Why Does Asparagus Make Your Pee Smell?

Asparagus contains asparagusic acid, a sulfur-containing compound that can make your pee smell. But it doesn’t happen to everyone and not everyone can smell it.

You may have noticed that after eating asparagus, your pee has a somewhat unpleasant scent.

This usually happens due to the metabolism of asparagusic acid, and the concept is referred to as asparagus pee.

However, this particular side effect of eating asparagus doesn’t happen to everyone, and some may have never smelled such a thing.

This article explains why eating asparagus makes pee smell, and why only some people can smell it.

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What’s asparagusic acid?

Asparagusic acid is a sulfur-containing compound that seems to be found exclusively in asparagus.

It’s a nontoxic substance that produces a sulfurous odor, which some say is similar to rotten cabbage.

Since a strong and pungent smell characterizes many sulfur-containing components, such as rotten eggs, natural gas, or skunk spray, scientists believe that asparagusic acid may be the cause of your pee’s funny scent after eating the vegetable (1, 2).

Asparagusic acid is a nontoxic, sulfur-containing compound that may cause your pee to have a distinct odor after eating asparagus.

How does it affect urine smell?

Once your body metabolizes asparagusic acid, it produces several sulfurous byproducts that are highly volatile — meaning that they vaporize easily (3).

When you pee, these compounds evaporate almost immediately, which enables them to travel from the urine up to your nose, allowing you to smell them.

Though scientists have not been able to determine whether one compound is responsible for the smell or if it’s due to the mixture of all of them, a compound called methanethiol is widely mentioned in the literature.

Methanethiol, also known as methyl mercaptan, is characterized by a strong and unpleasant scent that’s often associated with fecal odor and bad breath — and it happens to be the most common odorant found in urine after eating asparagus (4, 5, 6).

How long does the smell last?

Some people notice the rotten-like smell as early as 15–30 minutes after eating asparagus, and studies have determined that within 25 minutes, half of the asparagusic acid consumed has already been absorbed (7).

The fast absorption rate suggests that the effect of asparagus on urine smell can appear quite quickly, and recent studies also agree that it can last for more than a few hours.

One study in 87 people who ate 3–9 spears of asparagus found that the half-life of the asparagus smell was 4–5 hours (3).

The half-life of a substance tells you how long it takes for it to reduce to half of its initial amount. Generally it takes 4-5 half-lives for that substance to be completely eliminated from the body. Therefore, if the half-life of the asparagus smell was estimated at 4–5 hours, it means that the total effect could last up to 25 hours.

Yet, another study in 139 people who also consumed 3–9 asparagus spears reported the half-life of the smell to be 7 hours, meaning that the effect could even last up to 35 hours (7).

Either way, you can expect your pee to smell for quite a while.

When your body metabolizes asparagusic acid, it produces numerous smelly, sulfur-based compounds that give your pee a rotten-like smell that can last 8–35 hours.

It doesn’t happen to everyone

The effect of asparagus on urine scent is not universal, and a number of hypotheses try to explain this phenomenon.

One hypothesis — called the production hypothesis — suggests that only some individuals are capable of producing the sulfurous compounds responsible for the smell, while others are non-producers.

This hypothesis asserts that non-producers lack a key enzyme that helps metabolize asparagusic acid and are thus unable to produce the smelly byproducts (4).

For example, a small study in 38 adults determined that about 8% of them either didn’t produce the smell or produced it at concentrations that were too low to be detected (4).

The other hypothesis — called the perception hypothesis — states that everyone produces the smell, but some are unable to detect or perceive it (4).

In this case, researchers found a genetic modification that alters one or more of the olfactory receptors that should respond to the asparagus smell, causing what is known as asparagus anosmia, or the inability to smell asparagus pee (8).

In fact, research suggests that a large percentage of people can’t smell asparagus pee.

One study in 6,909 adults noted that 58% of men and 62% of women had asparagus anosmia, suggesting that this specific genetic modification is quite common (8).

Not everyone is familiar with asparagus pee, and researchers believe that it’s because some people either don’t produce the smell or are unable to perceive it.

The bottom line

The asparagusic acid in asparagus produces many sulfurous byproducts that give your pee a rotten-like smell.

The smell can be detected as early as 15 minutes after eating asparagus and may last up to 35 hours.

However, not everyone produces the smell, and the majority of people can’t smell it due to a specific genetic modification.