Monday, 25 May 2009

Ur-ine trouble if you've been eating asparagus

I was reading last week in Hugh Fearnley-Whittingstall's delightful food column that asparagus is in season again. I really enjoy this whole eating in-season produce and being eco-responsible lark (despite it being so fashionable it almost makes you cringe). It's great; you get to eat really fresh food which is also quite cheap, since there's loads of it around.

Asparagus stalks in the wild

So I've been tucking in to the zingy spears all week and have been loving it. I was surprised to learn from a friend who had been similarly glutting-out on asparagus, that he also always enjoys eating asparagus, since 'it always makes my wee smell all mental.' I was surprised by this as I had experienced no real bathroom shocks, asparagus-related or otherwise, over the past few days.

But enough of this cheap toilet humour. As usual I needed to quench my insatiable thirst for scientific knowledge (or something) so I set about finding out what causes these magic odours which some people can smell and some can't.

First off, it is clear that a chemical of some sort is responsible. Scientists generally agree the one causing the grievance in this case is methyl mercaptan.

As an interesting aside though, unlike our other senses, smell is not really that well understood by scientists. We know how things like vision, touch and hearing work in quite a lot of detail, but smell remains something of an enigma.

We do know that there are various receptors in our noses, and it seems obvious that these mediate smell in some way. So far though, it has proved difficult for scientists to work out their exact structures. (Conversely, we do know the structures of lots of other proteins, for example opsin, as shown in the recent benchtwentyone article on retinal). To add to the confusion several molecules which have similar structures have wildly different scents. Conversely, molecules of vastly different sizes and shapes can sometimes smell very similar (figure 1). As a result of this confusion, there have been several not-widely-accepted theories of smelling even relatively lately [1].

Most receptors in the body are large proteins. These mediate everything from the digestion of food to the degradation of neurotransmitters and work on the theory of 'lock and key'. This means the cleft in the center of the protein has a specific shape and size which binds only a particular guest. But if this is the case for odourants, we would expect similar molecules to smell the same - which is not always the case.

Figure 1

Methyl mercaptan (the guilty party derrived from asparagus) and ethane dithiol are both sulfurous compounds which smell awful; like rotten eggs. Acetophenone has a relatively small methyl group compared to benzophenone which is much larger. They both smell similar though - people tend to describe the odour as a lot like burnt almonds.

To get back to asparagus though, it seems there are two possible explanations for the discrepancy between mine and my friend's post-asparagus wee. The first option is that one of us has a particular enzyme or bacteria in our intestinal tract which breaks down a compound present in asparagus to methyl mercaptan and the other doesn't. This might make quite good sense, as naturally occurring amino acids which contain sulfur (such as methionine) are known to be broken down in the mouth to methyl mercaptan by bacteria [2], causing bad breath. The alternative is that we both have the compound in our urine, but I don't have the correct receptors in my nose to smell it.

Stopping short of inviting my contact into a slightly too close for comfort wee smelling investigation, I consuted the British Medical Journal and found that the experiment had already been done by the professionals [3]. The researchers in question found that the chemical was present in the urine of all the 203 asparagus-fed volunteers they investigated. Also, when presented with samples of an 'unknown liquid' (the researchers obviously felt it wouldn't be proper to reveal it's true identity) the volunteers who could smell the guilty chemical in their own urine could smell it also in everyone else's.

So like it or not, if you're like me and have been chowing down on asparagus all week, you're going to have some seriously funky bathroom smells sometime soon.

[1] L. Turin, Chem. Senses., 1995, 773 - 790

[2] T. Koga etal., Infection and Immunity, 2000, 68 (12), 6912-6916.

[3] M. Lison, S. H. Blondheim and R. N. Melmed, British Medical Journal, 1980, 281, 20 - 27.

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