Biology Through the Lens of Fish Odor Syndrome (Trimethylaminuria)

Body odor is a fact of life. For many, this is a simple fix, either through some deodorant, a quick shower, or a spritz of perfume. However, for some people, regardless of how much care they put into cleaning themselves, a fishy odor remains on their body. Although this may sound unbelievable to some, it is a reality for a select percentage of the population that have the genetic disorder Trimethylaminuria (TMAU), also known as “Fish Odor Syndrome”.

Learn more about Fish Odor Syndrome (TMAU), as well as the basics of the biological concepts behind it, such as enzymes, substrates, and denaturation.

What is Fish Odor Syndrome or Trimethylaminuria (TMAU)?

TMAU is a metabolic disorder where the body is unable to break down trimethylamine (TMA), which is a nitrogen-containing compound. TMA is what produces the fishy odor in the symptoms of TMAU, and it is produced when the body digests foods that are rich in compound such as choline, lecithin, and trimethylamine N-oxide. Examples of such foods are eggs, beef liver, certain legumes, and saltwater fish (Mann 2011). When TMA accumulates in the body, it is released through sweat, reproductive fluids, urine, and breath, which is what leads to the fishy smell. However, symptoms of TMAU vary from different cases, as not all individuals with TMAU have the characteristic odor (Genetic and Rare Diseases Information Center 2018).

Fish Odor Syndrome is an autosomal recessive disorder. That means in order to inherit Fish Odor Syndrome, you must receive both mutated copies of the FMO3 gene from your carrier parents!


The majority of cases of TMAU are caused by a mutation in the FMO3 gene. The FMO3 gene is important because it provides the instructions for the body to make a specific enzyme that performs a process of TMA molecules in the body. This chain of events is analogous to cooking. When you buy the wrong cookbook, there will be incorrect instructions to make the original cake you had in mind. Hence, following the erroneous instructions also means that an incorrect product will be created.

Fish Odor Syndrome = Mutated FMO3 Gene

In a person without TMAU, the normal FMO3 gene would create a normal enzyme that would build TMA up into Trimethylamine Oxide (TMAO), an odorless compound (Genetics Home Reference). To recap, people with TMAU are unable to convert TMA into TMAO, which causes the build up of TMA in their bodies.

Normal Person: TMA –> TMAO (odorless)

Patient with Fish Odor Syndrome: TMA –> (cannot convert to TMAO) so TMA builds up (Fishy smell)

With a better understanding of TMAU and its causes, let’s explore some of the key biology concepts brought up: genes, enzymes, and substrates.

What are Genes?

Inside the nucleus of every cell contains DNA, the carrier of genetic information in a living organism. In humans and most eukaryotic organisms, DNA is stored in a condensed molecule called a chromosome. Genes are a distinct sequence of ‘instructions’ in these chromosomes that tell the body to synthesize products such as protein or RNA.

Returning to the example of TMAU, the FMO3 gene creates a protein, which are long chains of amino acids. The protein created is an enzyme.

Enzyme and Substrates –> can make products! Credit: Wikimedia Commons


Enzymes are proteins that are catalysts; they speed up the rate of chemical reactions in our bodies. In these reactions, molecules are either created (built up) or broken down. The molecules that are the reactant of the enzyme’s chemical reaction are called substrates, and the new molecules created as a result of the reaction are called products. In other words, enzymes perform a chemical reaction of substrates, which results in products.

The structure of enzymes is very specific to its function. The way an enzyme performs a chemical reaction to a substrate is similar to how a lock and key functions. The key (substrate) is a specific shape, and if the lock (enzyme) has a keyhole where the key fits, the lock can be opened (the enzyme can perform the chemical reaction). In this analogy, the keyhole, or where the substrate fits into the enzyme, is called the active site of the enzyme.

Fish Odor Syndrome can manifest as a fish odor in urine, breath, and sweat.

FMO3 Gene in TMAU

In the case of individuals with TMAU, because the FMO3 gene has a mutation with incorrect instructions to create the enzyme, the enzyme will be a different shape and have an altered active site. This means that the substrate, TMA, is unable to fit into the shape of the active site, and the chemical reaction turning TMA into odorless TMAO cannot happen. An enzyme with an altered active site is similar to a lock with a distorted keyhole.

Time to shower! What treatments are available for Fish Odor Syndrome? Credit Source: Wikimedia Commons

Cures and Treatments for Fish Odor Syndrome (TMAU)

Currently, there has not been a cure for TMAU discovered or created yet. However, there are some ways to reduce the symptoms of body odor. For example, individuals could avoid foods that contain TMA and are high in choline, lecithin, and trimethylamine N-oxide. When washing, using soaps that have a moderate pH also helps to counteract the TMA that is secreted from the skin (Genetic and Rare Diseases Information Center 2018).

Although this metabolic disorder may sound strange for some, learning more about the major biological concepts behind TMAU can help with understanding more of the conditions that affect others.

Scrub, scrub, scrub! What interesting facts did you learn about body odor and Fish Odor Syndrome? 🙂 Credit: Flickr

Works Cited

Mann, D. (2011). If You Have Body Odor, It May Be in Your Genes. WebMD. Retrieved April 3rd, 2020, from

Genetic and Rare Diseases Information Center. (2018). Trimethylaminuria. National Center for Advancing Translational Sciences. Retrieved April 3rd 2020,

Genetics Home Reference. (n.d.). Trimethylaminuria. U.S. National Library of Medicine. Retrieved April 3rd, 2020,

© Copyright 2020 Moosmosis – All rights reserved

All rights reserved. This essay or any portion thereof
may not be reproduced or used in any manner whatsoever
without the express written permission of the publisher. 

Please Like our Facebook page to support! 🙂

7 replies »

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s