Oily fish omega-3 fatty acids—specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—are essential nutrients that the body cannot produce in adequate amounts. Found abundantly in species such as mackerel, salmon, and sardines, these long-chain polyunsaturated fatty acids support cardiovascular health, brain function, and inflammatory regulation. The NHS recommends at least one portion of oily fish weekly as part of a balanced diet. Unlike plant-based alpha-linolenic acid (ALA), EPA and DHA from oily fish are directly incorporated into cell membranes, making dietary intake particularly valuable. Understanding which fish provide the most omega-3, appropriate portion sizes, and population-specific guidance helps optimise nutritional benefit whilst managing potential contaminant exposure.

What Are Omega-3 Fatty Acids and Why Are They Important?

Omega-3 fatty acids are a family of essential polyunsaturated fatty acids that play crucial roles in human health. The term 'essential' indicates that the body cannot synthesise these compounds in sufficient quantities, making dietary intake necessary. The three main types of omega-3 fatty acids are alpha-linolenic acid (ALA), found primarily in plant sources, and the marine-derived eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are abundant in oily fish.

EPA and DHA are particularly important because they are incorporated directly into cell membranes throughout the body, where they influence membrane fluidity, cellular signalling, and gene expression. These long-chain omega-3 fatty acids serve as precursors for bioactive lipid mediators called resolvins and protectins, which help regulate inflammatory responses and support tissue repair. DHA is especially concentrated in the brain and retina, where it contributes to neuronal structure and visual function.

The body can convert ALA to EPA and DHA, but this process is inefficient—typically less than 10% of dietary ALA is converted to EPA, and even less to DHA. This limited conversion capacity makes direct dietary sources of EPA and DHA, particularly oily fish, especially valuable for maintaining optimal omega-3 status. For those following vegetarian or vegan diets, algae-based supplements can provide a direct source of EPA and DHA.

The Scientific Advisory Committee on Nutrition (SACN) recognises the importance of omega-3 fatty acids, particularly for cardiovascular health, with evidence for other benefits such as cognitive function and inflammatory regulation being more mixed. The NHS recommends that most people should aim to eat at least two portions of fish per week, including one portion of oily fish, to ensure adequate intake of these essential nutrients, though specific populations may require modified advice based on individual health circumstances.

Which Oily Fish Contain the Most Omega-3?

Oily fish are distinguished from white fish by their higher fat content, which is distributed throughout their flesh rather than concentrated in the liver. This characteristic makes them particularly rich sources of EPA and DHA. The omega-3 content varies considerably between species, with some providing substantially more than others per standard serving.

Fish with the highest omega-3 content include:

• Mackerel – One of the richest sources, providing approximately 2.5–3.0 grams of omega-3 per 100-gram portion

• Herring – Contains around 2.0–2.5 grams per 100 grams, whether fresh, pickled, or kippered

• Salmon – Provides approximately 1.5–2.5 grams per 100 grams, with wild salmon typically containing slightly more than farmed varieties

• Sardines – Offer about 1.5–2.0 grams per 100 grams and are often consumed with bones, providing additional calcium

• Trout (particularly rainbow trout) – Contains approximately 1.0–1.5 grams per 100 grams

• Fresh tuna – Provides around 1.0–1.5 grams per 100 grams

It is important to note that tinned tuna does not count as oily fish for nutritional purposes according to the NHS. This is partly because most canned tuna is made from species with lower fat content, and also because much of the omega-3 content is lost during the canning process. Other oily fish options include pilchards, sprats, and anchovies, all of which provide valuable amounts of EPA and DHA.

The omega-3 content can be influenced by factors including the fish's diet, whether it is wild-caught or farmed, the season of harvest, and preparation methods. Gentle cooking methods such as grilling, baking, or steaming generally preserve omega-3 content better than high-temperature deep frying, which may lead to some degradation of these beneficial fatty acids.

Health Benefits of Omega-3 from Oily Fish

The health benefits of omega-3 fatty acids from oily fish are supported by substantial epidemiological and clinical evidence, though the strength of evidence varies across different health outcomes. Understanding both the established benefits and areas of ongoing research helps inform balanced dietary recommendations.

