science A matter of taste

Catalyst — Spring 2005
If Brussels sprouts on your plate make you miserable, your DNA just might be at fault.

Benedicte Fontaine-Bisson was 24 years old when she discovered her genes were to blame for her dislike of bitter food. She’s a ‘supertaster,’ so sensitive to bitter flavours that it’s almost unbearable for her to taste broccoli, coffee, wine and beer. But some people’s genetic makeup excludes them from tasting what she is so responsive to.

Genetics is only one ingredient in the recipe for human flavour experience. The adage ‘you are what you eat’ is being recast as researchers uncover that what you eat mirrors who you are—genetically, historically and socially.

“Food preference is a wonderful example of a behaviour that involves every aspect of our being,” says John Prescott, a taste scientist at John Cook University in Cairns, Australia. For many years the genetics of food preference “wasn’t really anywhere and now it’s growing pretty rapidly.”

Fontaine-Bisson found out she was a supertaster when she participated in a simple experiment at the University of Toronto. She was given a piece of paper to taste that contained just micrograms of a bitter compound called PTC, or phenylthiocarbamide. It’s like a chemical present in vegetables like broccoli, Brussels sprouts and cabbage.

“Some other people tried it before me and they said, ‘Oh I can’t taste it, I can barely taste it,’ so I tried it, and oh my god, I had to spit it in the garbage. I became flushed, all red, I almost wanted to vomit it was so strong for me. And I couldn’t understand why other people could barely taste it.”

Research shows that between a quarter and a half of all people have a taste receptor on their tongue whose genetic sequence cannot detect the bitter PTC. The rest of us can. Then there’s the supertasters. Not only can Fontaine-Bisson taste PTC, she’s got extra bitter receptors in her taste buds that pack an extra punch.

“If you look at the tongues of non-tasters, tasters and super tasters you can see a progression from sparsely populated [taste buds], to more densely populated to extremely densely populated,” says Prescott.
‘Now people don’t push me to drink or eat bitter foods. I have a kind of explanation, something more scientific.’

More taste buds mean more signals to the brain when bitter compounds are present, which makes people overreact to bitter flavour. For Fontaine-Bisson, it’s a relief to find out it’s her genes that are at fault.

“Now people don’t push me to drink or eat bitter foods,” says Fontaine-Bisson. “I have a kind of explanation, something more scientific.”

But as Prescott points out, the genetics of flavour preference is still a young science. Genes for the five kinds of taste receptors — bitter, sour, sweet, salt and umani/savory — are only just coming to light. Currently, 24 different kinds of receptor genes for bitter taste are known. There have been several genes discovered for sweet, sour and umani taste receptors, but none for salt.

Although taste is made up of these five different components, our appreciation of food flavour hinges on 350 different types of odour receptors in our nose. The researchers who discovered this main gene family for odour receptors won a 2004 Nobel Prize for their work.

To further complicate matters, any given odour receptor can detect a multitude of odours; likewise, a single odour can trigger a multitude of odour receptors, says Dr. Charles Wysocki, an olfactory specialist at the Monell Chemical Senses Center.

Understanding how slight differences in the DNA of a person’s smell receptors can alter their experience of that smell is a focus of Wysocki’s research.

“We are comparing the genetic makeup of people who have different perceptions of androstenone, a chemical found in pork, celery and some mushrooms,” Wysocki says. “[Thirty five per cent of] people think it’s a very nasty stale underarm smell, or a stale urine smell. For other people (15 per cent), the very same odour at the very same concentration smells floral and about 50 per cent of the population cannot smell anything, even at very high concentrations.”

Although researchers are beginning to untangle subtle differences in taste and odour genetics, brain imaging research shows that human response to the flavour of food is more than just the sum of these individual taste and odour components.

“You can look at a [brain MRI] response to an odour, and then the response to a taste. But if you combine that odour and taste into a flavour that is presented together into the mouth you get a completely different [MRI] pattern of response,” says Prescott. “The brain takes different information from taste, from smell and [how a food feels] in the mouth and basically integrates them into a unitary perception.”

These perceptions—and consequent preferences—evolve throughout your life, but begin in the womb. “Taste and olfactory systems are developed before birth. Experience of flavour in amniotic fluid and mother’s milk modifies [later] response to that flavour,” says Julie Mennella, biopsychologist at the Monell Chemical Senses Center in Philadelphia.

