Fish oil is best known for lowering triglycerides and reducing inflammation, but its benefits extend to heart health, mood, and prenatal development. The two active components, EPA and DHA, work by embedding themselves into cell membranes throughout your body, where they influence everything from blood clotting to immune signaling. Here’s what the evidence actually supports.
How EPA and DHA Work in Your Body
EPA and DHA are long-chain omega-3 fatty acids that your body can’t make efficiently on its own. When you consume them, they physically insert into the outer membranes of your cells, changing how those membranes behave. EPA has a more rigid structure that stabilizes membranes and protects them from oxidative damage. DHA, which is slightly longer, increases membrane fluidity, which is especially important in brain and nerve cells where rapid signaling matters.
Once embedded in cell membranes, these fatty acids compete with omega-6 fats for the same enzymes. Omega-6 fats tend to produce compounds that promote inflammation, blood clotting, and blood vessel constriction. Omega-3s shift that balance, generating compounds that resolve inflammation and relax blood vessels instead. This competitive mechanism is the foundation of nearly every health benefit attributed to fish oil.
Lowering Triglycerides
The strongest evidence for fish oil is in triglyceride reduction. This is the one area where the American Heart Association specifically endorses prescription-strength omega-3s as a treatment option. At doses of 3 to 4 grams per day, EPA and DHA reduce triglycerides by roughly 27% on average, with a range of 21% to 35% depending on baseline levels. In people with very high triglycerides (around 900 mg/dL), reductions can reach 60%.
For people already taking statins, adding 4 grams per day of prescription omega-3s produces an additional triglyceride drop of about 21%. One important distinction: supplements containing both EPA and DHA can raise LDL cholesterol in people with very high triglycerides, while EPA-only formulations do not appear to have this effect. This is worth discussing with your doctor if your LDL is already a concern.
Heart and Blood Vessel Protection
Beyond triglycerides, omega-3s influence cardiovascular health through several pathways. They boost production of prostacyclin, a compound that prevents blood platelets from clumping together and helps blood vessels relax. At the same time, they limit production of thromboxane, which does the opposite. The net effect is a modest reduction in clotting tendency and improved blood flow.
Omega-3s also incorporate into the membranes of heart muscle cells and mitochondria, where they may help stabilize electrical activity and reduce the risk of irregular heart rhythms. The American Heart Association recommends eating two servings of fatty fish per week (about 3 ounces cooked per serving) as a baseline for cardiovascular health.
Reducing Inflammation
Fish oil consistently lowers several markers of systemic inflammation. A large umbrella meta-analysis found that omega-3 supplementation significantly reduces C-reactive protein (a general inflammation marker), along with two key immune signaling molecules involved in chronic inflammation. The anti-inflammatory effect on immune signaling was particularly pronounced in studies lasting 10 weeks or less, and the reduction in C-reactive protein was greatest in people with diabetes and those over 55.
The mechanism goes deeper than just competing with omega-6 fats. Omega-3s bind to a receptor on immune cells called GPR-120, which directly blocks a master switch for inflammatory gene activity. They also shift the balance of immune T-cells toward a less inflammatory profile and generate specialized compounds that actively resolve tissue injury rather than just suppressing the initial immune response. This resolution-promoting effect is distinct from how most anti-inflammatory drugs work, which simply block the inflammatory process without promoting healing.
Depression and Mood
EPA, specifically, shows promise as an add-on treatment for depression. In a clinical trial testing different doses, 4 grams per day of EPA produced a response rate of 64%, compared to 40% for placebo. Interestingly, lower doses of 1 or 2 grams per day performed no better than placebo, with response rates of 38% and 36% respectively. This suggests a dose threshold that most over-the-counter supplements fall well below.
The antidepressant effect appears to be strongest in people whose depression involves elevated inflammation, which aligns with what we know about EPA’s anti-inflammatory properties. Fish oil is not a replacement for standard depression treatment, but high-dose EPA may offer meaningful additional benefit for certain people.
Brain Health and Cognitive Decline
DHA is the dominant omega-3 in the brain, and it’s critical for neuronal structure and signaling. This has fueled widespread interest in fish oil for preventing dementia, but the clinical evidence is disappointing so far. Systematic reviews of trials using 1 gram per day or less of DHA found no benefit on cognitive function in cognitively healthy older adults. In people already diagnosed with Alzheimer’s disease, DHA supplementation did not slow symptom progression.
One possible explanation is dosing. Research suggests that standard supplement doses may not meaningfully increase DHA levels in the brain, particularly in people who carry a genetic variant (APOE4) that affects fat transport. Higher doses might be needed, but this hasn’t been confirmed in large trials. For now, there’s no strong evidence that fish oil supplements prevent or slow cognitive decline in adults.
Pregnancy and Fetal Development
DHA accumulates rapidly in the fetal brain during the last trimester of pregnancy, making maternal intake particularly important during this window. However, the evidence for supplementation is more nuanced than marketing suggests. A clinical trial of high-dose maternal DHA supplementation in preterm infants found no overall improvement in neurodevelopmental outcomes at 18 to 22 months.
There were notable exceptions in the most premature babies. Infants born before 27 weeks whose mothers received DHA scored higher on language assessments (mean score of 89 versus 83 in the placebo group). Separately, a combination of DHA and another fatty acid reduced severe retinopathy of prematurity by 50% in this very early-born group. The benefits appear most relevant for high-risk pregnancies rather than as a universal supplement.
Best Food Sources of Omega-3s
Fatty fish provides far more EPA and DHA per serving than supplements typically deliver. A 3-ounce serving of farmed Atlantic salmon contains roughly 1,830 mg of combined EPA and DHA. Here’s how common seafood options compare:
- Atlantic salmon (farmed): 1,830 mg combined EPA/DHA
- Atlantic herring: 1,710 mg
- Sardines (canned): 1,190 mg
- Atlantic mackerel: 1,020 mg
- Rainbow trout (wild): 840 mg
At the other end of the spectrum, popular choices like tilapia (150 mg), shrimp (240 mg), and canned light tuna (190 mg) provide relatively little. If you’re relying on fish for omega-3s, the species matters enormously. Two servings of salmon per week delivers roughly 3,600 mg, while two servings of tilapia delivers only about 300 mg.
Krill oil is marketed as a more bioavailable alternative because its omega-3s are bound to phospholipids rather than triglycerides. Some studies support slightly better absorption, but others show no difference. Krill oil capsules also tend to contain much lower total amounts of EPA and DHA per capsule than standard fish oil.
Side Effects and Cautions
At typical supplement doses, fish oil is well tolerated. The most common complaints are a fishy aftertaste, bad breath, and digestive issues like heartburn, nausea, or diarrhea. Taking capsules with meals or freezing them before swallowing can reduce these effects.
The more meaningful concern is bleeding risk. Fish oil reduces platelet aggregation, which is part of its cardiovascular benefit but can become a problem at high doses or when combined with blood thinners and antiplatelet medications. If you’re taking warfarin, aspirin, or similar drugs, the combination may increase bleeding risk. High doses may also slightly increase stroke risk. For people not on blood-thinning medications, standard supplement doses (1 to 2 grams per day) rarely cause problems.