Docosahexaenoic Acid (DHA)

DHA (docosahexaenoic acid, 22:6 n-3) is a long-chain omega-3 fat with 22 carbons and six double bonds. The NIH (US National Institutes of Health) says it is especially high in the retina and brain.

It is not one of the two essential fatty acids, which are ALA and linoleic acid. But the body turns ALA into DHA only slowly. In one tracer study young women converted 9% of an ALA dose into DHA, and in six young men no DHA was detected. That is why the NIH calls eating or supplementing DHA directly the only practical way to raise it.

Fish do not make DHA. Microalgae at the base of the marine food chain do, and fish collect it as they feed.

The brain cannot make DHA either. It imports it across the blood-brain barrier with a carrier protein called Mfsd2a. People from two families with broken copies of that gene had a lethal form of microcephaly, a very small head and brain.

The evidence is narrower than the reputation. The EU allows claims for brain function and vision at 250 mg of DHA a day. A Cochrane review, a careful summary of the trials, found fewer preterm births with omega-3 supplements in pregnancy.

But DHA did not protect older adults’ thinking in several large trials. American adults eat only about 63 mg of DHA a day from food. EFSA, the EU food safety agency, advises 250 mg for EPA and DHA together.

Health benefits of DHA

• ★★★Fewer preterm births, but mixed resultsOmega-3 supplements or foods in pregnancy (a mix, not DHA alone) cut births before 37 weeks from 13.4% to 11.9% across 26 trials. A large Australian trial of 900 mg a day found no cut in early preterm birth.
• ★★★No mental gain from DHA in infant formulaIn a review of 15 trials, formula with added DHA and arachidonic acid did not change mental scores at 18 months (4 trials, 661 infants). Eyesight results were mixed, and the authors could not recommend routine supplementation of formula for term infants.
• ★★★Little effect on deaths or heart eventsIn 86 trials, extra omega-3 (mostly EPA plus DHA capsules) made little or no difference to deaths (0.97 times as likely) or heart events (0.96). Triglycerides fell by about 15%.
• ★★★No change in children’s ADHD symptomsIn 16 trials of 1,166 children with ADHD, omega-3 and other fat supplements did not change parent-rated symptoms (not DHA alone). Side effects did not differ from placebo.
• ★★No protection against mental decline in older adultsIn 38 trials of 49,757 people, long-chain omega-3 (EPA and DHA) did not change new cognitive impairment (0.99 times the risk). In Alzheimer disease, 2 g of algal DHA a day for 18 months did not slow decline.

Foods high in DHA

DHA comes from the sea. Microalgae make it and fish collect it, so the richest foods are oily fish. In our USDA ranking per serving, a mackerel fillet leads with 1.57 g. Wild Atlantic salmon, farmed Atlantic salmon, herring and a can of sardines follow.

Per 100 g the order is the same, and per 100 kcal wild salmon, mackerel and herring lead. Leaner seafood has much less: oysters 0.21 g, tuna 0.16 g and cod 0.1 g in 3 oz, and one large egg has 0.03 g. The ranking below shows every amount.

Wild or farmed matters little for salmon in this ranking: the two are nearly identical at 1.12 g and 1.1 g per 100 g. The NIH says farmed fish usually have more EPA and DHA than wild-caught fish, but it depends on the feed.

Scottish farmed salmon lost EPA and DHA between 2006 and 2015, as marine ingredients in feed were replaced. Plants hold no DHA: flaxseed, chia and walnuts contain ALA, which the body converts poorly. Some eggs, yogurts, juices, milks and soy drinks are fortified with DHA, so check the label.

Our arithmetic for realistic servings: EFSA’s adequate intake for adults is 250 mg a day of EPA plus DHA, which is 1.75 g a week. Two 3 oz servings of salmon hold about 1.9 g of DHA alone, so they pass it. A can of sardines gives 0.47 g.

American adults eat much less. In survey data from 2003 to 2008, mean DHA intake from food was 63 mg a day, and 72 mg a day with supplements.

How DHA works

Most of the biochemistry below is well established. But the details of transport and signaling come from cell and animal studies. What extra DHA does in people is tested in the study cards above.

