The fishy smell - origin and chemistry

Fishy odor in the vagina - causes, chemistry and solutions

A characteristic and often bothersome vaginal odor reminiscent of spoiled fish is, for many women, the clearest sign of bacterial vaginosis (BV). The smell is caused by a volatile compound called trimethylamine (TMA), which is also responsible for the odor in saltwater fish when they begin to decompose. But how does this compound form in the vagina - and why does the smell disappear for some women, while it persists in others?

The chemistry behind the fishy odor

TMA is a small molecule with three methyl groups bound to a nitrogen atom: N(CH₃)₃. It also exists in an oxidized and odorless form called trimethylamine oxide (TMAO). Under normal circumstances, the body converts TMA into TMAO in the liver, which is then excreted via urine, sweat, saliva - and also vaginal discharge.

In people with the rare condition trimethylaminuria (“fish odor syndrome”), this conversion does not function properly, leading to a buildup of TMA in the body. But even in women without this genetic variation, TMA can accumulate locally in the vagina. This happens when bacteria convert otherwise odorless TMAO back into TMA within the vaginal environment.

What happens in the vagina with bacterial vaginosis?

Bacterial vaginosis is caused by an imbalance in the vaginal microflora, where the naturally dominant lactobacilli are displaced by other microorganisms. One of the bacteria most strongly linked to fishy odor is Mobiluncus spp. – which, according to Cruden and Galask (1988), is able to convert all available TMAO into odorous TMA. By contrast, the more well-known BV-associated bacterium Gardnerella vaginalis is not capable of this conversion.

This local biochemical activity means that even though the body has already converted TMA into odorless TMAO, the odor can still arise directly in the vagina - causing discomfort and concern for the woman affected.

The importance of pH for the odor

TMA is a volatile base, and its smell intensifies when the vaginal pH is elevated. At a lower pH, TMA is transformed into a soluble acidic form that is odorless. This means that even a slight lowering of vaginal pH can quickly result in a noticeable reduction of the odor. A simple way to check if your balance is disturbed is to measure the vaginal pH value with pH strips - so you know whether steps are needed to lower pH.

How do LadyBalance vaginal tablets work?

LadyBalance vaginal tablets contain milk sugar (lactose), which acts as a nutrient for the natural lactobacilli in the vagina. When these bacteria ferment the lactose, lactic acid is produced, lowering the pH value. This process begins within a few hours to a day, and over days to weeks the beneficial bacteria can once again dominate the vaginal microbiota.

This is not a “killing” agent against unwanted bacteria – such as Mobiluncus – but rather creates an environment where these bacteria cannot thrive. The low pH and the availability of fermentable sugar (lactose) give lactobacilli optimal growth conditions, gradually outcompeting the unwanted organisms.

Why does the odor worsen after intercourse?

Semen has a high pH and strong buffering capacity, which temporarily raises the vaginal pH. Many women therefore notice that the odor intensifies after unprotected intercourse. This is because the more alkaline environment promotes the presence and activity of bacteria such as Mobiluncus.

Where do TMA and TMAO come from?

TMAO is formed in the body as a breakdown product of specific dietary components – especially choline and carnitine. These substances are found in, among others:

• Egg yolk
• Soybeans, peas and cabbage
• Liver, kidney and other organ meats
• Red meat such as beef and pork
• Saltwater fish and shellfish

Normally, the liver ensures that these compounds are metabolized into odorless TMAO, which is excreted through the body’s secretions. But if TMAO is later converted back into TMA locally in the vagina by specific bacteria, the fishy odor can develop – regardless of the body’s initial metabolism.

Summary

Fishy vaginal odor is caused by the formation of trimethylamine (TMA), a compound produced locally in the vagina by certain bacteria, especially Mobiluncus spp., through conversion of TMAO. The odor intensifies at higher pH and quickly improves when the environment becomes more acidic.

LadyBalance vaginal tablets supply lactose to the vagina, supporting the lactobacilli in producing lactic acid and lowering pH. This reduces odor and helps restore a healthy vaginal microflora.

References

• Rehman HU (1999): Fish Odour Syndrome. Postgrad. Med J. 75: 451–452
• Cruden DL & Galask RP (1988): Reduction of TMAO to TMA by Mobiluncus strains isolated from patients with BV. Microb. Ecol. Health Dis. 1: 95–100
• Phillips IR, Shephard EA (2011): Trimethylaminuria. In: Pagon R, Adam M, Ardinger H, eds. GeneReviews®. University of Washington, Seattle