Why does asparagus make some people's urine smell but not others?

Observations of how what we eat can affect our urine can be traced back through history

There have been different theories put forward over the years (Getty)

Have you eaten asparagus and noticed that it can alter how your urine smells? Perhaps you’ve heard about the phenomenon but wondered why it doesn’t seem to affect you. Or maybe you’ve eaten beetroot and then been worried there appeared to be blood in your water. Why do certain foods change our urine and does it make a difference to our health?

Observations of how what we eat can affect our pee can be traced back through history, from the ancient Greeks including Antiphon and Theophrastes to an early edition of the medical journal The Lancet in 1836. But asparagus’s potential to affect urine was not formally described until 1735. This happened to coincide with the British agrarian revolution when fertilisers containing sulphur were first used on crops, although there is no real evidence to say if this effect is causal.

There have been different theories put forward over the years explaining why only some people notice a smell in their urine after eating asparagus. It was first thought that some people broke down the vegetable in a way that released a smelly chemical in the urine. In the UK, data suggested about half of the population were so-called “excretors” in this way. This led to a theory that the characteristic was carried by a dominant gene that only needed to be inherited from one parent.

Interestingly there have also been reports that women who are “non-excretors” can produce the odorous urine when pregnant. This suggested that any asparagus compound responsible for the smell could pass through the placenta and be converted to the smelly compound by the foetus that inherited the gene from their father.

Odour or smell?

The problem with the idea of a genetic ability to produce smelly urine is that there is not just one compound that always appears in urine after eating asparagus. Up to 29 different compounds from the vegetable are potential odourants, although methanethiol or methyl–mercaptin is the most predominant. The number of potential smelly molecules has given rise to an alternative theory, not that certain people don’t produce the chemicals but that some people lack the genetic ability to detect the smell.

One small study asked 38 people to eat asparagus and then sniff their own and each other’s urine samples to see if they could detect unusual odours, and around 8% of participants did not produce detectable odourants. A further 6% reported not being able to smell the asparagus odour in any of the samples they were asked to sniff.

When looking at possible genetic causes, the researchers not find any particular genes associated with production of odorants. They did however, find that the ability to detect the smell was linked to a specific DNA sequence that varied between different people. This means a single molecule change in the genes responsible for smell may be linked to the inability to smell asparagus odour in urine.

The problem is that there aren’t actually that many studies of this phenomenon and most of those that have been carried out are small so evidence is limited. This means it’s hard to say whether “asparagus pee” is an ability to produce the compounds or smell them, or a bit of both.

Blood red Shutterstock

Asparagus has also been reported to change the colour of urine, giving it a greenish tinge, something that is also associated with beetroot. In some individuals, the potentially disturbing effect can be pink or red urine. This can lead to the false impression of blood appearing in the urine (haematuria).

Known as beeturia, the cause is not thought to be a genetic characteristic, but related to the physical state of the person who experiences it. The beetroot’s red pigments only appear in the urine if they are not damaged by the digestive process and are then absorbed and re-excreted by the kidneys. It is thought that this can happen if the stomach is not acidic enough, or if the beetroot itself has high levels vitamin C in the beetroot itself. So, some people can produce beetroot-red urine some but not necessarily all of the time.

Other foods

Although asparagus and beetroot are the most commonly mentioned examples, it is actually likely that many foods have an effect on the chemicals that appear in urine. This allows doctors to use urine to assess a person’s dietary intake by analysing the different chemicals it contains.

However, most changes aren’t usually noticeable even if they are associated with colour or smell changes. For example, coffee isn’t usually thought of as something that strongly effects urine but a recent study has shown that drinking two cups can be enough to lead to the presence of compounds such as vanillin which are often associated with a vanilla-like sweet smell.

Perhaps next time you visit the bathroom, you may be able to see or smell a chemical marker of what you have recently ate or drank. It is usually normal for metabolites from food to appear in urine, and should not be anything to be worried about. If however, you think there is blood or a distinct change you should always seek a medical opinion.

Duane Mellor, Assistant Professor in Dietetics, University of Nottingham

This article was originally published on The Conversation. Read the original article.