Omega-3: What Your Body Actually Uses It For (And Why the Source Matters More Than You Think)

Long before omega-3 became a health trend, it was simply dinner. For centuries, Scandinavian diets have centred on fatty fish like salmon, herring, mackerel and trout, species that happen to be among the richest natural sources of the omega-3 fatty acids EPA and DHA. The tradition of cod liver oil, taken through dark winters by generations of Nordic families, was arguably the world's first targeted supplement, even if nobody called it that at the time.

The Nordic Nutrition Recommendations 2023 recommend consuming 300–450 g of fish per week, with at least 200 g coming from fatty fish, specifically because of the long-chain omega-3 content.¹ Fish consumption across the Nordic and Baltic countries varies considerably, with Norway and Iceland eating the most and intake in Denmark falling somewhere in the middle.²

But here's the part that often gets overlooked: even in Scandinavia, fish consumption has been changing. Younger generations eat less fish than their parents did. Convenience foods, plant-based shifts, and simple preference changes mean that the cultural safety net of a fish-rich diet is no longer something we can take for granted, even in countries surrounded by some of the best fishing waters in the world.²

That matters, because omega-3 fatty acids are essential in the most literal sense. Your body cannot produce EPA or DHA on its own. You have to get them from food or supplementation. And while some plant foods contain the omega-3 ALA (alpha-linolenic acid), your body's ability to convert ALA into the forms it actually needs, EPA and DHA, is extremely limited, typically less than 5–10%.³

Two Fats, Different Jobs: What EPA and DHA Actually Do

Most people think of omega-3 as a single thing. In reality, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are two distinct fatty acids with overlapping but meaningfully different roles in your body. Understanding the difference matters, both for choosing what to eat and for evaluating supplements.

EPA is primarily involved in modulating inflammation. It serves as a precursor to a class of signalling molecules called resolvins and protectins, which actively help your body resolve inflammatory processes rather than just suppressing them.⁴ EPA also influences how your immune cells communicate, and it plays a role in cardiovascular function, particularly in reducing blood triglyceride levels and supporting healthy endothelial function (the lining of your blood vessels).⁵

DHA is a structural fat. It's a major component of cell membranes throughout your body, but it's especially concentrated in two places: your brain and your retinas. DHA makes up roughly 40% of the polyunsaturated fatty acids in the brain and approximately 60% in the retinal photoreceptors.³ It contributes to membrane fluidity, which affects how efficiently your cells communicate with each other. This is why DHA is considered particularly important during pregnancy and early development, and why it continues to matter for cognitive function throughout life.⁶

Both EPA and DHA contribute to cardiovascular health, but the mechanisms differ. A large pooled analysis involving over 183,000 participants from 29 international cohorts, published in 2024, found that individuals with the highest circulating levels of EPA had a 17% lower risk of total stroke, while those with the highest DHA levels had a 12% lower risk.⁵ The evidence for fish consumption and cardiovascular protection is considered strong by the Nordic Nutrition Recommendations 2023, and this is largely attributed to the EPA and DHA content.¹

The Inflammation and Mood Connection

One of the most interesting areas of omega-3 research in recent years has been mood. And it's more specific than you might expect: the benefits appear to come primarily from EPA, not DHA.

A meta-analysis of 26 randomised controlled trials found that omega-3 supplementation had a significant overall effect on depressive symptoms. But when the researchers looked more closely, they found that the benefit was concentrated in formulations where EPA made up 60% or more of the total omega-3 content, at doses up to 1 g per day. DHA-dominant formulations did not show the same effect.⁷ A more recent 2023 systematic review confirmed this pattern, reporting significant reductions in depression severity with EPA-enriched interventions, but not with higher doses above 2 g per day, suggesting a threshold rather than a linear dose-response.⁸

Why EPA specifically? The leading hypothesis involves inflammation. EPA's anti-inflammatory properties appear to be particularly relevant for a subtype of depression characterised by elevated inflammatory markers. A 2024 study found that add-on EPA treatment improved depressive symptoms specifically in patients with higher baseline levels of C-reactive protein (a marker of inflammation), improving fatigue and sleep difficulties in particular.⁹ This aligns with the broader concept of "inflamed depression," the idea that for some people, low-grade chronic inflammation is a contributing driver of mood disruption, and addressing it directly can help.

This doesn't mean omega-3 is an antidepressant. But for people whose low mood may have an inflammatory component (and that's more common than many realise, particularly among women during hormonal transitions) EPA appears to play a genuinely supportive role.

Omega-3 and Women's Health

Beyond mood, there's a growing body of evidence that omega-3s matter for women's hormonal health in specific, measurable ways.

For menstrual pain, the evidence is relatively strong. A 2023 systematic review and meta-analysis of 12 studies involving 881 women with dysmenorrhoea found that omega-3 supplementation (300–1,800 mg per day over 2–3 months) produced a large effect in reducing menstrual pain. The same review found that 86% of studies measuring analgesic use reported a reduction, meaning women needed fewer painkillers during their period.¹⁰ A separate 2022 meta-analysis confirmed these findings and additionally noted that lower doses appeared to be more effective than higher ones, and that the benefit was more pronounced in younger women.¹¹

The mechanism is fairly well understood: omega-3s compete with omega-6 fatty acids (specifically arachidonic acid) for the same enzymatic pathways that produce prostaglandins, the compounds responsible for uterine contractions and the pain of period cramps. By shifting the balance toward less inflammatory prostaglandins, omega-3s can reduce both the intensity of contractions and the pain they cause.¹⁰

