As a journalist, and a classically polite Canadian, I don’t often stick my tongue out at the people I’m interviewing. It seems bad form, somehow. But I’m doing it now to Linda Bartoshuk, the grande dame of taste research. Fortunately, she doesn’t seem to mind.
“Oh, your tongue is gorgeous,” she gushes. She leans close and paints the tip of my tongue with a Q-Tip dipped in blue food coloring, which highlights the taste buds on my tongue. (To be accurate, they’re not actually taste buds, which are microscopic. Those mushroom-shaped bumps on the surface of the tongue that most people call taste buds are, technically speaking, really fungiform papillae, an impressive-sounding Latin term that means “mushroom-shaped bumps.”)
I hold up a mirror to see what Bartoshuk sees on my tongue. Tiny pink islands stand out in a sea of blue dye. “You see those red dots on the front? Those are fungiform papillae,” she says. “You have a lot. Oh, and you have them all the way back! You’re very close to a supertaster.”
Understanding this notion of the supertaster—that some people have a much more acute sense of taste than others—is what has brought me to Bartoshuk’s lab here at the University of Florida in Gainesville. It was Bartoshuk who first suggested, back in 1991, that people tend to fall into three groups, based on their ability to taste a bitter compound known as propylthiouracil, or PROP.
You may have encountered PROP in a high school biology lab or at a science museum somewhere. You’re handed a little piece of filter paper infused with a modest amount of PROP, which you put on your tongue. Some people—the nontasters—just shrug, tasting basically nothing apart from filter paper. Others—the tasters—notice an unpleasant bitter taste, while the third group experiences extreme bitterness. This third group, the supertasters, are easy to recognize: They’re the ones who make an anguished face and rush off to find something—anything—to wash that horrible taste out of their mouth. Bartoshuk often asks people to rate the intensity of PROP’s bitterness on a scale from 0 to 100, where 100 is the most intense sensation they’ve ever experienced—the pain of childbirth, say, or a broken bone, or the visual sensation of looking directly at the sun. Supertasters often rate the bitterness of PROP in the 60–80 range, nearly in broken-bone territory. Sure enough, I’d score it a 60: nasty, but not debilitating. “That’s into supertaster territory,” says Bartoshuk. “That’s in the area where you’re not screaming, but you’re definitely much higher than normal, and your tongue looks it.”
And it’s not just bitterness. Supertasters tend to rate sweets as sweeter, salt as saltier, and chili peppers as hotter. They even report that food aromas are more intense, says Bartoshuk—probably because taste and smell reinforce each other in the brain.
Before I get too smug about my taste acuity, though, Bartoshuk points out that supertasters tend to be pretty boring eaters. Most of them prefer to avoid the intense taste experiences that come with highly flavored foods, so their diets are often bland and narrow. (I knew a man once who lived on a habitual diet of lima beans and milk. I would bet good money he was a supertaster.) In particular, bitter greens and other vegetables don’t show up very often on the plates of most supertasters.
That’s where I start to get confused, because that doesn’t sound like me. I love collard greens, rapini, and other bitter vegetables; I always pick the hoppiest beer I can find; I drink my coffee black and without sugar; tonic water is my soft drink of choice—indeed, the only one I ever drink. In contrast, Bartoshuk—a nontaster—has very pronounced food aversions. She detests tonic water, for example. “When I first tasted it, I couldn’t believe it was a beverage,” she says. “I cannot stand greens. The bitter taste is just beyond belief to me.”
So what’s going on? It’s time to look more closely at this whole supertaster notion, which turns out to be more complex than it appears at first glance.
The ability to taste PROP turns out to be mostly a function of one particular bitter receptor, T2R38. There are two common variants of this gene: one version that responds strongly to PROP and one that doesn’t. This suggests that people with two copies of the nonresponding gene (one from each parent) are nontasters, those with two copies of the high-responding gene are supertasters, and those with one of each are normal tasters. And, indeed, researchers sometimes genotype people for T2R38 as a quick, objective way to determine their taster status.
But it’s not that simple. The T2R38 receptor recognizes just one group of chemicals: those that contain a particular set of atoms called a thiourea group. Your ability to taste those should have nothing to do with your ability to taste sweet, salty, or other kinds of bitter, let alone your perception of the burn of chili peppers, which involves an entirely different set of receptors and nerves. And it certainly shouldn’t affect the number of fungiform papillae on your tongue.
