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UW researchers show that the human genome is helpless in the face of chocolate

University of Washington

Knowing that extreme sensitivity to some bitter tastes is genetically-driven, researchers in the University of Washington School of Public Health and Community Medicine tried to find out if genetic taste markers might prevent some women from enjoying bitter chocolate or bitter espresso coffee. Dr. Adam Drewnowski, director of the school's Nutritional Sciences Program, says the study by graduate student Agnes Ly and himself showed that any aversion to bitter taste, genetic or not, was easily overcome by the addition of a little sugar or a lot of fat. The study was published in the January issue of Chemical Sense, an Oxford University Press journal.

"Human genetics is not destiny, particularly when it comes to a love for coffee and chocolate," Drewnowski said. "All the women tested, regardless of their ability to taste bitterness, enjoyed chocolate. But the ones who were more sensitive to bitter flavors would drink lattés, rather than straight espresso coffee."

The only genetic taste markers in humans involve a compound called 6-n-propylthiouracil or PROP. Some people find PROP extremely bitter and repulsive, whereas others cannot taste it at all. Though some genetic linkage studies have placed the PROP gene on human chromosome 5, the gene itself has not been described and its exact location is unknown. The women in the study, all UW students, were given pieces of blotting paper saturated with a solution of PROP. That is a common way to distinguish between PROP tasters and non-tasters. Past studies had shown that women who found PROP bitter to the point of revulsion were also more sensitive to some other bitter tastes, including the bitter taste of caffeine.

The study showed that PROP tasters were more likely to dislike solutions of pure caffeine in water. However, those differences were quickly eliminated by the addition of neohesperidine DC, an intense sweetener, derived from bitter oranges and supplied to the researchers by a company in Spain. There were no differences between PROP tasters and non-tasters in their evaluation or enjoyment of white, bittersweet, or bitter chocolate.

"Sensitivity to bitter can be managed by the addition of sugar and fat, as in mochaccinos or lattés." Drewnowski said. "And chocolate is the classic combination of complex bitter flavors, sweetness and fat."

Genetic taste markers did not affect the taste response to chocolate.

"The lessons from chocolate can be applied to Brussels sprouts," Drewnowski said. "There are things that can be done in the kitchen to make these bitter foods acceptable. As we already know, making bitter foods palatable is important in adding phytonutrients to the diet."

Phytonutrients, such as those found in bitter vegetables, green teas, coffee and chocolate, are associated with decreased risk of some ailments, including cancer and heart disease.

As for why people like chocolate, Drewnowski, who also holds an appointment at Fred Hutchinson Cancer Research Center, said it is not necessary to look for chemical or nutritional explanations for enjoying its complex flavor.

"The human genome is powerless in the face of chocolate," Drewnowski said. "We all eat it because we like it, and we don't need any scientific explanation to do that."


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