Why Does Asparagus Do That?

Have you ever noticed something "strange" after eating asparagus -- that is, after leaving the table and retiring to the small, private room? Well, it's not just you. The phenomenon has been observed for thousands of years, yet scientists still can't explain it completely. The question: why does your urine smell odd after you eat asparagus?

A stream of speculation about this effect has existed at least since the time of the ancient Egyptians. Scientists have pondered the question with such frequency and urgency that it has been known to awaken them in the middle of the night. If you are among those who have been unable to relieve themselves of anxiety over this issue, the following information may help.

Because some individuals notice this phenomenon and some don't, people tend to fall into two categories. Those who have experienced the effect may never have inquired about it, either because they think the question is a wee bit indelicate or because they think they may be the only ones who produce the odor. On the other hand, there are many who have never experienced the effect and don't care a whizz about the subject.

There are two competing theories about the cause of the asparagus urine phenomenon. Number one is that not all people produce the odorous chemicals, and number two is that everybody produces the chemicals but not everyone can smell them. Research on this issue has been fluid, favoring first one stream of thought and then the other. Only recently has a consensus begun to trickle forth, although it cannot yet be said that either theory enjoys the complete flush of success.

What is not in doubt is that asparagus contains certain compounds that, when metabolized, may produce certain sulfur-containing chemicals that have a strong, distinctive odor and are excreted in the urine. Identifying those chemicals, however, has been far from a piddling task.

In 1891 a chemist named Nencki reportedly identified the odor as the chemical methanethiol, and for a long time scientists believed that it was the offender. In 1975, however, Robert H. White at the University of California, San Diego took aim at the problem using modern analytical techniques and identified several sulfur-containing compounds as the odoriferous culprits but found no evidence of methanethiol.

Then, in 1987, Waring, Mitchell, and Fenwick at the University of Birmingham in England found as many as six completely different chemicals from White's, this time including the elusive methanethiol. It would appear that the identities of the odorous compounds are still as much up in the air as their vapors.

Scientists do generally agree, however, that the puzzle's solution lies in our genes. Some believe that only a certain number of people have the gene for producing the odorous chemicals; we'll call that the producer gene, or "P gene." Others believe that only certain people have the gene for smelling the pungent chemicals; we'll call that the smeller gene, or "S gene." To my knowledge, no studies have been done to determine whether people who have the P gene are more or less likely to have the S gene, or whether an S-gene person can detect the products of other people's P genes.

Allison and McWhirter (1956) found that 40 percent of 114 subjects apparently have the P gene, while Mitchell et al. (1987) found evidence of it in 43 percent of 800 subjects. On the other hand, a study by Lison et al. (1980) had concluded that everyone has the P gene, but only a minority of people have the S gene. A later study of 103 French subjects by Richer et al. (1989) detected the P talent in all of them. In their report, however, the French researchers neglected to describe certain essential details of their experimental methods, so their conclusions may be all wet.

To muddy the waters even further, a group of curious and uninhibited students at Carleton College in Minnesota recently concluded that every one of them produced the odor and that every one of them could smell it. Although the specimens were cross-mixed in a double-blind "whodunit" scheme, I believe their conclusions may not hold water because of flaws in their swapping procedure that I shall refrain from describing here.

The current state of understanding seems to be that both the production and the detection of odorous post-asparagus compounds are genetically determined, with most people being producers and fewer than half having the ability to detect the odor. But to my knowledge, none of the research has attempted to determine whether people who say they "smell it" are all detecting the same chemical compounds. In light of the mixed results, it may even be that different individuals produce different collections of asparagus metabolic products, and that different individuals have differing olfactory sensitivities to those different products. What a mess!

If that is indeed the case, it would be hard to think of a more frustrating set of circumstances for scientists who are trying to find a definitive solution to the Great Asparagus Piddle Riddle.