Tahlequah Daily Press

August 3, 2013

Study shows genetics can determine why we smell certain odors differently

By Meeri Kim
The Washington Post

— The next time you argue with a friend about the whiff of cilantro in your stir-fry that he finds disgusting, blame his genes.

All of us inhabit our own flavor world that is highly dependent on our genetic blueprint for being able to smell certain odors, according to two studies a New Zealand research group published online Thursday in the journal Current Biology.

The perception of flavor while eating includes what your taste buds can detect — sweet, salty, bitter, sour and umami (often called savory) — but they play a meek supporting role to your sense of smell.

That sensory experiences of food differ from person to person has always been known anecdotally. Think of strong, hoppy beer or black coffee that some find revoltingly bitter but others can drink like water. But the researchers wanted to know how much of that variability is genetically hard-wired.

They had 187 people smell 10 different compounds, taking note of who could smell them at a certain concentration level and who couldn't. They sequenced each person's DNA, scanning their genomes to pick out parts that could possibly explain the variations in smell sensitivity.

Out of 10 scents tested, four had a strong genetic basis — malt, apple, blue cheese and violet. They were also independent of one another, so even if you have a super-nose for blue cheese, you might fall short when it comes to smelling violets.

One study found a single gene change linked to the floral scent found in violets. The other study more generally hunted down spots within the genome connected to three additional food-related smells.

Odors typically consist of molecular building blocks called aroma compounds. For example, while blue cheese gives off a whole bouquet of aromas, 2-heptanone is the chemical that really "gives blue cheese its blue cheesiness," said lead researcher Richard Newcomb of the New Zealand Institute for Plant and Food Research.

The roof of our nasal cavity has a patch of millions of special neurons, each one covered with olfactory receptors that bind to odor molecules that travel through the air. But each neuron has a single type of odor receptor, meaning it can detect only smells that have the right structure.

For comparison, color vision only has three types of receptors, each corresponding to different range of the color spectrum; our nose has 400 types, each with a matching olfactory receptor gene within our DNA. Any variation in those genes will affect our sense of smell.

Since humans can detect 10,000 odors, and each one can be dialed up or down depending on that person's genetic makeup, smelling is a highly personalized experience.

"When people sit around and share a bottle of wine, they often describe it quite differently," Newcomb said.

Wines make up some of the most complex flavors out there, he said, and a single wine can give off hundreds, if not thousands, of distinct aromas.

Sometimes, your sensitivity can even completely change the notes of a scent. Newcomb and his colleagues found that the violet compound smelled "fragrant" and "floral" to those with a heightened sense, whereas less sensitive individuals described it as unpleasantly "sour" and "acidic."

Our genes are coded with similar likes and dislikes for cilantro.

"Some people get a floral note from it, whereas others get a real soapy sensation," Newcomb said.

Joel Mainland, a neuroscientist at the Monell Chemical Senses Center who was not involved in the studies, suspects the culprit is a bad-smelling chemical within the hundreds of molecules of cilantro that some super smellers can detect — the cilantro haters — but others cannot.

Mainland called the pair of studies "an advance in the field" and sees applications for the food and flavor industry.

"We can give [companies] a feel for the percentages of their target market that will be able to smell a given flavor by extrapolating out to all humans on the planet," Newcomb said.

Newcomb and his colleagues were careful to control for any olfactory "noise in the system" caused by mood, pregnancy or time of day.

Pregnancy can cause hypersensitivity to the point of disgust, and some believe overwhelming odors can even explain morning sickness. Being in a bad mood dulls our sense of smell for reasons unknown, he said, so data from gloomy subjects were left out. Even time of day has an effect; our nose works best before lunchtime, probably ramping up because of hunger.