Why asparagus makes our urine smell and other foods that do the same

Alexandra Thompson
·3-min read
Raw fresh asparagus on rustic background, close-up
Many notice distinct smelling urine after eating asparagus. (Getty Images)

Asparagus season has arrived in the UK, with many Britons rushing to enjoy the springtime vegetable.

The popular accompaniment is only on our doorstep for a limited time, with supplies running out by the end of May.

Read more: Asparagus: Health benefits and meals to make with the versatile spring vegetable

Whether you enjoy them roasted or steamed, many notice an unusual after-effect with the side dish – potent urine.

This is no new phenomenon. In 1731 the Scottish mathematician and doctor John Arbuthnot wrote asparagus gave his liquid waste “a foetid smell”.

The French novelist Marcel Proust said the vegetable transformed his “humble chamber pot into a bower of aromatic perfume”.

Why does asparagus make our urine smell?

“Eating asparagus can give your urine an unpleasant odour of rotting cabbage due to the breakdown of asparagusic acid,” Dr Sarah Brewer, medical director of Healthspan, told Yahoo UK.

“The culprit in smelly wee is thought to be cased by sulfur compounds such as methanethiol.”

While smelly urine is synonymous with asparagus, less than half of consumers are said to be affected.

“Interestingly, whether or not you produce these smelly products is a genetic trait that some researchers believe occurs in around 40% of people,” said Dr Brewer.

Read more: How to store asparagus (the best way)

A 1987 study of 800 people found 43% produced “odorous urine after the ingestion of asparagus”.

“Family studies suggest that the ability to produce the odorous urine is inherited”, wrote the scientists.

Dr Brewer added: “Another genetic trait means at least one in two of the population are unable to detect the smell of these asparagus breakdown products, even if they do produce them, due to a mutation in a cluster of genes that code for smell receptors.”

A 1980 study found 10% of 307 people could “smell it at high dilutions, suggesting a genetically determined specific hypersensitivity”.

“So, in order to smell asparagus pee after you have eaten asparagus, you must both be able to produce the culprit compounds and have inherited the odour receptors that allow you to detect them via your nose,” said Dr Brewer.

Drinking cappuccino, first person view
Too much coffee may similarly give your urine a potent odour. (Getty Images)

Other foods that make urine smell and when to see a doctor

If food has a strong odour on the plate, chances are you may notice it after using the toilet.

“Other foods that can affect urine odour include: Brussels sprouts, onion, garlic, coffee, fish – especially salmon - and foods that have a high content of vitamin B6,” said Dr Brewer.

Vitamin B6 is found in a variety of foods, including pork, poultry, eggs, milk and peanuts. Supplements containing the nutrient can also be to blame.

A healthy person should produce clear or pale yellow urine with a mild odour.

Smelly urine can also be a sign of dehydration or a side effect of certain medications.

Read more: How to grow and harvest asparagus at home

Although usually nothing to worry about, Dr Brewer urged people to see their GP if they continue to produce smelly urine.

“If the problem persists, seek medical advice in case you have a urinary infection or liver or kidney problems,” she said.

It is particularly important to see a GP if the smelly urine is accompanied by a sudden need to urinate or going more often than normal, pain when passing urine or blood in the toilet bowel.

Lower abdominal pain, fatigue, generally feeling unwell and becoming confused or agitated are also causes for concern.

Less commonly, persistently smelly urine can be a sign of type 2 diabetes, kidney stones or liver failure.