What exactly is that fishy smell?

We have such an association with vaginas and their fishiness that we often forget to ask why this fishy smell develops in bacterial vaginosis (BV).

The same process of fish smell developing is found in old fish and rotting food, but also in some infections, bad breath, and BV odour.

The fishy odour is caused by a chemical reaction between molecules: ammonia (produced by G. vaginalis and others) and methanol react, with the help of a catalyst, and become trimethylamine (TMA).

When your microbiome looks great but your vagina ain’t

You may have a full vaginal microbiome screen, which comes back as looking great – high in lactobacilli, hardly any pathogens, and all in low numbers that can be disregarded.

When there isn’t a pathogen to blame, we need to work out what’s going on. It could mean ammonia and methanol are reacting in your vagina and becoming TMA, which means they are in there when they normally wouldn’t be, possibly due to another metabolic process or a lactobacilli species contributing to the problem.

Fun fishy facts – the TMA freshness test

Fish freshness tests using a swab on the outside of a fish are seeking TMA, with low amounts used as a measure of freshness. This is why we are told that if a fish ‘smells fishy’ (read: has a lot of TMA) then it’s not very fresh. Over time, TMA develops and becomes more concentrated, indicating the fish has been out of the water longer (read: dead).

TMA is a product of decomposition in plants and animals, and in humans is mostly synthesised – put together – by gut microbes from nutrients in our food, specifically choline and carnitine. 

You can bring on this fishy smell by taking large doses of choline and carnitine. Foods that contain high levels of choline are eggs, milk, liver, red meat, poultry, shellfish and fish. Carnitine can be found mostly in red meat and dairy products.

You can’t avoid these nutrients, and nor should you – unless you have fishy odour syndrome or a metabolism disorder (explained below), you are not at any risk of these nutrients causing a fishy-smelling vagina.

Fishy odour syndrome

Fish odour syndrome is also known as trimethylaminuria, a rare genetic metabolic disorder that halts the production of an enzyme that stops the body from being able to properly convert TMA into TMAO (trimethylamine oxide).

TMA then builds up and is excreted in sweat, urine, and breath, causing a strong fishy odour. Excretion can occur through excessive TMA being secreted, which could be due to gut microbiota issues, altered metabolism, or a hormone issue.

How TMA reacts with our noses – why we hate it

TMA is also an olfactory receptor agonist, which in plain English means when we breathe in TMA and it hits our nostrils, we hate it because our noses have been trained, over time, to be receptive to this compound.

TMA causes an ‘attraction-aversion’ response, triggering off a neurological reaction: our brain fires up when we breathe in these amines, which doesn’t always happen with every odour we smell.

We are especially sensitive to some odours (molecules) over others, and this appears to be evolutionary: we are repelled by or attracted to certain molecules – delicious food over rotten food, for example.

TMA’s characteristic fishy odour is always an odour of decomposition in plants or animals, and therefore we are primed to reject it outright. It repels us. This is one reason why having a fishy vag is bad for intimacy – it repels because our animal brains are saying “Don’t eat this – it’s dangerous!” or “Don’t have sex with her! You don’t like her smell”. 

It is the same process by which pheromones are believed to operate: molecules that are communicating a message of safety or risk.

An interesting way to block a lover’s nose from TMA… Timberol

A perfuming product called Timberol apparently blocks the TAAR5 receptors in our nose that TMA sets off, about 96 per cent of the time.

It would block the fishy odour most of the time if we had access to it, except Timberol is not available to consumers – it’s a perfume developed for shampoo, shower gel, deodorants, and cleaning products. Timberol was developed by a company called Symrise.

Symrise says about Timberol:‘powerful modifier for woody accords; imparts a substantive woody-ambery characteristic; strengthens the floral side in feminine concepts at low use levels; underlines the masculinity of men’s fragrances at high use levels.’  It’s a shame we can’t use it, though I’m sure if you were into it, you could get your hands on some.

The Whiff Test

The whiff test that a doctor or scientist perform that kills bacteria and vaginal cells using potassium hydroxide (KOH). When BV is present, this test produces a fishy odour.

When used during a yeast infection, the whiff test is used to kill the cell and bacteria and leave just the yeast to detect.  



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