For PMS more broadly, a 2022 meta-analysis looking at the effect of omega-3s on premenstrual syndrome found a significant reduction in overall PMS severity, including both somatic symptoms (bloating, cramps, headache) and psychological symptoms (irritability, low mood, anxiety). The researchers noted that benefit increased with longer duration of supplementation and that older women saw somewhat less effect than younger women.¹²

There's also preliminary research linking omega-3 status to ovulatory function. A study published in Fertility and Sterility found that women with the highest omega-3 intake were significantly less likely to be anovulatory compared to those with the lowest intake. Higher omega-3 consumption was associated with increased progesterone levels and altered timing of follicular development.¹³ Women with PCOS may also benefit. Omega-3 supplementation has been shown to help reduce serum testosterone and improve menstrual regularity in some trials, likely through its effects on inflammation and insulin sensitivity.¹⁴

Not All Omega-3 Supplements Are the Same

This is where things get important, and where the supplement industry often falls short on transparency. When you buy an omega-3 supplement, three things determine whether it actually works: the form, the dose, and the quality.

The form refers to the molecular structure the EPA and DHA are delivered in. In whole fish, omega-3s naturally exist as triglycerides, three fatty acids attached to a glycerol backbone. Your digestive system is designed to recognise and process this structure efficiently. Most cheap supplements, however, use ethyl esters, a synthetic form created during the purification process, where the fatty acids are bonded to ethanol instead of glycerol. It's cheaper to produce, but your body has to work harder to absorb it.¹⁵

Research consistently shows that the triglyceride form is more bioavailable. A landmark bioavailability study led by Jørn Dyerberg (often called the father of fish oil research) found that re-esterified triglyceride forms produced significantly higher plasma concentrations of EPA and DHA compared to ethyl ester forms.¹⁵ A longer-term study found that triglyceride supplementation increased the Omega-3 Index, a validated blood measure of omega-3 status, to a greater extent than ethyl ester supplementation over six months.¹⁶ Perhaps most tellingly, early research found that ethyl ester omega-3s had only around 20% absorption when taken on an empty stomach, compared to substantially higher absorption for triglyceride forms regardless of meal timing.¹⁵

The dose matters more than the label often suggests. A supplement labelled "1,000 mg fish oil" may only contain 300 mg of actual EPA + DHA, with the rest being other fats. What you need to look at is the combined EPA + DHA per serving, not the total fish oil weight. For general health maintenance, most guidelines suggest 250–500 mg of combined EPA + DHA per day. For specific benefits like mood support or menstrual pain, the research has typically used doses of 1,000–2,000 mg per day.^7,10

Quality is the third factor. Omega-3 oils are polyunsaturated fats, which means they're prone to oxidation. A rancid fish oil supplement (and more exist on the market than you'd hope) is not just unpleasant; oxidised oils can introduce harmful compounds and may actually contribute to inflammation rather than reducing it. Look for supplements that disclose their TOTOX (total oxidation) value and are third-party tested for purity.¹⁷

The Omega-3 Index: A Better Way to Know Where You Stand

One of the more useful developments in omega-3 research is the Omega-3 Index, a blood test that measures the percentage of EPA and DHA in your red blood cell membranes. Unlike a snapshot blood test that fluctuates with your last meal, the Omega-3 Index reflects your average intake over the previous two to three months, making it a reliable marker of long-term status.¹⁸

An Omega-3 Index of 8% or above is generally considered optimal and has been associated with the lowest risk of cardiovascular events. Below 4% is considered high-risk. Most people who aren't regularly eating fatty fish or supplementing tend to fall somewhere between 3% and 6%.¹⁸

This test is particularly valuable because omega-3 status is influenced by more than just what you eat. Genetics, body weight, age, and sex all play a role in how efficiently you absorb and incorporate EPA and DHA into your cells. Two people eating the same amount of salmon each week may have meaningfully different Omega-3 Index levels. If you're investing in supplementation, testing is the most honest way to know whether it's actually working.

What This Means for You

If you're a woman living in Scandinavia, here's a grounded look at where the evidence points:

The Nordic Nutrition Recommendations 2023 recommend 300–450 g of fish per week, with at least 200 g from fatty fish like salmon, mackerel, herring, or trout. This translates to roughly 250–500 mg of EPA + DHA per day from food, which is a good baseline for general health.¹ If you're consistently eating 2–3 servings of fatty fish per week, you may already be well covered.

If you're not eating much fish, or if you follow a plant-based diet, supplementation is the most reliable way to maintain adequate EPA and DHA levels. Remember that ALA from flaxseed, chia, and walnuts, while healthy, does not meaningfully convert to the forms your body needs most.³ Algae-based supplements are available and provide DHA and some EPA directly, without the fish.

For general health, a supplement providing 500–1,000 mg of combined EPA + DHA per day in triglyceride form is a reasonable starting point. If you're supplementing specifically for mood support or menstrual pain, the research has used higher doses, typically 1,000–2,000 mg, with an emphasis on EPA-dominant formulations for mood and a balanced EPA/DHA ratio for pain.^7,10

If you're already taking vitamin D and magnesium, adding omega-3 completes what you might think of as the Nordic foundational trio. Vitamin D and omega-3 even share something in common: fatty fish is one of the few dietary sources that provides meaningful amounts of both. There's also emerging research suggesting that vitamin D and omega-3 may work synergistically in modulating inflammation and immune function, though this area is still developing.⁶

And if you want to take the guesswork out of it entirely, the Omega-3 Index test can give you a clear answer about where your levels actually stand.