Four ideas cover most of it. DHA is built into membranes, and it needs a carrier to reach the brain and eye. The body makes little of it from plant omega-3, and it becomes signaling molecules. Here is each one, and what has been seen in people.

It is built into the membranes of the brain and retina

DHA is a structural fat. Once it sits in membrane phospholipids, it changes how the membrane behaves. A review of membrane studies lists effects on chain order and fluidity, permeability, fusion and protein activity.

In the fetal brain, DHA builds up mainly in the last three months of pregnancy, a narrative review finds. It keeps accumulating at a high rate until the end of the second year of life, the same review finds. FAO and WHO, the UN food and health agencies, estimate that an average of 14 mg of DHA a day passes from mother to fetus. That runs across 40 weeks, with most of it in the last 12 weeks.

Mothers of exclusively breastfed infants pass on 110 mg a day in milk, FAO and WHO add. Across 65 studies of 2,474 women, mean breast milk DHA was 0.32% of fatty acids, with a range of 0.06% to 1.4%. It was highest in coastal populations that eat marine foods.

It needs a carrier to reach the brain and eye

In mice, Nguyen and colleagues found the major route for DHA into the brain. It is Mfsd2a, a protein in the cells lining brain blood vessels. It carries DHA bound to a lipid called lysophosphatidylcholine, not as a free fatty acid. Mice without it had much less brain DHA, neuron loss in the hippocampus and cerebellum, cognitive deficits and small brains.

The same carrier works in the retinal pigment layer. Mice without it had shorter rod outer segments and disorganized discs, though their visual function on electroretinography (an electrical eye test) was normal. In people, two families with inactivating MFSD2A mutations had a lethal microcephaly syndrome.

The body makes little of it from plant omega-3

The body can lengthen ALA into EPA, DPA and then DHA, mostly in the liver. The NIH puts reported conversion rates below 15%. Young women who took 700 mg of labeled ALA converted an estimated 21% to EPA, 6% to DPA and 9% to DHA.

In six young men, labeled EPA and DPA appeared, but no labeled DHA was apparent at any point up to 21 days. The authors suggest women may convert more than men, and that DHA from the diet may be critical for men.

FAO and WHO add that breast milk DHA cannot be raised by adding ALA to the diet. The milk of strict vegans holds among the lowest DHA amounts reported, they say.

It becomes signaling molecules

Cells also turn DHA into signaling lipids. In mice treated with aspirin and DHA, Serhan and colleagues found a family of DHA products they named resolvins. At nanogram doses they reduced white blood cell build-up in inflamed tissue by 40% to 80%. In human microglial cells (brain immune cells) they cut cytokine production, with a half-maximal effect near 50 pM.

The NIH adds that EPA and DHA compete with arachidonic acid for the enzymes that make eicosanoids, which tips the balance toward less inflammatory signals. These are lab results, so the human cards above are the test of whether they matter for health.

It is absorbed like other fats

The NIH says absorption is efficient, about 95%, like other ingested fats. In the gut, DHA is released from its carrier, taken up with bile-salt micelles and packed into chylomicrons (fat-carrying particles). These go into the lymph and then circulate in lipoprotein particles. The form you take changes how much reaches the blood, which is covered below.

What has been seen in people

Trial doses of DHA ran from 600 to 2,000 mg a day. That is about 2.4 to 8 times the 250 mg that EFSA sets for EPA plus DHA together (our arithmetic). It is also far above the 63 mg that American adults average from food. The chart puts them on one scale.

The clearest findings are in pregnancy and in blood fats, not in the brain of older adults. In pregnancy, 1,000 mg gave fewer early preterm births than 200 mg, mostly in women with low DHA at the start. But 800 mg did not lower postpartum depression.

In older adults, 900 mg improved one memory test in healthy people with mild memory decline. But 1,720 mg and 2 g did not slow decline in healthy older adults or in people with Alzheimer disease. DHA lowers triglycerides and raises LDL cholesterol. All of these are among the study cards above.

How much DHA do you need?

