Why does asparagus make urine smell

Asparagus is famous for its ability to make urine smell somewhat strange. That doesn’t mean there is anything wrong with Asparagus, in fact it’s very healthy for you to eat so why does Asparagus make urine smell?

Asparagus contains a sulphurous compound known as Mercaptan. This is the same compound found in rotten eggs, garlic and onions. When Mercaptan is broken down in the digestive system it releases certain by-products that cause the strange smell. However, not everyone suffers from this strange side effect of eating Asparagus. Only some people seem to have the gene required to make the enzyme that breaks down Mercaptan within their digestive system meaning these people do not suffer from strange smelling urine after eating Asparagus. The process of eating asparagus to the end result of your urine smelling odd is so quick it can occur 15 minutes after eating the Asparagus.

Everything we eat affects the digestive system including the smell and colour of our urine. Many people have reported red urine after eating a large amount of beetroot. This is simply because the chemical in the beetroot that is red in colour is not needed by the body so passes through our system completely. Different foods can affect our urine in different ways but some are more noticeable than others such as Asparagus.

A famous example of this affect is King George III (The Madness of King George). His doctors were under the mistaken belief that his blue urine was the product of his madness where as in actual fact it was well known that King George ate an excessive amount of blueberries. It is much more likely that his blue urine was just caused by eating far too many blue berries and the colouring for these blue berries passed through his system and coloured his urine.

It has been observed that young Asparagus is much more likely to produce this smell in the urine especially if they are cut when they are still white. The sulphurous compounds that Mercaptan breaks down into are mostly unique to this vegetable. These asparagusic acids and their pungent derivatives have a higher concentration within the younger asparagus which supports the observation that younger asparagus more often leads to the change in smell of urine.
It is suspected that while most people produce the gene required to break down the Mercaptan and produce the smell, most people cannot in fact smell this change. Studies have shown that while most people produce the smell after eating asparagus, only approximately 22% of people can smell the change. That means many people are still unaware of the effects of their foods on urine.
Another famous reference to this phenomenon is in the first Austin Powers movie. In this the main character Austin Powers hides behind a fountain and begins to pee as though the fountain is still working (he accidentally pulls out the plug). The guard fails to see him or suspect until he begins eating Asparagus and he notices almost immediately a strange smell in the room.