Cardiovascular health represents one of the most well-established areas of benefit. Consuming oily fish as part of a balanced diet is associated with reduced cardiovascular risk. However, it's important to note that NICE guidance (NG238) advises against routinely offering omega-3 supplements for primary or secondary prevention of cardiovascular disease. The prescription-only medicine icosapent ethyl (highly purified EPA) is recommended only for specific high-risk groups with elevated triglycerides despite statin therapy. While omega-3 fatty acids can help reduce triglyceride levels (particularly at higher supplemental doses), modestly lower blood pressure, and may affect cardiac rhythm, the benefits of dietary fish likely come from the whole food rather than isolated fatty acids alone. Oily fish provides additional nutrients including vitamin D, selenium, and high-quality protein.

Brain and cognitive function benefit from adequate omega-3 intake throughout the lifespan. DHA is a major structural component of neuronal membranes and is critical for foetal brain development during pregnancy. Observational studies suggest associations between higher fish consumption and reduced risk of cognitive decline, though there is no definitive evidence that omega-3 supplementation prevents or reverses established dementia. The mental health benefits remain an active area of research, with some evidence suggesting potential benefits for depression, though results are inconsistent.

Inflammatory conditions may be influenced by omega-3 intake due to the production of anti-inflammatory mediators. Some evidence suggests benefits for rheumatoid arthritis symptoms, though omega-3s should complement rather than replace conventional treatment. The role in other inflammatory conditions continues to be investigated.

Eye health is supported by DHA's concentration in retinal tissue, and adequate intake may contribute to reduced risk of age-related macular degeneration, though evidence remains observational rather than from definitive intervention trials.

Patients should be advised that while omega-3 from oily fish forms part of a healthy, balanced diet, it is not a substitute for prescribed medications or established medical treatments. High-dose omega-3 supplements may have different risk-benefit profiles compared to dietary fish, with some studies suggesting a potential increased risk of atrial fibrillation at high supplemental doses.

How Much Oily Fish Should You Eat Weekly?

The NHS recommends that most adults should eat at least two portions of fish per week, including at least one portion of oily fish. A standard portion is defined as approximately 140 grams of cooked fish. This recommendation aims to balance the nutritional benefits of fish consumption with considerations about environmental contaminants.

However, specific population groups require modified recommendations:

Men, boys, and women not planning pregnancy can safely consume up to four portions of oily fish per week.

Girls and women of childbearing age, those who are pregnant, breastfeeding, or planning pregnancy should limit oily fish intake to no more than two portions per week. This precautionary advice balances the benefits of omega-3 for foetal development against potential risks from contaminants such as dioxins and polychlorinated biphenyls (PCBs) that can accumulate in oily fish.

Additional specific guidance applies to certain fish species:

• Pregnant and breastfeeding women should avoid shark, swordfish, and marlin entirely due to high mercury content.

• Tuna consumption for pregnant and breastfeeding women should be limited to no more than two fresh tuna steaks (about 140 g cooked or 170 g raw) or four medium-sized cans of tuna per week (about 140 g drained weight per can).

It's worth noting that tinned tuna does not count as oily fish in UK dietary recommendations, though it still provides protein and other nutrients.

Children and adolescents can benefit from oily fish as part of a balanced diet, with boys able to eat up to four portions weekly and girls limited to two portions weekly. Portion sizes should be adjusted according to age and appetite.

For individuals who do not consume fish due to dietary preferences, allergies, or other reasons, alternative sources of omega-3 include plant-based ALA from walnuts, flaxseeds, and rapeseed oil, though conversion to EPA and DHA is limited. Algae-based supplements providing DHA and EPA are available for those following vegan diets.

Patients with specific health conditions, particularly those taking anticoagulant medications, should consult their GP or pharmacist before significantly increasing omega-3 intake or starting supplements, as high doses may theoretically affect bleeding risk, though this is rarely clinically significant at dietary levels.

When selecting oily fish, choosing a variety of species and considering sustainability certifications can support both personal health and environmental responsibility.

Frequently Asked Questions