According to EFSA, adults should get about 250 mg a day of DHA plus EPA. In pregnancy and breastfeeding, it advises 100 to 200 mg more DHA. The US has set no Daily Value for DHA itself.

Source	Group	Value	Notes
EFSA 2010	Adults	250 mg a day of EPA plus DHA	Adequate intake (AI) for the two together. EFSA 2012 notes advice for European adults of 250 to 500 mg a day.
EFSA 2010	Pregnancy and breastfeeding	Add 100 to 200 mg a day of preformed DHA	On top of the adult AI. “Preformed” means DHA itself, not DHA made from ALA.
EFSA 2010	Infants over 6 months and children under 24 months	100 mg a day of DHA	Adequate intake.
FAO/WHO 2010	Men and women who are not pregnant or breastfeeding	250 mg a day of EPA plus DHA	Not enough evidence to set a minimum for either alone; both should be eaten.
FAO/WHO 2010	Pregnant and breastfeeding women	At least 300 mg a day of EPA plus DHA, of which at least 200 mg DHA	Minimum intake for adult health and for fetal and infant development.
FAO/WHO 2010	Infants 0 to 6 months	0.1% to 0.18% of energy as DHA	The same as 0.2% to 0.36% of fatty acids, based on human milk.
FAO/WHO 2010	Infants and children 6 to 24 months	10 to 12 mg of DHA per kg of body weight a day	Adequate intake, at a “probable” level of evidence.
National Academies 2005	All ages	No value set for DHA	For age 1 and older the adequate intakes are for ALA. About 10% of the range of 0.6% to 1.2% of energy can be eaten as EPA and DHA, which is 0.06% to 0.12% of energy, about 130 to 270 mg a day on 2,000 kcal.

For babies, breast milk is the benchmark. Mean DHA in breast milk worldwide is 0.32% of fatty acids, with a range from 0.06% to 1.4%. FAO and WHO set their infant values from it. In the EU, infant formula and follow-on formula must contain 20 to 50 mg of DHA per 100 kcal.

In pregnancy the fetus takes DHA from the mother, an average of 14 mg a day over 40 weeks, mostly in the last 12 weeks. Breast milk takes about 110 mg a day. From these figures FAO and WHO reached an average requirement of 200 mg of DHA a day in pregnancy.

One trial that gave 1,000 mg or 200 mg found fewer early preterm births at the higher dose in women with low DHA status. It is among the cards above.

Intakes are well below the advice. In US survey data from 2003 to 2008, mean DHA from foods was 63 mg a day and 72 mg a day with supplements. The NIH quotes about 90 mg a day of EPA and DHA from food in adults in 2011 to 2012. By comparison, one serving of salmon holds nearly 1 g of DHA.

DHA deficiency: signs and who is at risk

The NIH says a deficiency of essential fatty acids can cause rough, scaly skin and dermatitis. Plasma and tissue DHA fall when omega-3 is lacking, it adds. It also says no cut-off for DHA or EPA is known below which vision, nerve or immune function is impaired.

Classic essential fatty acid deficiency is almost never seen in healthy people in the US. It appeared in patients on feeding solutions that lacked polyunsaturated fats. One 6-year-old girl on intravenous feeding low in linolenic acid (ALA) had numbness, weakness and blurred vision. These disappeared when her feeding emulsion was changed; it was an ALA deficiency, not a measured DHA deficiency.

Preterm infants are the clearest group at risk. DHA is laid down mostly in the last three months of pregnancy, so infants born before 29 weeks miss the normal supply. FAO and WHO list DHA as conditionally essential for infants, because the body makes too little from ALA.

In monkeys, a diet low in omega-3 fats from before birth cut blood DHA to 6% of control levels. It halved visual acuity by 12 weeks.

Vegans and vegetarians eat almost no preformed DHA. But in one sample of 165 vegans, the omega-3 index (EPA plus DHA in red blood cells) averaged 3.7%. That was similar to a group of omnivores who also eat very little.

In 46 vegans with an index below 4%, an algal supplement gave 254 mg of EPA plus DHA a day. That raised the index from 3.1% to 4.8% in 4 months.

