Fish Oil

Fish Oil Overview

Fish oil is an oil obtained from the tissues of fatty fish and is best known as a dietary source of the long-chain omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). These fatty acids are involved in many processes throughout the body, including cardiovascular function, cell membranes and inflammatory signaling.

EPA and DHA originate in aquatic food chains. Microalgae produce omega-3 fatty acids, which are then accumulated by fish as they consume algae and other marine organisms. Fatty fish such as salmon, sardines, herring, mackerel and trout are among the better dietary sources of EPA and DHA.

Fish is also a source of protein, vitamin B12, selenium and other nutrients, which is one reason health recommendations generally emphasize eating fish rather than relying entirely on supplements. The American Heart Association recommends eating two servings of fish per week, particularly fatty fish.

One consideration when choosing seafood is mercury. Large, long-lived predatory fish tend to accumulate more methylmercury than smaller fish. The FDA advice about eating fish lists salmon, sardines and many other commonly eaten fish among lower-mercury choices, while shark, swordfish, king mackerel, orange roughy, marlin, Gulf of Mexico tilefish and bigeye tuna are among the highest-mercury choices.

Fish Oil and Health

Fish oil has been studied for decades, but its effects are more specific than some older claims about omega-3 supplements suggest. Eating fish as part of a balanced diet and taking concentrated omega-3 supplements are also not necessarily equivalent. The dose, source of omega-3s and the health of the person taking them all matter.

Cardiovascular Health

The clearest evidence for omega-3 fatty acids concerns cardiovascular health and blood triglycerides. Regular consumption of fish and seafood is associated with a lower risk of cardiovascular disease, which is one reason fish is included in heart-healthy dietary patterns.

EPA and DHA can also lower triglyceride levels. For people with very high triglycerides, physicians may prescribe concentrated omega-3 products in therapeutic doses. These prescription products should not be confused with ordinary over-the-counter fish oil capsules. Routine use of nonprescription fish oil supplements has not consistently been shown to prevent heart attacks or strokes in otherwise healthy people.

Inflammation and Joint Health

EPA and DHA affect pathways involved in inflammation, and omega-3 supplements have been investigated for inflammatory conditions. Research suggests that long-chain omega-3 fatty acids may provide some benefit as an additional treatment for rheumatoid arthritis, although they are not a replacement for standard medical treatment and the evidence varies by condition.

Brain and Mental Health

DHA is an important structural component of the brain and retina, and omega-3 fatty acids continue to be studied in relation to mood and neurological health. Observational studies have sometimes associated higher fish intake with a lower risk of depression, but clinical trials of omega-3 supplements have produced mixed results. Fish oil should therefore not be presented as an established treatment for depression or other mental-health conditions.

Pregnancy and Early Development

DHA is particularly important during pregnancy and early development. Fish can provide DHA together with protein, iodine, vitamin D and other nutrients. Because mercury exposure is also important during pregnancy, the FDA and EPA recommend choosing a variety of fish that are lower in mercury rather than avoiding fish altogether.

Cancer Research

Omega-3 fatty acids have been investigated in relation to several types of cancer, but laboratory findings and observational associations should not be confused with proven prevention. Large randomized studies have not shown that ordinary fish oil supplementation significantly reduces overall cancer incidence. Fish oil should not be considered a cancer-prevention or cancer-treatment supplement.

Fish Oil Supplements

Fish oil supplements vary considerably. The amount of “fish oil” printed on the front of a bottle is not necessarily the amount of EPA and DHA it contains, so the supplement facts label is more useful when comparing products. Different supplements may provide omega-3s as natural triglycerides, re-esterified triglycerides, ethyl esters or other forms.

For most people, moderate doses of omega-3 supplements are generally well tolerated. Common side effects can include an unpleasant aftertaste, heartburn, nausea or digestive discomfort. Very high doses are a different matter: several clinical trials using about 4 grams per day found a small increase in atrial fibrillation among people with cardiovascular disease or a high cardiovascular risk.

Fish oil can also interact with medications, particularly anticoagulants and other drugs that affect blood clotting. Anyone considering high-dose omega-3 supplements or using them for a medical condition should discuss the dose with a health professional.

For more information on amounts and supplement use, see Dosage of Fish Oil.

Types and Sources of Omega-3 Oil

Fish Oil

Standard fish oil is generally made from the tissues of oily fish and contains varying amounts of EPA and DHA. Supplements may be sold as liquid oil or in softgel capsules. Quality depends on the raw material, processing, oxidation control and actual EPA/DHA content rather than on marketing terms alone.

Cod Liver Oil

Cod liver oil is obtained from cod liver rather than from ordinary fish tissue. In addition to omega-3 fatty acids, it naturally contains vitamins A and D. This makes cod liver oil nutritionally different from standard fish oil. Because excessive vitamin A intake can be harmful, particularly during pregnancy, cod liver oil should not simply be treated as an interchangeable high-dose source of omega-3s.

Prescription Omega-3 Products

Prescription omega-3 medications contain highly purified and standardized quantities of omega-3 fatty acids. They may contain EPA alone or combinations of EPA and DHA and are primarily used in specific medical situations, particularly for high triglyceride levels. The term “pharmaceutical grade fish oil” is sometimes used in marketing, but prescription omega-3 medication is a more meaningful distinction because it refers to regulated drug products with defined ingredients and doses.

Algal Oil

Algal oil is not technically fish oil, but it is an alternative direct source of DHA and, in some products, EPA. Because marine omega-3s ultimately originate largely from microalgae, algal oils can be useful for people who want long-chain omega-3 fatty acids without consuming fish.

Fish or Supplements?

For most people, eating fish regularly provides more than EPA and DHA alone. Fish also supplies protein, vitamins and minerals and can replace foods higher in saturated fat. This is why major dietary recommendations generally emphasize seafood as part of a balanced diet rather than recommending fish oil capsules universally.

Supplements can still be useful in particular circumstances, but more is not automatically better. The appropriate amount depends on diet, health conditions, medications and the specific EPA and DHA content of the product.

Related Dietary Oils

Omega-3 fats also occur outside fish oil, although different oils have very different fatty-acid profiles. Flaxseed is particularly known as a plant source of alpha-linolenic acid (ALA), a shorter-chain omega-3 fatty acid that the body can convert only in limited amounts to EPA and DHA.

If you are comparing dietary and culinary oils more broadly, you may also be interested in macadamia oil, flaxseed oil, rapeseed and canola oil, sunflower oil and safflower oil. Their nutritional composition differs considerably from marine fish oil, so they should not automatically be considered substitutes for EPA and DHA.

This page provides general information about fish oil and omega-3 fatty acids and is not intended to diagnose, treat or prevent any medical condition.

Fish Oil Overview |
Fish Oil and Health |
Supplements |
Types of Fish Oil |
Fish or Supplements?