Fish oil supplements provide two omega-3 fatty acids, EPA and DHA, best known for supporting heart and brain health. Their effect on gut bacteria is a newer research area, alongside other emerging areas like fasting-mimicking diets and your gut, and the picture that's emerging is modest changes rather than a full overhaul.
In an eight-week randomized trial, 22 middle-aged adults took a daily combination of 2 grams of EPA and 2 grams of DHA. At the broadest level, overall microbial diversity did not shift meaningfully, individual variation between people outweighed the supplement's effect.[1] Research on gut enterotypes finds that a person's baseline bacterial community shapes how much any single supplement can shift things, which may help explain why the effect size here was modest. But at the genus level, specific changes did appear.
The changes reversed once people stopped taking the supplement, both the increases and the decreases returned to baseline levels.[1] The researchers also found that a liquid drink formulation produced larger increases in short-chain-fatty-acid-producing bacteria like Roseburia than the same dose in capsule form. How the omega-3s are delivered may matter as much as the dose itself. Some participants experienced mild digestive symptoms, including diarrhea, during the trial.
What to do with this
If you take fish oil for heart or brain health, a secondary, modest, and reversible shift in a handful of gut bacteria genera is a reasonable expectation, not a transformation of your microbiome. The trial dose, 2 grams EPA plus 2 grams DHA daily, sits at the higher end of common supplement labels. Check your product's actual EPA and DHA content rather than assuming "1000 mg fish oil" delivers that amount. If digestive upset shows up after starting, taking it with food or splitting the dose across two meals is a reasonable first adjustment before stopping altogether. For a fiber-focused supplement with a different gut mechanism, see psyllium husk and your gut.
