Why Does Carnitine Smell like Fish??

Key Takeaways:

  • Carnitine itself does not have a fishy odor, but some people taking high doses of carnitine supplements can develop a fishy smell.
  • This fishy odor is caused by a condition called trimethylaminuria, also known as fish odor syndrome.
  • Trimethylaminuria results from the body’s inability to break down trimethylamine (TMA), a fish-smelling compound produced from L-carnitine.
  • L-carnitine is found naturally in meat, fish, and dairy. Carnitine supplements increase the amount of L-carnitine in the body.
  • People with trimethylaminuria lack sufficient levels of the enzyme FMO3 which breaks down TMA into odorless TMAO. This causes TMA buildup and the fishy smell.


Carnitine is a popular nutritional supplement touted for its potential health and performance benefits. However, some individuals report developing an unpleasant fishy odor when taking carnitine supplements. This article will provide a comprehensive explanation of why carnitine can cause a fishy smell in some people.

We will evaluate the science and mechanisms behind this phenomenon. Key topics covered include: the structure and sources of carnitine; trimethylaminuria – the condition responsible for the odor; the role of the liver enzyme FMO3; bacterial production of trimethylamine from carnitine; and risk factors for experiencing fishy odor symptoms.

Understanding the intricate processes by which carnitine intake can lead to fishy body odor will empower readers to make informed decisions regarding carnitine supplementation. Equipped with this knowledge, those who wish to use carnitine can take steps to reduce any chances of malodor development. Ultimately, the aim is to provide a deep scientific explanation of how and why carnitine can smell like fish.

What Is Carnitine and Where Is It Found?

Carnitine is a quaternary ammonium compound biosynthesized from the amino acids lysine and methionine. It plays a key role in energy production, shuttling long-chain fatty acids into the mitochondria to be oxidized and generate ATP energy. Carnitine is found in almost all cells in the body, but is most concentrated in skeletal and cardiac muscle.

The human body can synthesize carnitine to some degree, but it is also obtained through dietary sources. Foods highest in carnitine include:

  • Red meat
  • Dairy products
  • Fish
  • Poultry
  • Tempeh

Carnitine is also widely available as an oral supplement, often marketed as improving exercise performance and weight loss. Supplemental doses can far exceed normal dietary intake levels.

What Is Trimethylaminuria?

Trimethylaminuria (TMAU), also known as fish odor syndrome, is a rare metabolic disorder causing the body to emit a strong fishy odor. This condition prevents the normal breakdown of trimethylamine (TMA) – a chemical produced from the digestion of foods containing carnitine and choline.

TMA has an overpowering fishy smell, even at low concentrations. When TMAU is present, TMA accumulates in tissues and bodily fluids instead of being further metabolized into non-odorous compounds. This buildup is ultimately secreted in sweat, urine, reproductive fluids, and breath – leading to the characteristic fishy smell.

Research indicates trimethylaminuria affects only around 1% of the general population. However, mild or transient forms of the disorder may be more common.

The Role of FMO3 Enzyme in Trimethylamine Breakdown

The root cause of trimethylaminuria is deficient activity of the flavin-containing monooxygenase 3 (FMO3) enzyme in the liver. FMO3 is responsible for oxidizing trimethylamine into odorless trimethylamine N-oxide (TMAO) which is easily eliminated from the body.

When FMO3 is nonfunctional, trimethylamine accumulates and gets transported in the blood to other tissues. FMO3 is the primary safeguard against trimethylamine buildup and fishy odors in most people.

However, some individuals have genetic defects in the FMO3 gene which reduce enzyme activity. Researchers estimate around 1 in 10,000 people have mutations causing complete FMO3 deficiency. More have partial deficiencies which can lead to mild or intermittent symptoms.

Bacterial Production of Trimethylamine from Carnitine

Carnitine itself has no fishy odor. But in the setting of FMO3 deficiency, carnitine supplementation can provide more substrate for bacterial production of smelly trimethylamine in the gut.

Gut microbiota like Escherichia coli possess enzymes to metabolize carnitine into trimethylamine. This microbial conversion occurs primarily in the large intestine. Trimethylamine is then absorbed into the bloodstream and transported to the liver for processing by FMO3.

When high doses of oral carnitine are ingested, more unabsorbed carnitine reaches the colon. This allows gut bacteria to generate larger amounts of trimethylamine from the excess carnitine.

