Asparagus Urine Odor: Not Producing It and Not Smelling It Are Different

Some people notice a change in the smell of their urine after eating asparagus, while others have no idea what they mean. Does the difference lie in producing the characteristic odor, or in being able to smell it?

A study that examined the two separately identified both people whose samples had an odor that was difficult to detect and people who had difficulty identifying the characteristic odor in other people’s samples. In addition, a variant in a region containing a cluster of olfactory receptor genes was associated, in this analysis, with the ability to perceive the odor rather than the ability to produce it.

Smelling only your own urine cannot reveal the reason

The study started by reconsidering how earlier surveys had asked their questions. Asking whether someone has ever noticed an unusual smell after eating asparagus cannot distinguish people who do not produce the characteristic odor from those who produce it but cannot smell it.

Similarly, comparing post-asparagus urine with water might allow people to answer using ordinary urine odor as a cue, rather than the distinctive asparagus-related odor.

The team therefore compared urine collected before and after eating asparagus, or after eating bread, and had both the donors and other participants smell it. The main aim was to separate individual differences in producing the characteristic odor from differences in detecting it within the same study.

An asparagus day and a bread day

Thirty-eight adults participated. Because not everyone completed every stage, there were 37 urine donors and 31 odor assessors. The study comprised two experiments conducted about four months apart, with no overlap in participants.

Urine was collected on two days at least three days apart. On one day, participants ate asparagus weighing 125 g raw, prepared with oil and salt; on the other, they ate 72 g of bread with the same amounts of oil and salt. Water intake was also standardized, and urine was collected before eating and two hours afterward. The order of the asparagus and bread days was counterbalanced.

Samples were frozen, then thawed and brought to room temperature for evaluation. The sides of the jars and other visible features were covered to make visual judgments difficult. Participants and experimenters wore cotton gloves to avoid contamination from scented cosmetics and similar products.

Choosing the jar containing the characteristic odor

Assessors first smelled a particular person’s post-asparagus sample as a reference. They then compared two jars containing samples from that same person.

The pairs consisted of samples after asparagus versus after bread, or before versus after asparagus. Participants had to choose which was the post-asparagus sample even if they were unsure. Each pair type was presented three times, yielding six responses for each urine donor.

How accurately other people select a donor’s sample provides a measure of how detectable its characteristic odor is. Conversely, how accurately one assessor selects the right samples across many donors provides a measure of that assessor’s ability to smell the odor. Comparing performance with the 50% chance level of a two-choice task is also important.

Individual differences in both producing and smelling the odor

For three of the 37 donors, post-asparagus samples were not identified at a level significantly above chance. The characteristic odor was distinguishable in the remaining donors’ samples, although detectability varied.

Even those classified here as unable to produce the odor were not shown to have absolutely none of the substances responsible. This was not a chemical analysis measuring particular molecules. It tested whether the odor was sufficiently detectable under a defined intake, two-hour sampling time, and smelling procedure.

Among the 31 odor assessors, two could not distinguish the asparagus-related odor in other people’s samples better than chance. The remaining assessors also varied in how well they performed.

There was no significant correlation between how readily people produced the characteristic odor and how readily they perceived it. Nor was there a clear association with age or sex. At a minimum, the results did not explain differences between participants solely in terms of either production or perception.

Checking whether the difficulty extended to odors in general

To determine whether people who struggled to identify asparagus odor simply had a generally weaker sense of smell, the researchers measured detection thresholds for phenylethyl alcohol, which has a rose-like odor.

All participants fell within the study’s criteria for the normal range, and sensitivity on this test was not associated with asparagus-odor identification performance. This supports an explanation involving reduced sensitivity to a particular odor.

The later experiment also included a task comparing ordinary urine with urine to which basil aroma had been added. Participants identified the samples almost perfectly, reducing the likelihood that poor asparagus-odor performance arose from misunderstanding the two-choice instructions.

The genetic association appeared on the perception side

DNA was collected from cheek cells, and the researchers examined a single genetic variant, rs4481887, reported in earlier research. This site lies in a region containing about 50 olfactory receptor genes, between the genes OR2M7 and OR14C36.

To correspond with the earlier research, the genetic association analysis was performed in the subgroup of participants who self-identified as white. It found an association in which people carrying the A allele identified the characteristic odor more accurately. In contrast, the association between the same variant and the detectability of the odor produced in samples was not significant.

Because nearby variants in this region tend to be inherited together, an association at this one site cannot establish that OR2M7 is the receptor directly detecting the odor. The study’s contribution was to separate production from perception in an association previously based on self-report, and to show a link with the latter.

The odor-causing substances and timing remain open questions

The original paper reviews earlier reports of several sulfur compounds and other substances that may produce the characteristic odor after asparagus consumption. This study itself did not identify a single definitive causal molecule. Differences in the quantities present or in volatility may also influence how easily samples can be distinguished.

Urine was collected only two hours after eating. If individuals differ in when the odor first appears or reaches peak intensity, results at another sampling time could differ. Habitual asparagus intake was not investigated either, leaving the relationship between odor experience and genetically influenced sensitivity for future research.

Summary

After asparagus consumption, individual differences were found both in how detectable the characteristic odor was in samples and in people’s ability to smell it. Under these conditions, samples from three of 37 donors were not adequately distinguishable, and two of 31 assessors could not adequately identify the characteristic odor in other people’s samples.

A variant near a cluster of olfactory receptor genes was associated with perceiving the characteristic odor rather than with producing it. The same experience of not noticing an odor can have different explanations; separating the production and reception sides makes those explanations more specific.

The original paper states that the research was funded through institutional resources at the Monell Chemical Senses Center.

References

Pelchat ML, Bykowski C, Duke FF, Reed DR. (2011). Excretion and Perception of a Characteristic Odor in Urine after Asparagus Ingestion: a Psychophysical and Genetic Study. Chemical Senses, 36, 9–17. Original research article.