Image via iStock.

By: Duane Mellor, University of Nottingham .

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.

Why do certain foods change our urine? (Image via iStock.)

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 per cent of participants did not produce detectable odourants. A further 6 per cent reported not being able to smell the asparagus odour in any of the samples they were asked to sniff. (Post continues after gallery.) 

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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.

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).

Noticed a change in the colour of your wee? (Image via iStock.)

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. (Post continues after gallery.) 

10 foods you never knew were healthy

Bread

KATIE THOMSITT, Dieticians Australia spokesperson says: "Lots of people shy away from bread to achieve weight loss. But bread is such an important source of fibre, and plays such a helpful role in weight loss and preventing bowel cancer. A couple of slices of a dark, seedy bread per day is absolutely fine."

Cheese

KATIE SAYS: "Cheese is definitely a higher fat source of dairy, but still a good dairy fat. Choose low-fat options if you have it regularly, otherwise consume it in moderation. One serve of dairy is 30g, so a couple of slices is absolutely fine."

Chocolate

KARA LANDAU, The Travelling Dietician says: "Studies have been able to show that dark chocolate can improve insulin sensitivity, reduce blood pressure in healthy individuals, protect against heart disease, and improve HDL good cholesterol (not a bad start for such a tasty food!). Dark chocolate also contains polyphenols, whilst white chocolate does not. These polyphenolic compounds are able to relax blood vessels allowing for better blood circulation, and thereby assisting with the maintenance of healthy blood pressure levels."

Coffee

KATIE SAYS: "Coffee is a really good snack, in particular if its milk-based to get the dairy in - for example, a skim latte. As long as its in moderation - daily guidelines say a maximum of three per day! And you want lots of water on top of that."

Eggs

KARA SAYS: "Yes, even the yolks are good for you! Healthy fat soluble vitamins, as well as half the protein found in an egg, are all found in the yolk!"

Avocado

KATIE SAYS: "Avocado is a healthy fat. Lots of people think its high-fat and not good for you, but its rich in mono unsaturated fats, which give you protection against heart disease. It also helps with controlling lower cholesterol levels."