Asparagus urine odor detection

A cluster of smell-receptor genes decides whether you can detect the distinctive sulfur smell in urine after eating asparagus. Some people simply can't smell it.

The trait is about smelling, not producing

Asparagus contains sulphur compounds that the body breaks down into small volatile molecules, chiefly methanethiol and related sulphides, which appear in urine within about fifteen minutes of eating it. Almost everyone produces them.

What varies is whether you can smell them. This variant sits near a cluster of olfactory receptor genes, including OR2M7, and is associated with asparagus anosmia, the inability to detect the odour. So the old question of whether asparagus affects your urine is mostly a question about your nose.

How strong the evidence is

The association was found in a 2010 study that surveyed roughly 10,000 people through a consumer genetics company and tested 22 common traits, an early example of web-based research design. In this site's evidence file, the association with smell perception draws on 9,126 people of European ancestry with a p-value below 1e-23.

It has held up as a real signal in a small literature. Later work has found additional variants in the same olfactory region, so this one variant is a marker for the region rather than the whole story.

How to read your result

As a curiosity, and one you can test directly in about fifteen minutes. If your result and your experience disagree, trust your nose: the variant explains part of the difference between people, not all of it, and smell perception also depends on concentration, dilution and simple attention.

Why olfactory genes vary so much

Humans carry around 400 working olfactory receptor genes and several hundred more that have decayed into pseudogenes, and this gene family is among the most variable in the genome. Many people are effectively blind to one specific smell while detecting everything else normally, a condition called specific anosmia, and asparagus is the best-known example.

What each rs4481887 genotype means

rs4481887 has three possible genotypes: GG, AG and AA.

Can't smell asparagus odor (rs4481887 GG). You carry the version near these smell receptors linked to not detecting asparagus pee, the distinctive smell some people notice in urine after eating asparagus. You may simply not smell it, even though your body still makes the odor.

Can smell asparagus odor (rs4481887 AG). You carry one copy of the variant that lets you detect the asparagus urine smell, so you can probably smell it after eating asparagus. It's purely about your nose, not your kidneys.

Can smell asparagus odor (rs4481887 AA). You have two copies of the variant tied to smelling asparagus pee, so you almost certainly notice the distinctive odor in urine after eating asparagus. Everyone's body makes the smell; this gene is about whether your nose picks it up.

Evidence & sources

Across global populations, about 5% of people carry two copies of this variant, and about 64% carry none.

This variant appears in 1 published genetic study: PubMed 20585627.

Common questions

Does asparagus not affect my urine? It almost certainly does. Nearly everyone produces the sulphur compounds; the difference is whether you can smell them, and this variant is associated with that. You can test it directly.

Can I learn to smell it? Specific anosmias sometimes shift with repeated exposure and attention, and detection also depends on concentration, so a strong dose in a small volume is easier to notice. There is no reliable way to train it.

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