Twins Days Festival in Twinsburg, Ohio. Each pair—including 74 sets of identical twins—had grown up together.
Because the twins shared the same home environments, the researchers could largely rule out differences in influences before birth, childhood diets, and culture. Identical twins also share almost the same genetic inheritance. Fraternal, or nonidentical, twins have only about a 50 percent overlap in genes, the researchers note. Indeed, Paul M. Wise, who led the study, points out that fraternal twins are no more genetically similar than any other pair of siblings.
Consequently, if similar taste thresholds showed up dramatically more often in identical twins than in fraternal twins, a person's perception of that flavor could probably be traced to genetics. However, if similar thresholds were just as likely to occur in identical- and fraternal-twin pairs, the effect more likely could be laid to the pair's common environmental influences.
In a report published online for an upcoming issue of Chemical Senses, the Monell scientists describe examples of each situation. In general, Wise's group reports, identical twins were far more likely to share a threshold for sour sensitivity than were fraternal twins. Both groups, however, proved about equally likely to share a similar taste threshold-sensitivity for salt.
What's that taste?
Recruits to the study ranged in age from 14 to 72 years old. Members of each pair were deliberately seated away from each other as they began taking part in a series of taste tests.
Tiny cups contained water with increasingly higher concentrations of either table salt or citric acid, the source of a lemon's sour taste. The volunteers slowly downed each drink in succession, stopping when they encountered something tasting different than plain water.
The participants weren't told what flavor to expect. Once a person tasted something, he or she had to say whether the drink tasted somewhat sweet, sour, salty, or bitter.
The taste thresholds varied dramatically among the participants, Wise notes. There was more than a 1,000-fold difference in detection limits for volunteers who were most sensitive versus those least sensitive to sour tastes. More than a 100-fold difference separated individuals who were most and least sensitive to salty tastes.
In general, however, identical twins were far more likely to share a sour-taste threshold than fraternal twins were. In fact, genetic factors appeared to account for slightly more than 50 percent of the difference between twins, the Monell group calculates. Its analyses attribute roughly 20 percent of individuals' differences in salt-taste sensitivity to environmental influences.
Besides environment and genetics, what else may be contributing to taste-threshold differences? Unique influences from health and other factors can affect a single member of a twin pair, Wise explains, however, the design of this study didn't allow his team to explore such influences.
The findings appear consistent with what I've witnessed at the family dinner table. Although my husband selects dry—even slightly acidic—wines, his twin sister will reach only for syrupy sweet ones. Their musical preferences are equally divergent. Do you suppose one might have gotten the opera gene, while the other received DNA that fosters a preference for Streisand and Sinatra?
Paul Breslin and Paul M. Wise
Monell Chemical Senses Center
3500 Market Street
Philadelphia, PA 19104
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