T2R38 probably has nothing to do with supertasting, at least not directly. Your T2R38 genes determine whether you have the genetic ability to taste PROP at all—but if you do, the amount of bitterness you experience probably depends on how well the rest of the taste machinery in your mouth and brain responds. The genes that control that machinery are what really make the difference between a taster and a supertaster—and if you can taste PROP at all, the amount of bitterness you experience is a decent measure of how sensitive the rest of your machinery is. That’s probably why people who rate PROP as intensely bitter also tend to rate salt as saltier, sugar as sweeter, and chili peppers as hotter than people who find PROP less bitter. If so, people with broken T2R38 genes might still be supertasters for anything that doesn’t require that bitter receptor. They just need to find a different way to prove it.
One way might be to measure the density of fungiform papillae, which is why Bartoshuk painted my tongue blue. Each papilla contains several smaller clusters of cells bearing taste receptors. These clusters are the real taste buds, technically speaking, and the cells within them send nerve impulses up the taste nerves to the brain, signaling which of their receptors has encountered its particular taste quality. It makes sense that tongues with more papillae would generate stronger nerve signals and hence experience more intense tastes. Sure enough, most studies do support that hunch—although there are a few annoying studies that fail to find a link between number of papillae and taste perceptions.
So what determines how many papillae you have on your tongue? Nobody knows for sure, but there are intriguing hints that a protein called gustin might be involved in stimulating the formation of fungiform papillae. People with one particular variant of the gustin gene have abundant, normal papillae, while those with a different variant have large, misshapen, sparsely scattered papillae. No doubt, too, there are plenty of other genes that affect overall taste sensitivity and thus help to define whether you’re a supertaster, an ordinary taster, or a (relative) nontaster. But the science doesn’t seem to have caught up with our curiosity on this matter.
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Fortunately, scientists do know a fair bit about the genetics that underlie some of the differences in people’s taste perceptions—enough, in fact, to make it clear that each of us lives in a unique world of flavor. Genetic differences likely explain some (though not all) of why former president George H. W. Bush hated broccoli, why a gin and tonic is ambrosia to one person and anathema to another, or why some of us put sugar in our coffee. I wanted to learn more—and, especially, I wanted to know where my own taste perceptions fit into the picture. That brought me to the Monell Chemical Senses Center in Philadelphia.
In particular, I wanted to see Danielle Reed, who has done a lot of the best work on genetic differences in taste perception. A few months before my visit, I had drooled into a vial and shipped it off to Reed for genetic analysis. (Saliva contains enough cells that geneticists no longer need blood samples or even cheek swabs to run their DNA tests.) Now it’s time to see how my sense of taste compares with everyone else’s.
Reed’s taste-test procedure couldn’t be more low-tech. Her assistants hand me a box containing several numbered vials of liquid, plus a large plastic cup to spit into. Starting with vial 1, I sip the liquid, swish it around in my mouth, and spit into the cup, indicating on a questionnaire how sweet, salty, sour, and bitter I found the sample; how intense the sensation is; and how much I like it. And then I go on to vial 2. It’s a bit like a wine tasting, but without the pretentiousness. And without the wine.
A few hours later, test scores in hand, it’s time to sit down with Reed to see how they match up with my genes. In person, Reed is a short, plump, cheerful woman with frizzy dark hair who clearly thinks unpacking someone’s genes is a bit like unwrapping a present. She must have done this hundreds of times by now, if not thousands, but the excitement is still there.
The first test turned out to be a bit of a trick: Vial 1 held plain old distilled water. I’m relieved to see that I scored its taste intensity to be “like water”; rated it dead neutral on the liking spectrum; and detected no sweet, salty, sour, or bitter tastes. At least I’m not tasting stuff that isn’t there. Now on to the real tastes, and the genes.
First up, T1R3, the gene that contributes to the receptors for sweet and umami. Reed had tested my genome for a variant that, other researchers had found, affects sweet perception. These genetic variants are like spelling changes in the genome. Just as changing a single letter—“dog” to “dig,” say—can alter a word’s meaning, changing a single letter in the DNA sequence of a gene can alter the resulting receptor protein. For the T1R3 variant, people with a T at one particular spot are less sensitive to sweet taste, and like it more, than those with a C. “It’s like they can’t taste sweet as much, so they are choosing the higher concentrations,” says Reed.
