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February 15, 2009

A Matter of Taste

Brussel Sprouts. Credit: WikipediaHang out with people who study human diet long enough, and eventually you'll be asked to lick a strip of paper. That's what happened to me on Friday, when I joined the speakers from The Evolution of Human Diets session for lunch after their seminar. Anne Stone of Arizona State University handed me a strip from a vial, as if she were handing out toothpicks after a meal. When I licked it, I tasted only a mild acrid flavor, like licking the glue on an envelope. I was a "nontaster," she said. Not so for the co-organizer of the session, paleoanthropologist Peter Ungar of the University of Arkansas. He told me that when he licked the paper, it was intensely bitter--he grimaced to show me just how awful. He was definitely a "taster."

It seems that we all were licking paper dipped in phenylthiocarbamide, or PTC, which is a bitter organic compound. PTC does not occur in food, but people who can taste PTC are supersensitive to bitter compounds in foods such as broccoli and Brussels sprouts. Stone brought the PTC to Chicago to show how humans vary in their ability to taste different foods.

Where did this sensitivity come from?
 
Researchers have known since 2003 that the ability to taste PTC depends on whether a person inherits a single gene--TAS2R38--that codes for a taste receptor on the tongue. Soon after, biological anthropologist Stephen Wooding of the University of Texas Southwestern Medical Center in Dallas fed chimpanzees apples doused in PTC and found that some chimps are tasters and some are not. This suggested that humans and chimpanzees had inherited the PTC gene from an ancient ancestor.

But when Stone sequenced variants of the gene in humans and chimpanzees, she found that they had inherited different variants of the gene--in other words, the ability to detect bitter fruit had evolved independently in both species. This genetic adaptation may have spread among some human and chimpanzee populations because "there are natural occurring bitter compounds that should be avoided for potential toxicity," such as quinine found in bark, says biologist Claude-Marcel Hladik of the Muséum National d'Histoire Naturelle in Paris.

At the meeting, Hladik also reviewed his own landmark studies that found that humans vary in their ability to taste sugar and salt--differences that arose as modern humans spread out around the globe in the past 100,000 years and ate different foods. In particular, Hladik noted that the Inuit of Greenland are far more sensitive to salt than most other humans, perhaps because they have to drink large amounts of fresh water to wash out the urea that accumulates in their blood from eating a diet rich in seal meat and fat. If the Inuit could detect small amounts of salt in the first sip of water melted from sea ice, that would protect them from consuming water polluted with salt and from getting sick. So, the genetics of dietary adaptations explain why it would be unkind to salt the food of an Inuit--or to pass the Brussels sprouts to Ungar.

--Ann Gibbons