Forms of DHA: absorption and effects

Forms include natural fish-oil triglycerides, ethyl esters, re-esterified triglycerides, free fatty acids, krill-oil phospholipids, algal oil, cod liver oil, whole fish and fortified foods.

The honest bottom line is that every form raised blood levels. Ethyl esters were absorbed somewhat less well than triglyceride forms in two trials. We found no trial that compared forms for a heart or brain outcome. Most comparisons use EPA plus DHA mixes and measure blood or red cell levels, not benefits.

We found no trial that compared the same triglyceride DHA oil taken with and without a fatty meal. The authors of one trial of free fatty acid and ethyl ester products say the free fatty acid form depends on gut lipases. They say it is absorbed best with a high-fat meal.

The trials of key outcomes above used DHA-rich fish oil (pregnancy and older adults) and algal DHA (Alzheimer disease and schoolchildren). Infant trials used formula with DHA and arachidonic acid, and very preterm infants got an emulsion fed into the gut.

Form	What it is	What studies show	Source
Natural fish oil triglycerides	Oil as it occurs in fish	The reference in a 72-person trial. Re-esterified triglycerides gave 124% of its EPA plus DHA uptake, ethyl esters 73% and free fatty acids 91% (not significantly different).	1, 3
Ethyl esters	Made from the natural triglycerides by swapping the glycerol for ethanol; the form of some prescription EPA plus DHA products	Lower uptake than natural oil (73%) in one trial, and a smaller rise in the omega-3 index than re-esterified triglycerides over 6 months (171% versus 197%). Under a low-fat diet, a branded free fatty acid product gave about 3 times the EPA plus DHA exposure of a branded ethyl ester product in 52 volunteers; the abstract does not state who funded it.	1, 2, 3, 4
Re-esterified triglycerides	Ethyl esters converted back to triglycerides	124% of natural fish oil in one trial, and the omega-3 index rose to 197% against 171% with ethyl esters in 150 volunteers.	1, 2, 3
Free fatty acids	Fatty acids without glycerol or ethanol attached	91% of natural fish oil, not significantly different. About 3 times the exposure of an ethyl ester product on a low-fat diet in one brand comparison.	1, 3, 4
Phospholipids (krill oil)	Omega-3 fats bound mainly in phospholipids	The omega-3 index rose more after krill oil than fish oil in 24 volunteers (two authors work for the krill oil maker). A single-dose study found similar absorption from krill oil and a phospholipid-enhanced fish oil, and a pilot trial in 47 people with high triglycerides found no significant difference. The NIH says some studies suggest higher bioavailability and others do not, so “krill oil is better absorbed” is unproven.	3, 5, 6, 7
Algal oil	DHA from cultured microalgae, usually as triglycerides; some products also hold EPA	In 32 adults, 600 mg of DHA a day from algal-oil capsules matched cooked salmon for DHA in blood. In vegetarians, 0.94 g a day lowered triglycerides by 23% and raised LDL cholesterol. The NIH says algal capsules usually hold about 100 to 300 mg of DHA.	3, 8, 9
Cod liver oil	Oil from cod liver	Also supplies vitamin A and vitamin D, according to the NIH. Dyerberg’s trial included a single-blind cod liver oil arm; the abstract gives no separate result.	1, 3
Icosapent ethyl (prescription)	EPA only, as an ethyl ester	Holds no DHA, so its heart trial (REDUCE-IT) says nothing about DHA. See our EPA page.	3
DHA in infant formula	DHA added to formula, usually with arachidonic acid	EU rules require 20 to 50 mg of DHA per 100 kcal. In a trial of 343 infants, formula with 0.32% of fatty acids as DHA (plus arachidonic acid) gave better visual acuity at 12 months than DHA-free formula, but a Cochrane review found no consistent neurodevelopment benefit.	10, 11, 12
Plant omega-3 (ALA)	ALA from flaxseed, chia, walnuts and some oils	Not DHA. Young women converted an estimated 9% of ALA to DHA, and none was detected in young men.	3, 13, 14

Here are the form-specific and head-to-head studies, strongest evidence first, 5 in all.

