Fish oil contains the long chain polyunsaturated fats (n-3 PUFAs), Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DPA). These are also called Omega 3’s. These were studied as it was noted that Greenland Eskimos whose diet was largely seafood (hard to grow broccoli in Greenland!) had very low rates of heart disease. The fats are felt to convey the cardiovascular benefits of fish and are what comes in your fish oil capsule.
These fats are incorporated into cell membrane phospholipids within a couple weeks and alter membrane fluidity and the protein receptor responses. Blood vessels become more compliant, and responses of the blood vessels to stress hormones and resultant vasoconstrictors are less, so blood pressure is lower, and there is less stress on the heart muscle. The uptake is non-linear, with the greatest uptake at the lower doses, and less as higher doses are consumed. EPA also is metabolized to prostaglandin E3, and eicosanoid that mediates inflammation, which may be why high dose fish oils have been shown to improve outcomes in at least one autoimmune kidney disease. When you stop fish oil, the membrane changes take about 2-3 months to revert, though levels in your blood start to fall in a few days.
In studies, as little as 1 gm/day of fish oil, containing 250-500 mg of EPA and DHA decreases the risk of cardiovascular “events” (stroke, heart attack, arrhythmias, heart failure and peripheral vascular disease) with reduction in total mortality of 30% in early studies of patients with vascular disease. Later studies have shown less benefits, but we are using other medications now in these populations that hugely impact mortality, so the effects of fish oil are less noticeable. In populations without vascular disease, it requires larger and longer studies to see benefits of fish oil, simply because it the cardiovascular events occur less often.
Higher doses of fish oil, 3-4 gms daily, reduce blood pressure in those with hypertension by 4.5 mm/Hg for the systolic blood pressure (top number) and 3 mm/Hg for the diastolic blood pressure (bottom number). This is as good as some medications. Heart rate also lowers by 1 to 2.5 beats per minute. For those without hypertension, fish oil can lower the systolic blood pressure by 1.5 mm/Hg.
This dose of fish oil can also reduce triglycerides (bad fats in your blood) by 25-30%. In the case of triglycerides, the fish oil response is linear, in other words, the more you take, the more your triglycerides come down.
Higher doses of fish oil, up to 12 g/day, have not been shown to cause any adverse effects.
Eating oily fish 1-2 times a week has also been shown to improve cardiovascular outcomes. The fish with the highest levels of omega 3 are wild salmon, trout, mussels, sardines and albacore tuna. A serving can contain up to 500-1500 mg of the EPAs. Please note that albacore tuna can contain high levels of mercury, which should be avoided by pregnant women, women who may become pregnant, and children. Wild salmon is recommended over farmed salmon, as it has less saturated fat that farmed salmon, and though both have omega 3s, those in farmed salmon may be of lower quality.
Eating fish in addition to your fish oil also gives you protein, various vitamins and selenium.