I find DHA fascinating because it’s one of those nutrients where the evolutionary story actually matters. Archaeological evidence suggests that access to DHA-rich seafood may have been crucial for human brain development. Our brains are unusually large relative to body size compared to other primates, and DHA appears to have played a role in that development. 1

That said, the question of whether taking DHA supplements today will meaningfully improve your health is more complicated than the evolutionary backstory might suggest. I’ve gone through the research, and the picture is mixed.

What is DHA?

DHA (docosahexaenoic acid) is an omega-3 long-chain polyunsaturated fatty acid. It’s often mentioned alongside EPA (eicosapentaenoic acid), which is another omega-3 fatty acid found in fatty fish. The two are related but serve somewhat different functions in the body.

What makes DHA particularly interesting is its concentration in the brain. It accounts for over 90% of the omega-3 fatty acids in the brain, representing roughly 10-20% of total brain lipids. You’ll also find high levels in the retina. This distribution isn’t random; DHA plays structural roles in cell membranes and is involved in neurotransmitter function, gene expression, and inflammatory regulation.

The body can technically convert the plant-based omega-3 ALA (found in flaxseed and walnuts) into DHA, but the conversion rate is quite poor. Most estimates put it at under 5%, which is why fish oil supplements remain popular among people who don’t eat much seafood.

6 Evidence-Based Benefits of DHA

Age-related macular degeneration is a leading cause of vision loss in Western countries, affecting roughly 8% of the population. The disease starts with changes to the macula (the central part of the retina) and can progress to irreversible vision loss. 2

A large prospective study following over 113,000 participants for up to 28 years found that higher dietary intake of omega-3 fatty acids, particularly DHA, was associated with a 40-50% reduction in early macular degeneration risk. 3

The mechanism likely relates to DHA’s anti-inflammatory properties and its ability to inhibit abnormal blood vessel growth. However, I should note that this was observational data. People who eat more fish tend to have other healthy habits too, so we can’t say with certainty that DHA itself was responsible for the benefit.

For those concerned about eye health, you might also want to read about lutein, which works through different mechanisms to support vision.

2. May support cognitive function in healthy older adults

Cognitive decline is one of the more distressing aspects of ageing. Memory problems rank as a top concern for many people, second perhaps only to cardiovascular disease.

A follow-up study lasting 9.1 years found that older adults without dementia who had the highest blood DHA levels had a 47% lower incidence of all-cause dementia. 4

A separate double-blind controlled trial lasting 24 weeks in 485 healthy middle-aged and older adults found that DHA supplementation improved learning and memory function. 5

These results are promising, but here’s the catch: a systematic review of 10 randomised controlled trials involving 2,327 older adults found that DHA supplementation showed no significant benefit for memory, executive function, working memory, or attention in people who already had age-related cognitive decline. 6

My interpretation? DHA might help prevent cognitive decline in healthy people, but it’s probably not going to reverse decline once it’s already happening. If you’re interested in supplements for brain health, our article on dementia-supporting supplements covers this topic in more detail.

3. Has anti-inflammatory effects

Inflammation is a normal immune response, but chronic low-grade inflammation is implicated in numerous conditions including Alzheimer’s disease, atherosclerosis, type 2 diabetes, and cancer. 7

A randomised, double-blind controlled study in 154 abdominally obese individuals with low-grade inflammation found that DHA had notable anti-inflammatory effects, reducing interleukin-18, C-reactive protein (CRP), and tumour necrosis factor-alpha (TNF-α). Interestingly, DHA appeared to have stronger anti-inflammatory effects than EPA in this study. 8

This is relevant for conditions like interstitial cystitis and painful bladder syndrome, where chronic inflammation plays a central role. While DHA won’t treat these conditions on its own, its anti-inflammatory properties might provide some benefit alongside other approaches.

4. May benefit attention deficit hyperactivity disorder (ADHD)

ADHD is one of the most common neurodevelopmental disorders, with an estimated prevalence of 7.2%. It’s more common in males and typically emerges in childhood, though it can persist into adulthood. Symptoms include difficulty sustaining attention, hyperactivity, and impulsivity.

One randomised, double-blind, placebo-controlled trial followed 66 patients aged 6-18 years with ADHD for six months. Those receiving DHA supplementation showed greater improvements in cognitive domains. 9

This is encouraging, but I want to be honest: a single trial with 66 participants isn’t strong evidence. The effect might be real, but larger studies are needed before we can be confident. If your child has ADHD, DHA supplementation is unlikely to cause harm and might help, but it shouldn’t replace evidence-based treatments like behavioural therapy or medication when those are warranted.