In healthy individuals, the higher trimethylamine loads are efficiently degraded by FMO3. But in those with defects in this enzyme, trimethylamine readily accumulates and leads to fishy odors.

Who Is At Risk For Fishy Odors From Carnitine?

While carnitine supplements themselves are odorless, certain individuals appear more prone to developing fishy body odors when ingesting high doses:

  • Individuals with genetic FMO3 defects – mutations in FMO3 are the primary cause of persistent trimethylaminuria.
  • People with reduced FMO3 activity – this can be acquired from liver disease, kidney disease, hormonal influences, and unknown causes. Even mild FMO3 impairment puts one at risk.
  • Women – research shows female sex hormones like estrogen can temporarily inhibit FMO3 activity. This may make women more susceptible around puberty, menstruation, pregnancy and menopause.
  • Individuals with higher levels of trimethylamine-producing gut bacteria – some naturally harbor more bacteria equipped to generate trimethylamine from dietary carnitine.
  • Patients taking medications that impair FMO3 – certain prescription drugs are known to inhibit FMO3 and increase trimethylamine levels.

How To Prevent Fishy Odors When Taking Carnitine Supplements

For those concerned about malodorous side effects, there are several precautions one can take to minimize chances of fishy smells from carnitine supplementation:

  • Consult your physician before use – this allows assessment of personal risk factors like liver dysfunction.
  • Start with a low dose and gradually increase – this gives the body more time to adapt and process additional carnitine.
  • Take carnitine with a meal – food decreases gut bacterial activity and absorption into the bloodstream.
  • Choose an encapsulated form over liquid – encapsulation prevents bacterial breakdown in the mouth and esophagus.
  • Use an acid-resistant capsule – this delays release until the small intestine where absorption is high.
  • Improve liver function – diet and lifestyle changes may optimize FMO3 activity in at-risk individuals.
  • Take activated charcoal – can reduce absorption of trimethylamine from the gut and lower systemic levels.
  • Maintain good oral hygiene – promotes a healthy oral microbiome composition.

FAQs About Carnitine’s Fishy Smell

Why does carnitine make your pee smell funny?

Carnitine can make urine have a fishy or ammonia-like odor in people with trimethylaminuria. Carnitine generates trimethylamine as a metabolic byproduct, which builds up and gets excreted in urine when FMO3 is deficient. The more carnitine taken, the more trimethylamine secreted into fluids like urine.

Does L-carnitine break down into TMA?

Yes, L-carnitine can break down into trimethylamine (TMA) via bacterial metabolism in the gut. Gut microbiota contain enzymes to convert L-carnitine into TMA. This breakdown occurs primarily in the large intestine where carnitine is exposed to high bacterial populations.

Can vegetarians develop fish odor syndrome?

Yes, vegetarians can still develop trimethylaminuria fish odor syndrome since the condition is caused by impaired FMO3 function rather than diet alone. Vegetarians obtain carnitine from dairy products. Gut bacteria can produce TMA from even small amounts of carnitine. However, the condition may be less common in vegetarians than meat eaters.

Does cooking or freezing carnitine foods reduce the smell?

Cooking or freezing does not prevent the fishy odor associated with carnitine intake in people with trimethylaminuria. These preparation methods do not alter the fundamental biochemistry in which gut bacteria metabolize carnitine into smelly TMA. Avoiding dietary carnitine altogether is the only way to reduce odor.

Can you smell it if you have a build up of TMA/TMAO?

Yes, individuals with trimethylaminuria do notice their own fishy odor from TMA buildup. However, they often adapt and become less sensitive to their own smell. The odor is more noticeable to unaffected people. TMA has an extremely low odor threshold, so even small amounts can produce significant odors.


In summary, carnitine supplements themselves do not possess any unpleasant fishy aromas. However, high carnitine intake allows more substrate for bacterial production of trimethylamine – the direct source of fishy odors in affected individuals. Defective metabolism of trimethylamine due to inadequate FMO3 enzyme function prevents degradation into non-odorous compounds. The resulting buildup is secreted throughout the body, leading to the characteristic fishy smell.

While relatively rare, this phenomenon is important for carnitine users to understand. Those concerned with malodorous side effects can take proactive steps like starting with low doses, taking with meals, and using encapsulated forms. Additionally, improving liver function may provide a long-term solution by optimizing trimethylamine metabolism. With proper precautions, patients need not deny themselves potential benefits of carnitine supplementation.


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