Roughly 60% to 80% of people can smell the distinctive odor in urine after eating asparagus, depending on the study. The wide range exists because this isn’t just one question: some people don’t produce the smell at all, and others produce it but lack the ability to detect it. Those two factors, production and perception, combine to create a surprisingly complex picture of a seemingly simple dinner-table question.
Two Separate Traits, Not One
For decades, scientists debated whether the divide was between “producers” and “non-producers” or between “smellers” and “non-smellers.” It turns out both camps were right. A psychophysical study published in Chemical Senses tested both sides by collecting urine samples and running smell tests on the same group of participants. About 8% of people failed to produce the odor at all after eating asparagus. Separately, about 6% of people who were exposed to asparagus-tainted urine could not detect the smell. One unlucky participant fell into both categories.
This means the majority of people both produce and smell it. But the reason your friend swears their pee never smells after asparagus could be either explanation: their body might not break down asparagus into smelly compounds, or their nose might simply be blind to those compounds. Without a controlled test, there’s no way to tell which one from personal experience alone.
What Causes the Smell
Asparagus contains a compound called asparagusic acid that your body breaks down during digestion. The byproducts are a cocktail of up to six sulfur-containing chemicals, including methanethiol and dimethyl sulfide, the same family of compounds responsible for the smell of rotten eggs and cooked cabbage. These molecules are volatile, meaning they evaporate easily at body temperature and waft upward the moment you urinate.
The smell appears fast. Most people notice it within 15 to 30 minutes of eating asparagus, and it can linger for several hours. One study tracked it for up to 14 hours after a single serving.
The Genetics of “Asparagus Anosmia”
The inability to smell asparagus pee has its own name: asparagus anosmia. A large genome-wide study published in The BMJ pinpointed the genetic roots, identifying 871 genetic variations linked to the trait. All of them sit in a cluster on chromosome 1 that contains a family of smell receptor genes. Three independent genetic markers in that region showed the strongest associations.
Two of the most likely culprits are variations in a gene called OR2M7, which codes for a smell receptor protein. People with certain versions of this gene appear to have receptors that can’t pick up the sulfur compounds in asparagus urine. A variation in another receptor gene, OR2L3, also plays a role. These receptors are involved in binding odor molecules and triggering the nerve signals your brain reads as “smell,” so when they don’t work properly for these specific compounds, the odor is invisible to you.
This is a genuinely specific blind spot. People with asparagus anosmia typically have a perfectly normal sense of smell otherwise. They just carry receptor variants that happen to miss this particular group of sulfur molecules.
Why Study Estimates Vary So Much
If you search this topic, you’ll find numbers ranging from 20% to 40% of people who “can’t smell it,” which sounds like it contradicts the 6% figure above. The difference comes down to study design. Earlier studies relied on self-reporting: researchers simply asked people whether they noticed a smell after eating asparagus. Self-report surveys tend to find higher rates of non-smellers because they mix together people who truly can’t smell the odor, people who don’t produce it, and people who just never paid attention.
The more controlled studies, where researchers actually put asparagus urine under people’s noses and asked them to identify it, find much lower rates of true anosmia. The 6% figure comes from that kind of direct testing. The BMJ’s larger genetic study, which surveyed over 6,000 participants, found a higher proportion of self-reported non-smellers, but that study was designed to find genetic associations rather than pin down a precise prevalence.
The most accurate answer based on controlled data: around 92% of people produce the odor, and around 94% of people can detect it when it’s present. So the vast majority of people both make and smell asparagus pee. The fraction who genuinely cannot smell it, even when it’s right in front of them, is relatively small and genetically determined.
Why Some People Think They’re Immune
Given that only about 8% of people are true non-producers, many people who believe asparagus “doesn’t affect them” are likely producing the odor without realizing it. If you carry the receptor variants for asparagus anosmia, you’d never know your own urine smelled different. You could go your entire life eating asparagus and never once notice anything unusual, while everyone else who uses the bathroom after you gets the full experience.
Concentration also matters. How much asparagus you eat, how hydrated you are, and individual variation in metabolism all affect how strong the odor is. A few spears with a large glass of water might produce a faint smell that even a capable nose could miss, while a full plate of roasted asparagus on a mildly dehydrated evening will be unmistakable to anyone with functioning receptors.