Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is a primary structural component of the human brain, cerebral cortex, skin, and retina. It can be synthesized from alpha-linolenic acid or obtained directly from maternal milk (breast milk), fish oil, or algae oil.
DHA's structure is a carboxylic acid (-oic acid) with a 22-carbon chain (docosa- is Greek for 22) and six (hexa-) cis double bonds (-en-); with the first double bond located at the third carbon from the omega end. Its trivial name is cervonic acid, its systematic name is all-cis-docosa-4,7,10,13,16,19-hexa-enoic acid, and its shorthand name is 22:6(n-3) in the nomenclature of fatty acids.
Most of the DHA in fish and multi-cellular organisms with access to cold-water oceanic foods originates from photosynthetic and heterotrophic microalgae, and becomes increasingly concentrated in organisms the further they are up the food chain. DHA is also commercially manufactured from microalgae: Crypthecodinium cohnii and another of the genus Schizochytrium. DHA manufactured using microalgae is vegetarian.
Some animals with access to seafood make little DHA through metabolism, but obtain it in the diet. However, in strict herbivores, and carnivores that do not eat seafood, DHA is manufactured internally from ?-linolenic acid, a shorter omega-3 fatty acid manufactured by plants (and also occurring in animal products as obtained from plants). Limited amounts of eicosapentaenoic and docosapentaenoic acids are possible products of ?-linolenic acid metabolism in young women and men, and though DHA is difficult to detect above dietary background in males compared with females, this illustrates the importance of DHA production for the developing fetus and healthy breast milk. Rates of conversion are 15% higher for women, with those taking oral contraceptives demonstrating 10% higher DHA levels.
DHA is a major fatty acid in brain phospholipids and the retina. While the potential roles of DHA in the mechanisms of Alzheimer's disease are under active research, studies of fish oil supplements, which contain DHA, have failed to support claims of preventing cardiovascular diseases.
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Central nervous system constituent
DHA is the most abundant omega-3 fatty acid in the brain and retina. DHA comprises 40% of the polyunsaturated fatty acids (PUFAs) in the brain and 60% of the PUFAs in the retina. Fifty percent of the weight of a neuron's plasma membrane is composed of DHA. DHA is richly supplied during breastfeeding, and DHA levels are high in breastmilk regardless of dietary choices.
DHA modulates the carrier-mediated transport of choline, glycine, and taurine, the function of delayed rectifier potassium channels, and the response of rhodopsin contained in the synaptic vesicles, among many other functions.
DHA deficiency is associated with cognitive decline. Phosphatidylserine (PS) controls apoptosis, and low DHA levels lower neural cell PS and increase neural cell death. DHA levels are reduced in the brain tissue of severely depressed patients.
Metabolic synthesis
In humans, DHA is either obtained from the diet or may be converted in small amounts from eicosapentaenoic acid (EPA, 20:5, ?-3) via docosapentaenoic acid (DPA, 22:5 ?-3) as an intermediate. This synthesis had been thought to occur through an elongation step followed by the action of ?4-desaturase. It is now considered more likely that DHA is biosynthesized via a C24 intermediate followed by beta oxidation in peroxisomes. Thus, EPA is twice elongated, yielding 24:5 ?-3, then desaturated to 24:6 ?-3, then shortened to DHA (22:6 ?-3) via beta oxidation. This pathway is known as Sprecher's shunt.
Metabolism
DHA can be metabolized into DHA-derived specialized pro-resolving mediators (SPMs), DHA epoxides, electrophilic oxo-derivatives (EFOX) of DHA, neuroprostanes, ethanolamines, acylglycerols, docosahexaenoyl amides of amino acids or neurotransmitters, and branched DHA esters of hydroxy fatty acids, among others.
The enzyme CYP2C9 metabolizes DHA to epoxydocosapentaenoic acids (EDPs; primarily 19,20-epoxy-eicosapentaenoic acid isomers [i.e. 10,11-EDPs]).
Potential health effects
Neurological research
Large-scale human trials showed that DHA did not slow decline of mental function in elderly people with mild to moderate Alzheimer's disease.
In one preliminary study, algal DHA taken for six months decreased heart rate and improved memory and learning in healthy, older adults with mild memory complaints. In another early-stage study, higher DHA levels in middle-aged adults was related to better performance on tests of nonverbal reasoning and mental flexibility, working memory, and vocabulary.
One study found that the use of DHA-rich fish oil capsules did not reduce postpartum depression in mothers or improve cognitive and language development in their offspring during early childhood. Another systematic review found that DHA had no significant benefits in improving visual field in individuals with retinitis pigmentosa. A 2017 pilot study found that fish oil supplementation reduced the depression symptoms emphasizing the importance of the target DHA levels.
Pregnancy and lactation
It has been recommended to eat foods which are high in omega-3 fatty acids for women who want to become pregnant or when nursing. A working group from the International Society for the Study of Fatty Acids and Lipids recommended 300 mg/day of DHA for pregnant and lactating women, whereas the average consumption was between 45 mg and 115 mg per day of the women in the study, similar to a Canadian study. Despite these recommendations, recent evidence from a trial of pregnant women randomized to receive supplementation with 800 mg/day of DHA versus placebo, showed that the supplement had no impact on the cognitive abilities of their children at up to seven years follow-up.
Other research
In one preliminary study, men who took DHA supplements for 6-12 weeks had lower blood markers of inflammation.
Nutrition
Ordinary types of cooked salmon contain 500-1500 mg DHA and 300-1000 mg EPA per 100 grams. Additional rich seafood sources of DHA include caviar (3400 mg per 100 grams), anchovies (1292 mg per 100 grams), mackerel (1195 mg per 100 grams), and cooked herring (1105 mg per 100 grams).
Discovery of algae-based DHA
In the early 1980s, NASA sponsored scientific research on a plant-based food source that could generate oxygen and nutrition on long-duration space flights. Certain species of marine algae produced rich nutrients, leading to the development of an algae-based, vegetable-like oil that contains two polyunsaturated fatty acids, DHA and arachidonic acid, present in some health supplements.
Use as a food additive
DHA is widely used as a food supplement. It was first used primarily in infant formulas. In 2004, the US Food and Drug Administration endorsed qualified health claims for DHA.
Some manufactured DHA is a vegetarian product extracted from algae, and it competes on the market with fish oil that contains DHA and other omega-3s such as EPA. Both fish oil and DHA are odorless and tasteless after processing as a food additive.
Studies of vegetarians and vegans
Vegetarian diets typically contain limited amounts of DHA, and vegan diets typically contain no DHA. In preliminary research, algae-based supplements increased DHA levels. While there is little evidence of adverse health or cognitive effects due to DHA deficiency in adult vegetarians or vegans, breast milk levels remain a concern for supplying adequate DHA to the developing fetus.
DHA and EPA in fish oils
Fish oil is widely sold in capsules containing a mixture of omega-3 fatty acids, including EPA and DHA. Oxidized fish oil in supplement capsules may contain lower levels of EPA and DHA.
Hypothesized role in human evolution
An abundance of DHA in seafood has been suggested as being helpful in the development of a large brain, though other researchers claim a terrestrial diet could also have provided the necessary DHA.
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