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DocSpot: Smelly urine

Dear Dr Margaret

When I eat asparagus, my urine smells disgusting. It happens really quickly. But my friend says it doesn’t happen to her.

Several people have asked me about this recently – perhaps this was a specially good summer for asparagus.
 
Scientists are not absolutely sure about the cause of the smell, but the most likely culprit is a substance called asparagusic acid. This acid protects the asparagus plant against parasites – perhaps they don’t like the smell either. There is a lot of asparagusic acid in young asparagus plants and, of course, it is the young shoots that we like to eat. Your body can convert the asparagusic acid into some very smelly chemicals, such as methanethiol and dimethyl sulphide, and these are passed out in your urine.
 
Most people find the smell appears very quickly. Your urine can become smelly within about 20 minutes of eating asparagus. This is not surprising – it shows how efficiently we can make use of the food that we eat.
 
You say your friend does not have this problem. In fact, about 50% of people say they produce smelly urine after eating asparagus, and 50% say they do not. Whether you do or don’t seems to be inherited. Perhaps only half the population have the gene for the enzyme that converts asparagusic acid into the smelly chemicals, or perhaps half the population are unable to absorb asparagusic acid from the gut.
But there could be another explanation. Professor Steve O’Rahilly, Professor of Metabolic Medicine at the University of Cambridge, UK, tells me that some people are simply unable to detect the smell. This inability is inherited. So maybe asparagus produces smelly urine in everyone, but not everyone can smell it. This means that your friend might be producing the smelly urine, but doesn’t know it.
There is nothing you can do to prevent the smell, except stopping eating asparagus. And of course it does not mean that you are unhealthy in any way.
 
If you want to know more, look at an interesting article by Dr Steve Mitchell of Imperial College School of Medicine, UK. This article was the source of some of my information on this topic. You can read it at http://dmd.aspetjournals.org/cgi/content/full/29/4/539
 

Last updated; Monday, May 8th 2017


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