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Sep. 24, 2009

Why does asparagus make my pee smell funny?

by Molly Nickerson

Asparagus-induced aromatic pee is an event that has always amazed me because it happens so quickly. When you eat asparagus, it goes into the stomach to be broken down by acids in the stomach, just like any other food. The nourishing elements of the meal are absorbed into the blood stream and the food molecules travel through the bloodstream to the liver and kidneys for purification. This is all normal and good. Waste that is collected in the kidneys is excreted in urine. Asparagus, unlike other vegetables, contains asparagusic acid. It is the breakdown of asparagusic acid that creates smelly byproducts.

Here are the details….digestive enzymes break down ingested asparagus and produce methanethiol (or methyl mercaptan), which is believed to be the aromatic culprit in stinky asparagus urine. (However, some scientists believe that a different family of thioesters that are the byproducts of acid reacting with sulfur-containing alcohol are responsible.) The methanethiol waste product is then (somewhat mysteriously) transmitted through the kidney to the urine. Ironically, asparagus is a diuretic, which contributes to the rapidity with which the scented urine exits the body. The aromatic nature of the methanethiol is due to the fact that it is released as a gas when you urinate.

Interestingly, a similar process is employed by skunks when they make their musk. The difference with skunks is that they have specific glands for storing and concentrating sulfur-containing chemicals, which they can create from more products than asparagus. Methanethiol is also released from decaying organic matter. It is truly a wonderful aroma.

There has been some controversy as to whether everyone has stinky urine after eating asparagus. I am aware of no recent, controlled clinical study on this topic, but there is an alternative hypothesis. It is quite possible that, while we all conduct this digestive process, we do not all have smell receptors that are sensitive tor methanethiol. We all have a very unique array of olfactory receptors and it turns out that the ability to smell asparagus is a dominant genetic trait and is therefore not universal. We are born with about 400 olfactory receptors out of a possible 1000. This does not mean that we can only smell 400 different aromas; a single odorant binds with varying affinities to different olfactory receptors, creating a distinct pattern of nerve impulses to the brain that define the scent. Another odorant may bind similar olfactory receptors with different affinities, creating a unique pattern of nerve impulses to the brain and a completely unique sensory representation. The brain then interprets that scent and associates it with the object. We can smell thousands of different smells, based on the unique combination of smell receptors rendered active by a particular odorant.

Our olfactory systems are a very primitive part of our brain and tightly linked with the hippocampus and our memories. This is why certain smells evoke strong memories. Perhaps you can conduct a controlled trial of the genetic patterns of methanethiol sensing in your family at the next family gathering. That will certainly create some smell-associated memories!

About Molly Nickerson

The views expressed are those of the author and are not necessarily those of Science Friday.

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