Asparagus Odor

Asparagus Odor Photo Credit: Clipart.com.

Genetics influence whether you smell a distinctive odor in your urine after eating asparagus.


Explaining asparagus pee. I'm Bob Hirshon and this is Science Update.

Many people, but not all, smell a distinct odor in their urine after eating asparagus. Now, researchers at the Monell Chemical Senses Center in Philadelphia are finding out why. Marcia Pelchat and her colleagues found that some people don't produce the smelly asparagus by-product, while others can't smell it. The latter was linked to a variation in their olfactory genes. Pelchat says this insensitivity to one particular odor is called a "specific anosmia."

And the suspicion is that everyone has a few of them. So that everyone has his or her own sensory world.

She also notes that the asparagus odor is a sulfur compound, similar to the rotten-egg-like odor added to natural gas so we can detect it. And it's worth finding out if people who can't smell asparagus pee also might be less likely to notice a gas leak. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

This may seem like a weird little study, and in some ways, it is. But it also relates to larger issues about the way we perceive the world. Chances are there's a kind of food you like that at least one friend finds disgusting, and vice versa. The same goes for smells. And while many factors undoubtedly affect our personal tastes, scientists are learning that things simply smell and taste different to different people.

Take cilantro, for example: an herb that turns up often in Mexican, Vietnamese, and Thai food, among others. To some people, it's delicious; to others it tastes like soap, or worse. There's no definitive answer as to why—it hasn't been formally researched—but informal observations suggest that some people may be better able to smell all of the herb's flavor chemicals than others. And since a food's flavor mostly comes from its smell (our tongues actually taste only the basic sweet, salty, bitter, or sour elements), differences in smell determine differences in taste. (Surprisingly, the difference of opinion on cilantro may be because some people can't taste the herb's good flavors, so they taste only the unpleasant soapy ones.)

The cilantro insensitivity may be another example of a "specific anosmia," like the one that keeps some people from smelling the asparagus odor. ("Anosmia" means "inability to smell;" some people have total anosmia, which is like smell-blindness.) Our olfactory (smell) system can respond to thousands of different chemicals, so it's not surprising that genetic variations can make us insensitive to some chemicals and not others. The end result is that each of us has a unique sensory world: even if you and a friend sniff the same item, your experience of it may be different from your friend's. Very roughly speaking, it's like each of us is listening to a piano with different keys missing—so the same concerto will sound different when played on one piano versus another, especially in places that rely on those keys. 

This doesn't mean that chocolate, for example, tastes radically different from person to person. It's likely that most people have receptors that respond to all the flavor compounds in chocolate, and even if we're missing one or two, it won't make the chocolate taste like tomato soup or garlic bread. But it's interesting to note that our differing perceptions of taste and smell start at a very basic level—whether or not our brains even detect everything that's there. 

Now try and answer these questions:

  1. What is a specific anosmia?
  2. How does a specific anosmia affect whether or not you smell the asparagus odor?
  3. What else can affect your perception of an odor?
  4. To people with red-green color blindness, red and green look the same. How is a specific anosmia for one particular chemical like red-green color blindness? How is it different?

You may want to check out the related the Science Updates Smelling Diseases and Cancer Sniffing Dogs focus on the emerging science of screening for illnesses by using smells.

For Educators

The Science NetLinks Science Update lessons Smelling Diseases and Cancer Sniffing Dogs focus on the emerging science of screening for illnesses by using smells.

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