I turn out to be a TT—one T from each parent—which should make me a classic sweet craver. But that really didn’t make sense, I told her. Just that morning, I’d been given a sweetened iced coffee at Starbucks by mistake, and I had ended up pouring most of it out, because it was much too sweet to drink. As far as I’m concerned, it’s also no big deal to skip dessert after dinner—it’s not important to me. Had something gone wrong with the genotyping?
Reed turned to my taste-test result and burst out laughing. “Oh, look at you! You’re not so far off here.” I’d rated the 12 percent sugar solution—roughly equivalent to a (flat) soda—as only moderately sweet, and highly pleasant. Reed herself—a CC—finds it disgustingly syrupy. Clearly, the link among genes, taste perceptions, and actual food choices is not a simple one.
That complexity is also evident in some of my bitter receptor genes that Reed tested. One of these was the bitter receptor T2R19, which detects quinine, the bitter chemical found in tonic water. I had the low-responding gene variant, according to the genetic test. Sure enough, when I sipped Reed’s quinine solution, I scored it only mildly bitter and not very intense. That squares nicely with my liking for tonic water. But it doesn’t explain Reed’s fondness for gin and tonic, because she carries the high-intensity gene variants. “I taste gin and tonic as very bitter,” she says, “but I love it!”
Then there’s our old friend T2R38, the bitter receptor that determines sensitivity to PROP, as well as the bitter compound phenylthiocarbamide (PTC) and the bitter thiourea compounds in broccoli and brussels sprouts. The genetic test backs up what I already knew from talking with Bartoshuk: I’m one of the “lucky” ones who reacts strongly to these bitter chemicals. And when I tasted the PTC solution, I scored it as intensely bitter.
So why does Dani Reed like gin and tonic, which she finds intensely bitter? Why am I drawn to the foods and drinks I taste as bitter, instead of avoiding them?
“What you taste isn’t always what you like,” says Reed. “I always say, ‘It’s the brain, stupid!’ You can learn! Within the correct context, it’s very much beloved.” Indeed, we quickly learn to find pleasure in flavors—even ones we initially find repulsive—that are paired with attractive rewards. The bitter coffee that delivers a wake-up jolt soon becomes pleasant in its own right. Same for the bitter beer or gin and tonic that accompanies an evening with good friends.
There may be another dimension to taste preferences, too, says Beverly Tepper, a sensory scientist at Rutgers University in New Jersey. Some of us are what Tepper likes to call “food adventurous.” That means there are really two kinds of supertasters, according to Tepper. Those who are not food adventurous are the classic, picky eaters: they don’t like things too sweet, too hot, too fatty, too spicy. “They know what they like, and their food choices are guided by their previous experiences. They’re a little bit finicky,” says Tepper. Mr. Lima-beans-and-milk presumably falls into that category.
On the other hand, supertasters who are food adventurous are willing to be surprised, even by intense tastes, and will try something again even after a disconcerting first experience. Because they’re not put off by intense experiences, this category of supertasters resembles nontasters in their food preferences. “I’m a supertaster, and I actually like a lot of the foods that theoretically I shouldn’t like. But I’m also food-adventurous,” says Tepper. That describes me to a T, too. I get the intense sensory jolt from a highly flavored food—but I like the stimulation.
These few genes that I had tested are probably just the tip of the iceberg when it comes to genetic differences in taste perception. Reed thinks there could be dozens—perhaps even hundreds—of genes that affect our taste acuity and our perceptions of particular tastes. In addition to the taste receptor genes themselves, many other genes probably affect how our cells respond once a taste receptor has been stimulated, how readily signals are sent to the brain, and every other step of the taste-sensing pathway. My flavor world, it seems clear, is different from yours. We can serve ourselves from the same bowl of soup and have different taste experiences. And taste is only one part of the flavor equation.
Excerpted from FLAVOR: The Science of Our Most Neglected Sense by Bob Holmes. Copyright © 2017 Bob Holmes. With permission of the publisher, W. W. Norton & Company. All rights reserved.