With summer comes asparagus, and with asparagus comes a unique visit to the bathroom – for some anyway. 

Head of Re: News Simon Day digests the science behind the smell and tries to understand why asparagus can make some people’s pee smell. 

In my life the onset of summer is defined by one distinct fragrance. 

It’s not the scent of fresh grass clippings on Saturday morning. It’s not the smoke from barbeques wafting over the neighbourhood on the first sunny day in December. 

It’s the nutty, grassy and slightly sulphuric smell of asparagus urine. 

I’m fascinated by the science of it. I’ve met distinct categories of people who respond to asparagus in different ways: those who create the fragrant urine, and those who don’t, and then there are those who can detect the smell of asparagus urine and those who cannot. 

I also represent a rare category of people who really enjoy the smell. I find the rich pungent notes of sulphur moreish and appealing. 

So, I spoke to Auckland University food scientist Peter Swedlund to find out what our different responses to asparagus say about who we are.

“All of those things are true. It’s an interesting interplay of genetics of biochemistry and perception,” Swedlund said.

But despite asparagus urine having been the subject of many studies, what actually causes the different responses is hard to pin down. 

Our body has to deal with a lot of chemicals within our food. 

It has certain pathways and processes that break down the different compounds of our diet. 

Asparagus is high in sulphur because of asparagusic acid - a sulphur compound unique to the summer vegetable. 

While the acid itself doesn’t smell or taste sulphuric, after eating it the body starts to release the potent potential of the acid. 

“There are myriad metabolic pathways that our body breaks down unfamiliar chemicals. This is done by enzymes. The enzymes that break down the native compound in asparagus are proteins and coded for by genes,” Swedlund said. 

So far research has been unable to pin down the precise differences in genetics and metabolism that might cause some to produce the scent and others to not. 

It’s also unclear why some people can spell asparagus piss and others can’t. 

A 2016 US study found 40% of the 7000 people surveyed could strongly detect the smell of asparagus urine. 

“Perception has a genetic component of course. Whether we have the type of receptor is genetic,” Swedlund said. 

The study showed the group that was unable to smell asparagus shared more than 800 genes but the precise genetic variation that prevents the receptors detecting the smell could not be isolated.  

The study also suggested more work needed to be done in this area to help those who couldn’t smell asparagus “discover what they are missing”. 

I agree. Those who cannot perceive the beautiful summer perfume of asparagus urine are most definitely missing out. 

“It is very interesting how this demonstrates the individuality of perception and metabolism,” Swedlund said.  

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