If you’ve ever wanted to blame the sweet tooth you inherited from your great-great-grandma for that second scoop of ice cream you just ate (and thus your big, child-bearing hips), well, now you can. Kind of. You can, at least, forgive yourself for a moment of weakness.

The science of genetics is opening a window – just a crack – onto our food preferences, including that sweet tooth, and the view is fascinating. I discovered this firsthand recently, when I got updated results from an ancestry.com DNA test I took a few years ago. The test uncovered the genetic roots of my sensitivities to sweet, bitter and savory tastes, and I learned more about my love of cilantro and the reason I hold my breath in the bathroom after eating asparagus.

Other companies also do this kind of testing, but I’ve been a member of ancestry.com for eight years. It’s where I keep my still-growing family tree, begun the old-fashioned way by my father in small-town records and remote, overgrown graveyards, and continued by me using online resources. Genealogy is addictive. As Barry Starr, director of scientific communication at ancestry.com, humorously put it in a recent interview, each new discovery – another relative identified on a 19th-century census record or a cause of death uncovered on a 1930s-era death certificate –  gives you a little hit of dopamine. So when the DNA tests finally fell into a price range I could afford, my curiosity won out and I ordered one. I spit into a little tube and sent my genes off to the lab.

Initially the test just tracked my ethnicity. If it proved anything beyond a shadow of a doubt, it’s that I’m one of the whitest people on the planet, despite my longing for a more diverse background. A whopping 57 percent of my genes can be traced back to England, Wales and northwestern Europe. Another 29 percent hails from Ireland and Scotland. I  have an 11 percent serving of Germanic Europe and just a dollop of Norway in my chromosomes. Initially my test also showed traces of North African and Persian DNA. But here’s the great thing about the test: As the pool of people submitting their spit to ancestry.com grows, and the organization’s scientists learn more from their DNA, they can narrow down your own family origins and give you updates. The updates are free; they’re just another little dopamine hit when you turn on your computer and discover new information in your account. In my case – much to my disappointment – the traces of North African and Persian DNA fell away.

Then, not long ago, I signed onto my account and discovered a new section on genetic traits, ranging from genes that affect my appearance to those that affect how my body processes vitamins. Most interesting to me were the sensory traits, many of which deal with food preferences. By testing two olfactory receptor genes on chromosome 11, for example, the test confirmed that I love cilantro – I’m not one of those people who think the herb tastes like dish soap.

Sure, they taste delicious, but, er, no way to put this delicately, does your urine stink after you eat asparagus? It’s genetic. Shawn Patrick Ouellette/Staff Photographer

But I am one of those people who have always noticed a certain, um, odor in the bathroom after eating asparagus. When we digest asparagus, our body produces asparagusic acid, which breaks down into funky-smelling compounds in our urine. The ability to smell these compounds is a dominant trait, so I inherited it from one of my parents (gee, thanks, Mom and Dad). People who cannot detect these metabolites are said to have “asparagus anosmia.”

Nature vs. nurture

Genetics aren’t the only thing that determine whether you prefer sweet over savory, love or hate cilantro, or can smell your stinky urine, however.

“Basically what we say is genetics loads the gun but environment fires it, pulls the trigger,” says Danielle Reed, a taste geneticist who is also associate director of the Monell Chemical Senses Center in Philadelphia, a nonprofit dedicated to basic research on taste and smell. “You have a certain landscape of what you can and cannot taste that is determined genetically, then you have a whole life experience that fills in the gaps that drive you toward what you like and don’t like to eat.”

My genetics say, for example, that I am extra-sensitive to the taste of bitter foods, which should mean that every time I take a bite of Brussels sprouts or broccoli, which contain bitter natural compounds called glucosinolates, my face should scrunch up in a look that says “This is revolting.” But I love broccoli, bitter greens and cauliflower. I confess I hated Brussels sprouts when I was a kid, but I think that had more to do with the way my mother prepared them than their natural flavor.

So what’s up? The ancestry.com test examines three markers in the TAS2R38 gene, which plays a key role in how we perceive bitter flavors. But Reed and Starr both point out that other genes may be at play as well. Reed says humans have 25 bitter receptors, each tuned to different bitter compounds. Some people are blind to certain bitter tastes, she said, but not all of them. Others may be able to taste the bitterness of, say, Brussels sprouts, but they like it.

