Why does fish smell?

THAT'S THE WHYIF YOU'VE ever encountered a fish that has been out of the sea for a while, you'll know the whiff that hits you from it – as you pull that fillet of salmon or cod out of the fridge, it can set your nose wrinkling.

Why does fish emit such a distinctive smell?

It’s down to a chemical compound called trimethylamine N-oxide, or TMAO, in saltwater fish. TMAO itself doesn’t have an odour, and it seems to be quite useful for ocean fish and other marine creatures; it helps them to counteract the cellular effects of living in such a salty environment.

But when the fish are no longer alive, TMAO gets broken down to trimethylamine, and it’s this latter compound which belts the nose with its characteristic fishy odour.

Interestingly, bacteria in our intestines can produce trimethylamine as they help digestion. But an enzyme called flavin-containing monooxygenase 3 (FMO3) converts the fish-smelling compound trimethylamine to the odourless TMAO, which we then excrete.

In a rare condition called trimethlyaminuria, mutations in the FMO3 gene can result in people not effectively converting trimethylamine to TMAO, and so there may be a fishy odour from their urine, sweat and breath.

“The symptoms of trimethylaminuria can be improved by changes in the diet to avoid precursors, in particular TMAO which is found in high concentrations in marine fish,” notes a 2011 paper in The Clinical Biochemist Reviews.