If the sweaty guy standing in line next to you smells like vanilla—or urine—you may have whiffed a steroid in his body odor called androstenone.
The chemical can take on either a vanilla or "woodsy" urine scent, depending on which version of a mutated odor gene carried by you, the person doing the smelling, a new study has shown.
"People who express different variants of this receptor perceive this odor differently," said study co-author Leslie Vosshall, a molecular neurobiologist at Rockefeller University in New York City. "This is the first study linking differences in how a chemical smells down to a gene."
Potpourri of perception
When people sniff androstenone, Vosshall said, roughly a third detect vanilla, a third report urine and the final third smell nothing at all.
"The polarization of sensitivity is really also quite extreme," she said. "Some people are overwhelmed by an amount 10-billion-fold dilution while some can't smell anything at all."
Vosshall said she's in the "top 5 percent" of sensitive smellers and picks up a stinky urine smell.
"If someone sneaks a tissue into my office that's even been around androstenone, it smells like my head is in the armpit of a guy who's been running for 100 days without showering," she told LiveScience.
Vosshall and her colleagues' findings on the odor gene, called OR7D4, are detailed online today by the journal Nature.
Take a whiff
To discover the genetic link to androstenone's smell, Vosshall and her colleagues at Duke University in North Carolina exposed about 400 volunteers to 66 different chemicals, including nutmeg, anise, vanilla and androstenone. Each subject noted the intensity and presumed smelly identity of each chemical, then researchers took blood samples to see how androstenone affected the subjects' DNA in a separate experiment.
Those with the more common version, or allele, of the OR7D4 odor-receptor gene described androstenone as "sickening," while those with a slightly different allele detected vanilla.
The gene now explains about 30 percent of the variation in people's perception of androstenone—leaving researchers uncertain what other biology affects perceiving the chemical—but Vosshall said that the link was very strong compared to other genetic mysteries
"Thirty percent is extremely high when you consider genes responsible for breast cancer explain only 10 percent of breast cancer cases," Vosshall said. "There are probably other odor genes responsible for the differences in sensitivity and perception of androstenone."
The discovery is important because smells are a big part of being human, Vosshall said. While androstenone may be a guy thing, mainly because it's derived from the hormone testosterone, female pig studies show a whiff of it can have profound effects on biology.
"Farmers spray it on female pigs in heat, and they'll get in position for mating almost instantly—no questions asked," Vosshall said. "In human females, studies show it may cause arousal, sweating and a surge in stress hormones. Women are also much more sensitive to it near ovulation."
She explained that while androstenone's "funny effects" on women are intriguing, the jury is still out on whether it's a pheromone. "It seems like the human brain has some filters against it, but it's doing something different from smelling cheesecake or strawberries," she said.
Vosshall said such signals may be important in social settings, such as landing a date in a club or politely rejecting a tuna sandwich, but research has been limited.
"We want to see what other things are going on in human genes when we smell things, because how we experience odor is a big part of the human experience," Vosshall said. "Knowing how much of our perception was drummed into us as kids and how much of it is genetic is a really important topic in science."
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