Why does Asparagus and Beetroot affect your urine?

With the Christmas holidays almost upon us, it seems we just can’t stop thinking of food (or our readers can’t at least).  Some foods seem to affect our bodies in some pretty weird ways after we’ve eaten them; with the smell and colour of our urine being the most distinctive change imaginable. Let’s find out why…

Why does Asparagus cause our urine to smell?

Asparagus causes our urine to smell a distinctive way because among the many things eaten by us, it is the only food known to contain a compound called asparagusic acid. This is a small molecule that is broken down by our digestive processes to produce even smaller molecules. Many of these small molecules are passed from the blood into the urine where they can evaporate easily and hence, get up our noses. The particular culprit for the unpleasantness of the smell is the element sulphur. Just about any small compound containing sulphur that evaporates will stink like crazy. We have evolved an aversion to this substance so that we can avoid it and possible sources, in order to preserve our life. For example, the simplest compound of sulphur, H2S (hydrogen suphide) is just about as toxic as cyanide. We can detect exceedingly low levels – well below the level where they become toxic. However, this is not a reason to stop eating asparagus!Asparagus

The distinctive smell of asparagus wee is the combination of various sulphurous compounds produced by the digestion of asparagusic acid, including methanethiol (the stuff they put in gas supplies so we can smell leaks, also known as a mercaptan), dimethyl sulfide (also found in heavily boiled cabbage) and the products of the reaction between methanethiol and oxygen, dimethyldisulfide, dimethylsulfone, and dimethylsulfoxide. Interestingly, not everyone is genetically equipped to detect the smell at such low levels. You just must be unlucky!

Why does Beetroot cause our urine to change colour?

The reason beetroot pee is red is much simpler:

Foods have different colours because of the dyes and colour-giving molecules they contain. Normally when we eat them, their colour-giving molecules are degraded by digestion. But if this doesn’t happen fully, then some of the colour can get through into our urine.Beetroot

Some types of beetroot have such strong colours (i.e. strongly coloured molecules, and lots of them), that sometimes not all of it gets broken down by the body. This can be particularly the case if eaten alongside other foods that contain what are called ‘reducing agents’. One example is oxalic acid and is found in many green vegetables, such as broccoli and rhubarb for example. Or, another reason for strongly coloured molecules not being broken down is if the stomach is less acidic than it usually is – perhaps because you are taking medication for acid reflux or ulcers (proton pump inhibitors such as omeprazole also known as Losec, for example).

If the colour components in beetroot (‘betanin’) are protected from acid and/or oxidative degradation in the stomach, and if they don’t spend a great deal of time in the digestive system (if they are part of a meal loaded with roughage and chilli) then they will get through to the blood. The water-soluble betanin can then come out in your pee, making it look like it is dyed red.

So, if you want to have purple pee, the prescription is: pop a proton pump inhibitor pill, have a lentil and broccoli vindaloo, followed by rhubarb and beetroot pie, wash it down with plenty of water, and wait…..! All in the interests of science! (Note from editor: please don’t.)

Answer by Frank Mair

Photo Credit: thebittenword.com via Compfight, erix! via Compfight


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Article by Frank Mair

December 9, 2014

Dr Frank Mair is a Senior Lecturer in Inorganic Chemistry at the University of Manchester and Director of the School of Chemistry's Outreach effort. As well as teaching undergraduate and supervision postgraduates, he burns things and blows stuff up in order to catch the attention of what will become the next generation of chemists. But he also uses metal complexes to encourage small molecules to join up to make rubber. He has a wife and two sons, and a black powder firing musket.


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