That contradiction could be due to the confounding environmental factor, which can include cultural differences. Starr once tested the perception of bitter on a variety of people using strips coated with a synthetic chemical similar to glucosinolates. “Most people responded in horror at the taste of it,” Starr said. “But a friend of mine who grew up in a part of Asia where there were a lot of bitter foods in the diet actually didn’t mind at all. She thought it was fine.”

Sensitivity to bitter tastes is a dominant trait, which means at least one of my parents must be a “taster.” I’m guessing it’s my father, who won’t let a single bite of broccoli get past his lips but loves collard greens because he was raised in the South, where collards are regularly on the dinner table. Eating a food you hate until you like it, or at least tolerate it, is an example of the “mere exposure effect,” Reed explained, “and it means that you come to like things that you’re constantly exposed to.”

Most Americans would be disgusted by natto, fermented soybeans with a “snot-like” texture that are enjoyed in Japan, says scientist Bob Holmes, author of the 2017 book “Flavor: The Science of Our Most Neglected Sense.” And Japanese consumers have, historically, snubbed cheese, which is beloved by many westerners. “That seems to be cultural,” Holmes said, “because cheese is becoming more popular in Asia, and certainly second-generation Asian-Americans are perfectly happy with cheese.”

Holmes is a lot like me. He had his DNA tested through Monell, then followed up with a panel of taste receptor tests and discovered he’s very sensitive to bitter tastes. That means he should be pushing them away. In fact, he loves bitter greens, especially rapini (or broccoli rabe). He chalks up the discrepancy between his genes and taste buds to life experiences.

“Nobody likes the first taste of coffee” because of the bitterness, Holmes said, “but we quickly learn that it comes with all sorts of pleasurable consequences. You get that buzz from it, and you start to associate coffee with being out with friends, so you learn to love the bitter.”

Holmes also cited experiments by the Oxford psychologist Charles Spence that illustrated just how important environment is in flavor perception.

“He had people eat oysters listening to the sounds of the seashore, like crashing waves and calling seagulls, versus when they were listening to cattle mooing,” Holmes said. “They rated that the oysters tasted better, and tasted more of the sea, when they could hear the sounds of the seashore.”

Sweet deal

You might think that someone whose genes indicate they are sensitive to sweet tastes would have a huge sweet tooth. But it’s actually the opposite. People who are sensitive to sweets “just want a small piece of cake instead of a big piece of cake,” Starr said. “There’s less required to feel like you’ve had enough sweets.”

According to my test results, I’m “a little more sensitive” to sweets. That makes sense to me because I occasionally crave sweets (especially ice cream and chocolate), but once I indulge, I can sometimes go days or weeks without more. And I’m hyper-sensitive to the sugary syrups used at a lot of coffee houses and bars. A little sugar goes a long way.

Humans are wired for sugar, scientists say, perhaps because our ancient ancestors learned that honey or fruit provided a quick source of energy. (Similarly, humans may have evolved to reject bitter tastes because it helped them avoid poisonous foods.)

“We have wonderful videos of infants in utero when their mothers have glucose in their amniotic fluid because their mother is diabetic,” Reed said. “You can see that the infant is drinking the sweetened solution in the amniotic fluid. So babies can taste it in utero.”

But environment affects how we perceive sweet as well. Holmes’ tests, for example, showed that he is less sensitive to sweet flavors, which means it should take more sweets to satisfy him. But the very morning he found out that bit of information about himself, he had to pour out a sweetened iced cappuccino he bought at Starbucks. “I couldn’t drink it because it was too sweet,” he said. “And I have no problems skipping dessert. So behaviorally I don’t have a sweet tooth, even though my genes and my lab tests suggest that I do.”

Holmes suspects his palate leans this way because his family rarely ate dessert when he was a child.

Reed said the “reward factor” also plays a big role in our desire for sweets. “Food is very rewarding for some people,” she said, “and they just cannot let go of the reward value of sugared foods. That’s probably a combination of both their genetic makeup and their experience. Some people have a lifetime of experience eating sugar to comfort themselves.”

Sensitivity to savory flavors, or umami, works the same way. My test showed I’m “a little less sensitive” to umami, which may explain my love of aged cheeses and ripe tomatoes.

Can anything practical be learned from all of this, or is a genetic exploration of our flavor preferences just a bit of fun? It’s too early to say how such knowledge could help people meet nutritional goals or otherwise improve their health, Reed said. But knowing, for example, that your genes say you can’t taste sweet foods especially well could encourage you to treat yourself with more compassion, she says.

“It will help you understand why you’re putting four teaspoons of sugar in your coffee, right?” Reed said. “It’s no longer a moral weakness. It’s just there’s a biology there.”

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