5. Brain development during pregnancy (results are complicated)

Here’s where the story gets interesting. You’d expect DHA to be important for foetal brain development. Both animal and human studies have established that DHA is needed for central nervous system development and affects cognition, behaviour, and executive function. 10

However, a randomised clinical trial following 543 children for 7 years found that when pregnant women took DHA supplements during the second half of pregnancy, their children did not have significantly better IQ scores, language abilities, or learning skills compared to placebo. In fact, the DHA group actually showed slightly more executive dysfunction and behavioural problems. 11

This surprised me. The obvious explanation is that women who eat a reasonable diet probably already get enough DHA for normal foetal brain development, and extra supplementation doesn’t add much benefit. Another possibility is that the relationship between DHA and brain development is more nuanced than we thought.

For pregnant women, the NHS recommends eating two portions of fish per week, including one oily fish, rather than relying on supplements. NHS Guidance

6. Potential anti-cancer properties (preclinical evidence only)

Research suggests DHA may induce DNA oxidative damage in cancer cells, potentially promoting cancer cell death. It might also improve the sensitivity of tumour tissues to anti-cancer drugs. 12

I want to be very clear here: this evidence comes primarily from laboratory studies on cell cultures. We don’t have good human trials showing that DHA supplements prevent or treat cancer. It would be irresponsible to suggest taking DHA as a cancer treatment or prevention strategy based on current evidence. At best, eating more fish as part of a balanced diet is unlikely to cause harm and aligns with general dietary recommendations.

What about bronchopulmonary dysplasia in preterm infants?

You might come across claims that DHA helps prevent bronchopulmonary dysplasia (BPD), a chronic lung condition in premature babies. A 2020 systematic review of 5 randomised controlled trials involving 1,966 preterm infants found that DHA supplementation did not reduce BPD risk. 13

It also didn’t significantly reduce other neonatal complications including death, necrotising enterocolitis, ventricular haemorrhage, severe retinopathy, or sepsis. So despite theoretical reasons to think DHA might help, the evidence doesn’t support routine supplementation for this purpose.

Side Effects of DHA

DHA is generally well tolerated. The most common side effects are gastrointestinal:

Loose stools
Stomach upset
Fishy burps or aftertaste
Nausea
Hiccups

Taking supplements with meals usually reduces these issues.

One consideration: DHA products often contain EPA as well, and EPA has blood-thinning properties. At high doses or with long-term use, this could theoretically increase bleeding risk, though this isn’t commonly seen at standard supplementation levels.

According to the Mayo Clinic, fish oil supplements containing DHA are generally considered safe at doses up to 3 grams daily, though higher doses should be discussed with a healthcare provider. Mayo Clinic

3 Contraindications and Safety Precautions

1. Blood pressure and blood sugar medications

DHA may have mild blood pressure and blood sugar lowering effects. If you’re taking medications for hypertension or diabetes, be aware of the potential for additive effects that could cause hypotension or hypoglycaemia.

2. Bleeding disorders and anticoagulant medications

People with coagulation disorders, those taking anticoagulant medications (like warfarin), and those planning surgery (within the next 2-3 weeks) should exercise caution. The blood-thinning effect of omega-3 fatty acids could increase bleeding risk.

3. Pregnancy, breastfeeding, and children

While dietary DHA from food is considered safe and beneficial during pregnancy and breastfeeding, supplements are a different matter. Pregnant women, nursing mothers, and children should consult a healthcare provider before starting any supplement regimen. This isn’t because DHA is dangerous, but because dosing and quality control matter, and professional guidance ensures appropriate use.

Foods Rich in DHA

If you’d prefer to get your DHA from food rather than supplements, fatty fish is your best bet:

Salmon: approximately 1-1.5g DHA per 100g serving
Mackerel: high DHA content, though watch mercury levels
Sardines: good DHA source and lower on the food chain, so less mercury
Herring: another excellent option
Trout: provides meaningful amounts

The NHS recommends eating at least two portions of fish per week, with one being oily fish. NHS Guidelines

For vegetarians and vegans, algae-based DHA supplements are available. Since marine animals get their DHA from eating microalgae in the first place, going straight to the source makes sense. Krill oil and cod liver oil are other supplement options, though krill oil tends to be more expensive.

The Bottom Line

DHA is genuinely important for brain and eye health. The evolutionary evidence for its role in human brain development is compelling, and the observational studies linking higher DHA intake to better cognitive outcomes in older adults are encouraging.

That said, I wouldn’t oversell supplementation. The randomised trial evidence is mixed. DHA supplements probably won’t hurt you, but they also might not provide dramatic benefits for most people eating a reasonable diet. The strongest case for supplementation is for people who don’t eat fish regularly and are concerned about cognitive health as they age.

If you’re considering DHA supplements, start with improving your diet first. Eating fatty fish twice a week is the approach supported by most health authorities. If you can’t or won’t eat fish, algae-based supplements are a reasonable alternative.

As always, discuss supplements with your healthcare provider, especially if you have underlying health conditions or take medications.