Fish oil is a popular supplement derived from the tissues of fatty fish, valued for its high concentration of omega-3 fatty acids, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These polyunsaturated fats support cardiovascular health and reduce inflammation. Despite these health advantages, a frequent complaint among users is the unpleasant side effect of a fishy aftertaste or odor. This unwelcome smell stems from two distinct biological processes related to the oil’s chemical properties.
Digestive Sources of Fishy Odor
The most common manifestation of the fish oil smell is “fish burps” or reflux, which originates in the upper gastrointestinal tract. This odor occurs when the oil-filled capsule breaks down prematurely in the stomach, releasing its contents. The oily contents float atop the stomach acid, allowing volatile compounds to escape back up the esophagus, causing the characteristic fishy taste and smell.
A significant factor contributing to this discomfort is the quality of the oil, particularly its susceptibility to oxidation or rancidity. Omega-3 fatty acids are highly unsaturated, making them chemically reactive when exposed to air, light, or heat. This reaction generates breakdown products that possess a much stronger, more pungent fishy smell than fresh oil.
If the oil is already oxidized before consumption, the resulting volatile compounds are released immediately upon the capsule dissolving, exacerbating burping and reflux. Independent testing has shown that a notable percentage of supplements may exceed recommended limits for oxidation, meaning many users are unknowingly consuming rancid oil. Taking the supplement on an empty stomach further increases the likelihood of reflux, as the oil is processed rapidly in the acidic environment.
Metabolic Causes of Systemic Odor
A less common but more pervasive form of odor occurs when a fishy smell is noticeable on the breath, sweat, or in the urine, indicating a systemic issue. This happens after the omega-3 fatty acids have been absorbed and metabolized by the body. The systemic odor is caused by the excretion of volatile organic compounds (VOCs) released as byproducts of processing the oil.
When large quantities of fatty acids are consumed, the body’s metabolic pathways break them down, and some resulting volatile chemicals are expelled through various routes of elimination. These compounds are carried through the bloodstream to the lungs for exhalation and to the skin and kidneys for excretion in sweat and urine. This process is a normal part of fat metabolism, but the concentration can become noticeable with high-dose supplementation.
In rare instances, a distinct, strong odor of rotting fish may be linked to a metabolic disorder called trimethylaminuria, also known as Fish Odor Syndrome. This genetic condition impairs the body’s ability to properly process trimethylamine (TMA), a compound derived from the intestinal breakdown of certain foods. Instead of being converted into an odorless compound by the liver, the unoxidized TMA is excreted through sweat, breath, and urine, causing the severe, persistent smell.
Strategies for Eliminating Fish Oil Smell
Fortunately, several practical strategies exist to mitigate or eliminate the unpleasant odors associated with fish oil supplementation. Adjusting the timing and manner of ingestion can slow the breakdown of the capsule in the stomach.
Ingestion Adjustments
- Taking the supplement with a meal, especially one containing other fats, helps to emulsify the oil and minimizes the likelihood of reflux.
- Freezing the capsules temporarily hardens the gel shell and delays its dissolution until it has passed further into the digestive tract.
- Splitting the daily dose and taking smaller amounts multiple times throughout the day eases the burden on the digestive system.
When selecting a product, attention to quality can significantly reduce the risk of odor. Consumers should look for supplements verified by an independent third-party testing organization, as this certification confirms the oil has low levels of oxidation and meets purity standards. Additionally, choosing capsules with an enteric coating is an effective strategy, as this special shell resists stomach acid and only dissolves once the capsule reaches the small intestine.
For individuals who continue to experience systemic odor or who wish to avoid fish products entirely, switching to supplements derived from algae is an alternative. Since the omega-3s in fish originally come from the algae they consume, these supplements provide EPA and DHA without relying on fish oil.