• Question: Is it true that if you eat asparagus it will make your urine smell? But you have to have the gene for it to smell and the gene to be able to smell it?

    Asked by bethanyknight to Charlie, Eoin, Jemma, Julian, Steve on 22 Mar 2011.
    • Photo: Charlie Ryan

      Charlie Ryan answered on 21 Mar 2011:

      hi bethany good question although i don’t like asparagus so i dont know the answer! Do you like asparagus?? Maybe you could run an experiment if you do???! Although please don;t tell me the results – i really don’t want to know!!

    • Photo: Stephen Moss

      Stephen Moss answered on 22 Mar 2011:

      Hi Bethany
      Yes it’s true, to both of your questions. I like asparagus, and I have the gene so I can smell the methanethiol. Not the nicest smell in the world, but interesting from the genetic and biochemical point of view.

    • Photo: Julian Rayner

      Julian Rayner answered on 22 Mar 2011:

      Hi bethany. This is a really cool example of a simple genetic trait. Asparagus contains a lot of a chemical called asparagine, and when you eat enough, the excess that your body doesn’t need gets excreted in your urine. Everyone excretes it, but some people express the gene that enables them to smell it (the gene encodes a receptor in the brain), and others don’t.

      Other examples of relatively simple genetic traits like this include eye colour, whether your ear lobes are attached to the side of your head, and ability to roll your tongue. However, most traits like height or weight, as well as most common diseases like diseases like diabetes and heart disease, seem much more complex. In those cases there are many many genes that impact the overall outcome, and each one seems to have a relatively small effect – the gene with the biggest effect on height, for example, only makes a few cm difference. In these cases, how high you are, or your risk of diabetes, depend on the interactions of a whole lot of genes, as well as the impact of the environment. So the asparagus wee is a cool story, but not really indicative of how most of genetics works!

    • Photo: Jemma Ransom

      Jemma Ransom answered on 22 Mar 2011:

      Yes it’s true, there is a specific smelly pee associated with eatng asparagus

      The body metabolises (breaks down) asparagus producing a stinky chemical. Most likely a sulphur based compound. There has been a big debate as to whether we all produce this odorous chemical and if we do, whether we can all smell it.
      Several studies in the 80’s concluded however that whilst most of us can generate ‘asparagus pee’, only about 22% of the population have the correct genes to dettect it.

    • Photo: Eoin Lettice

      Eoin Lettice answered on 22 Mar 2011:

      Yes it is true!

      Around 40 or 50 % of people who eat it develop this smelly urine and you’re right: the production of the smelly pee and the ability to smell it are both genetic traits.

      Only people with a certain gene can break down the asparagus into those smelly chemicals and only those with a specific gene can detect the odour.

      For the record, and after a series of scientific tests 🙂 I am part of the 40-50% who can both produce and smell the asparagus urine!

      Eoin

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