Is DHA safe? Supplements, dose and interactions

DHA from food is not a safety concern for healthy people, and no official body has set an upper limit. The US National Academies set none in 2005. In 2012 EFSA found the data insufficient to set one for EPA, DHA or DPA.

EFSA still gave guidance. Supplemental DHA alone up to about 1 g a day raises no safety concern for the general population. Up to about 5 g a day of EPA plus DHA does not appear to raise bleeding risk. Nor does it seem to affect glucose control, immune function or lipid oxidation, as long as the oils are not oxidized.

The NIH says the FDA judged supplements with up to 5 g a day of EPA plus DHA safe when used as recommended. The FAO/WHO report notes an FDA level of 3,000 mg a day set in 1997. The EU triglyceride and blood pressure claims require supplements to warn against more than 5 g a day of the two combined.

• Cholesterol and heart rhythm. EFSA found that 2 to 4 g a day of DHA raises LDL cholesterol by about 3%. That may not have an adverse effect on cardiovascular risk. In 11 trials of algal DHA, LDL rose 0.23 mmol/L while triglycerides fell 0.20 mmol/L. A meta-analysis of 7 large trials found more atrial fibrillation with marine omega-3 supplements (1.25 times the risk), mostly above 1 g a day. The NIH says two trials at 4 g a day showed a slight rise. People with heart rhythm problems or heart disease should ask a doctor before taking high doses.
• Side effects and blood thinners. Side effects are usually mild: unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhea, headache and odorous sweat, per the NIH. With 4 g a day of EPA plus DHA in the STRENGTH trial, 24.7% had stomach or bowel side effects against 14.7% on corn oil. The National Academies noted that 2 to 15 g a day of EPA and DHA might lengthen bleeding time. Fish oil might also raise the INR, a clotting test, in people on warfarin. Most research finds 3 to 6 g a day of fish oil does not change anticoagulant status. But drug labels advise periodic INR checks, so tell your doctor if you take a blood thinner.
• Fish contaminants and cod liver oil. The NIH says omega-3 supplements have not been found to contain methylmercury, because it is removed during processing. Fish are another matter. The FDA and the US Environmental Protection Agency give advice for people who are pregnant or breastfeeding. They suggest 2 to 3 servings of 4 oz a week from the “Best Choices” list. That list includes salmon, sardines, herring, Atlantic mackerel, trout and cod. They advise avoiding shark, swordfish, king mackerel, marlin, orange roughy, bigeye tuna and tilefish from the Gulf of Mexico. In a UK cohort, children of women who ate more than 340 g of seafood a week did better than those whose mothers ate none. Cod liver oil supplements also supply vitamin A and vitamin D.
• Pregnancy, babies and children. Talk to a midwife or doctor before taking DHA in pregnancy. In the largest trial, 900 mg of omega-3 did not lower early preterm birth, and more babies were very large for their gestational age. The Cochrane review found more pregnancies lasting past 42 weeks. In babies born before 29 weeks (N3RO trial), 60 mg/kg of DHA a day raised lung disease, 49.1% against 43.9%. A pooled analysis of that trial and one other later found no clear difference in severe lung disease (34.4% against 31.9%, relative risk 1.06). These were doses given in neonatal units under medical care.

How to get more DHA

• Eat oily fish. A 3 oz serving of wild or farmed Atlantic salmon holds about 0.95 g or 0.94 g of DHA in our ranking. A mackerel fillet gives 1.57 g and a can of sardines 0.47 g. If you are pregnant or breastfeeding, choose from the “Best Choices” list of the FDA and the US Environmental Protection Agency.
• Use algal oil if you eat no fish. Algal capsules usually hold about 100 to 300 mg of DHA. In a 2-week trial, 600 mg of DHA a day from algal oil matched cooked salmon for blood DHA. And 254 mg a day raised the omega-3 index of vegans from 3.1% to 4.8% in 4 months.
• Do not count on plant omega-3. Flaxseed, chia and walnuts hold ALA. In the tracer studies young women made 9% of it into DHA, while young men made none we could detect. Eat them for ALA, not for DHA.

History of DHA

The interest in marine omega-3 fats began with fish-eating peoples. In 1971 Bang, Dyerberg and Nielsen described the blood fats of Greenland west-coast Inuit in The Lancet (the paper’s title says “Eskimos”).