Fetuses exposed to unusual flavours in the womb from their mother’s diet are more likely to be accepting of that flavour. The reason? “We learn which foods are safe from our mothers.”

Mennella recently found early exposure to unusually bitter or sour flavours in infant formula can make babies develop preferences for tastes adults can’t stand. The infant formula tastes bitter and sour because its ingredients are broken down to help with digestion.

While most babies dislike bitter flavours—an evolutionary advantage since many poisons are bitter—three-month-old infants given the bitter formula over three and seven months learned to like it. Other research finds kids need nine straight days of trying a new food before they’ll accept it. These studies suggest food preferences can be learned, given a little time and patience.
‘As adults we’ll try things because it’s a ‘happening’ food or drink, and we’ll develop a certain liking for food because it fits in with our self image.’

Prescott believes learning to like the taste of something by exposure continues throughout adulthood. The only difference is the motivation for eating an unusual or disliked food. Kids are forced to eat broccoli by their parents, while adults are pressured by peers or health concerns.

“As adults we’ll try things because it’s a ‘happening’ food or drink, and we’ll develop a certain liking for food because it fits in with our self image,” Prescott says. “Adults think, ‘I’ve got to start drinking scotch,’ which they don’t usually like, or eat olives … and then they develop liking for it.”

For Fontaine-Bisson, wine is what she wishes she liked. “When you go to someone’s place for dinner it is socially acceptable to drink wine,” she says. “But I don’t really appreciate it that much. I force myself.” Fontaine-Bisson says she recently tried a rosé wine that she liked, “probably because it was very sweet and fruity.”

Severe exposure is just one form of conditioning that can have an impact on taste preference. People who pair unknown or disliked flavours with those they enjoy—like salt or sweetness—will also change their preferences, says Prescott.

But there’s an additional, unconscious conditioning that changes food preferences based on the health impact of the food or flavour — such as providing energy or an essential vitamin or mineral.

“Post-ingestive conditioning pairs a novel flavour with the [consequent] effects such as the energy provided by something like a sugar or fat,” says Prescott. “The body recognizes these substances as being good, ‘Oh, this novel flavour is a signal that my body is getting energy, I’d better start to recognize and like this novel flavour.’”
tongue

This is one reason Prescott thinks people might like food with MSG (mono-sodium glutamate). “When you introduce glutamate into the diet [of rodents] it actually gets preferentially taken up by the body, in contrast to the body’s own glutamate, and used for synthesis of other amino acids and utilization of carbohydrates.”

Prescott’s lab showed that pairing a novel flavour with glutamate helped increase human acceptance of the new flavour.

“If you ingest glutamate, compared to just tasting it, you develop a high degree of liking for a novel flavour, and by analogy with how we think sugars and fats act post-ingestionally. We think that the body recognizes MSG, and is rewarding [the new flavour] because of MSG’s usefulness.”
‘The idea is to find a compound that we could put in a carbonated soft drink so that two percent sugar tastes like ten or 12 percent sugar.’

Prescott’s research is just one example of how food preferences are linked with health. “When you ask people to eat some sorts of [healthy] foods and not others, the single biggest barrier to adopting those recommendations is how the food tastes,” says Prescott. “Crap foods taste terrific.” Controlling flavour — or human response to it — will hopefully lead to healthier and tastier diets.

Nutrition researchers are trying to find compounds that sensitises odour receptors in the elderly, to compensate for the fewer receptors found in their noses as they age. Scientists at Linguagen, a taste biotechnology company, are looking for a compound that sensitises sweetness receptors, to decrease sugar content in food and beverages. “The idea is to find a compound that we could put in a carbonated soft drink so that two per cent sugar tastes like 10 or 12 percent sugar,” says Richard McGregor, a senior scientist with Linguagen.

Whatever her predispositions are, Fontaine-Bisson finds being a supertaster difficult. “I would like to eat everything,” she says.

Linguagen also developed a compound that blocks bitter receptors in the tongue from sending taste signals to the brain. Supertasters like Fontaine-Bisson may soon be able to subdue their strong sensory reaction to bitter foods.