Try this experiment. (I know it sounds yucky, but try it anyway.) Eat some asparagus--a lot if you can stomach it. Then, the next time you go for a tinkle, sniff the air. Do you smell an odor something like cooked cabbage? If you do, you are not alone. Even good old Ben Franklin conducted this experiment and achieved a positive result: "A few stems of asparagus eaten shall give our urine a disagreeable odor," he wrote. Proust apparently liked the odor better. "[It] transforms my chamber-pot into a flask of perfume," he wrote.
If you've taken a look at Brain Sense, you know I'm partial toward studies of the chemical senses, perhaps because we know so little about them. How we smell and taste is better understood today than ever before, but we still have a lot to learn. So, this week when a new study on what we might call the Asparagus Phenomenon came online, I wanted to sniff around and learn more about it.
Here's what scientists at the Monell Institute reported in last week's online edition of Chemical Senses. Realizing that most people, like Franklin and Proust, are aware of the odor--while a relatively few others are not--the researchers set out to determine whether the odor-unaware minority doesn't produce the odor, doesn't recognize the odor, or both.
In a series of tests with 38 volunteers who consumed (and smelled the results of) both asparagus and bread, the scientists found that approximately 8 percent of us don't produce the odor, while 6 percent make it but can't smell it. One person in the study group neither produced nor detected the odor.
Studying DNA samples from the volunteers, the researchers determined that the inability to smell asparagus-metabolite odor was linked to genetic variation within a family of olfactory (smell) receptors. Specifically, the reduced ability to smell the asparagus metabolites in urine turned out to be related to a single DNA difference near a known olfactory receptor gene called OR2M7. The gene lies within a large cluster of olfactory genes on chromosome 1.
"This is one of only a few examples to date showing genetic differences among humans in their sense of smell," said study co-author Danielle Reed, a Monell behavioral geneticist. "Specifically, we have learned that changes in an olfactory receptor gene can have a large effect on a person's ability to smell certain sulfurous compounds."
Results of the study also provide an example of ways in which normal people differ in their metabolism. "Although seemingly just a curiosity, the individual differences in metabolism could be important in other realms," said study lead author Marcia Levin Pelchat. "Additional studies are needed to determine whether the inability to produce the odor is associated with other metabolic traits or disorders," Pelchat added.
For More Information:
Faith Brynie. Brain Sense. Amacom, 2009.
Marcia Levin Pelchat, Cathy Bykowski, Fujiko F. Duke, and Danielle R. Reed. "Excretion and Perception of a Characteristic Odor in Urine after Asparagus
Ingestion: a Psychophysical and Genetic Study." Chemical Senses Online, Sept. 27, 2010,