The NIH says epidemiological research from the 1970s found low rates of heart attack among Greenland Inuit and other fish-eating populations. That spurred the research on EPA and DHA.

The case for DHA in the eye and brain came from animals and a patient. In 1982 Holman and colleagues described a 6-year-old girl on intravenous feeding with numbness, weakness and blurred vision. These cleared when her feeding emulsion supplied linolenic acid.

In 1984 Neuringer and colleagues showed that infant rhesus monkeys fed little omega-3 fat had falling blood DHA and poor visual acuity.

DHA moved into the food supply through infant formula. The NIH says manufacturers have added DHA and arachidonic acid to most US infant formulas since 2002. In the EU, a 2016 regulation made DHA mandatory in infant and follow-on formula at 20 to 50 mg per 100 kcal. That happened even though a Cochrane review in 2017 could not recommend routine supplementation of term formula.

More recently, in 2014 Nguyen and colleagues found the Mfsd2a carrier that moves DHA into the brain. In 2015 a human disease from its mutations was reported.

Frequently asked questions about DHA

How much DHA should I take in pregnancy?

EFSA advises adding 100 to 200 mg of preformed DHA a day to the adult 250 mg of EPA plus DHA. FAO and WHO advise at least 200 mg of DHA a day (300 mg of EPA plus DHA). In a US trial, 1,000 mg of DHA gave 1.7% early preterm births against 2.4% on 200 mg, mostly in women starting low in DHA. A trial of 900 mg of omega-3 found no drop in early preterm birth; ask your midwife or doctor, and eat low-mercury fish.

What does DHA do for the brain and eyes?

DHA is a major brain and retina fat, and the EU allows claims for normal brain function and vision at 250 mg a day. In babies, formula with 0.32% DHA gave better visual acuity at 12 months in one trial. But a Cochrane review found no consistent gain in mental scores for term infants. In older adults, 38 trials of long-chain omega-3 found no protection against cognitive decline.

Is DHA better than EPA?

The trials do not show that one is better overall: in 56 men, 4 g of purified DHA lowered blood pressure and EPA did not. In a meta-analysis of depression trials, EPA-rich formulas helped and DHA-pure ones did not. EFSA found that 2 to 4 g of DHA a day raises LDL by about 3%. Up to 4 g of EPA a day has no clear effect. See our EPA page.

Can vegans get enough DHA?

Yes, from algal oil. The body turns little ALA into DHA: young women made 9% of a dose, and none was detected in young men. In vegans, 254 mg a day of algal EPA plus DHA raised the omega-3 index from 3.1% to 4.8% in 4 months. And 600 mg a day of algal DHA matched cooked salmon for blood DHA over 2 weeks.

Does DHA raise LDL cholesterol?

Yes, modestly. In 11 trials of algal DHA (median 1.68 g a day), LDL cholesterol rose 0.23 mmol/L and triglycerides fell 0.20 mmol/L. EFSA found that 2 to 4 g a day raises LDL by about 3%. The large heart trials we found used EPA and DHA together or EPA alone, not DHA alone.

Which foods are highest in DHA?

Oily fish: in our USDA ranking a mackerel fillet leads with 1.57 g, then wild and farmed salmon, herring and canned sardines. Per 100 g the order is the same. Plant foods have no DHA, only ALA.

Do babies need DHA in formula?

Breast milk contains DHA, at a mean of 0.32% of fatty acids worldwide. EU rules require 20 to 50 mg of DHA per 100 kcal in infant formula. The evidence for benefit is thin: a Cochrane review of 15 trials found a 0.06-point difference in mental scores at 18 months. In very preterm babies, high-dose DHA may have raised the risk of lung disease in one trial.

Does DHA improve memory or prevent dementia?

Not in most trials: in healthy older adults with mild memory decline, 900 mg a day improved one learning test over 24 weeks. But 2.3 g a day of fish oil for 18 months did not help, nor did 2 g of DHA daily slow Alzheimer disease. People with the highest blood DHA had about half the dementia risk in a cohort, but that cannot show cause.