| IUPAC name
(4Z,7Z,10Z,13Z,16Z,19Z)-docosa-4,7,10,13,16,19-hexaenoic acid; Doconexent
| Other names
cervonic acid, DHA
|3D model (Jmol)||Interactive image|
|Molar mass||328.488 g/mol|
|Melting point||−44 °C (−47 °F; 229 K)|
|Boiling point||446.7 °C (836.1 °F; 719.8 K)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|(what is ?)|
Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is a primary structural component of the human brain, cerebral cortex, skin, sperm, testicles 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 α-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 α-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 sperm, 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.
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.
In humans, DHA is either obtained from the diet or may be converted in small amounts from eicosapentaenoic acid (EPA, 20:5, ω-3) via docosapentaenoic acid (DPA, 22:5 ω-3) as an intermediate. This synthesis had been thought to occur through an elongation step followed by the action of Δ4-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 ω-3, then desaturated to 24:6 ω-3, then shortened to DHA (22:6 ω-3) via beta oxidation. This pathway is known as Sprecher's shunt.
How it is metabolised
Potential health effects
Alzheimer's disease and decline of mental health
Preclinical studies indicate that DHA improves memory, can slow the progression of Alzheimer's disease neuropathology in mice, sparking interest in additional research. However, the first large-scale human trials showed that DHA did not slow decline of mental function in elderly people with mild to moderate Alzheimer's disease. These trials were part of a large US National Institutes of Health (NIH) intervention study to evaluate DHA in Alzheimer's disease.
Researchers from the National Institute on Aging supported Alzheimer's Disease Cooperative Study and conducted a clinical trial comparing DHA and placebo over 18 months in 402 people (average age=76) diagnosed with mild to moderate Alzheimer's disease. Treatment with DHA increased blood levels of DHA, and appeared to increase brain DHA levels, based on a measured increase of DHA in study participants' cerebrospinal fluid.
However, DHA treatment did not slow the rate of change on tests of mental function, global dementia severity status, activities of daily living, or behavioral symptoms in the study population as a whole. Treatment effects did not differ between the mild and moderate Alzheimer's patients, leading study authors to conclude that the results do not support the routine use of DHA for patients with Alzheimer's.
Animal studies in the various transgenic mouse models of Alzheimer's disease had linked dietary DHA to decreases in amyloid plaques and tau. Animal studies also showed, when DHA was combined with arachidonic acid (AA or ARA, also present in fish oil), plaque formation was greater with the arachidonic acid compared to DHA alone.
DHA deficiency is under study for its potential role in decline of mental function in healthy adults, indicated in a study from 2010 conducted at 19 U.S. clinical sites on 485 subjects aged 55 and older who met criteria for age-associated memory impairment. The study found algal DHA taken for six months decreased heart rate and improved memory and learning in healthy, older adults with mild memory complaints. These findings indicate the importance of early DHA intervention and provided a statistically significant benefit to cognitive function in individuals over 50 years of age.
Higher DHA levels in middle-aged adults is related to better performance on tests of nonverbal reasoning and mental flexibility, working memory, and vocabulary. DHA is the majority component of a Souvenaid, an oral supplement developed by MIT scientists that has been shown to modestly improve memory in subjects with early Alzheimer’s disease while showing insignificant benefits in other tests of cognitive function.
The use of DHA by persons with epilepsy could decrease the frequency of their seizures. Studies have shown that children with epilepsy had a major improvement, i.e. decrease in the frequency of their seizures, but another study showed mixed results with 57 adults taking DHA supplementation. The 57 subjects demonstrated a decreased frequency of seizures for the first six weeks of the study, but for some it was just a temporary improvement.
In mice, DHA was found to inhibit growth of human colon carcinoma cells, more than other omega-3 PUFAs. The cytotoxic effect of DHA was not caused by increased lipid peroxidation or any other oxidative damage, but rather a decrease in cell growth regulators. However, different cancer lines may handle PUFAs differently and display different sensitivities toward them.
DHA was shown to increase the efficacy of chemotherapy in prostate cancer cells in vitro, and a chemoprotective effect in a mouse model was reported. By contrast, one case-control study nested within a clinical trial originally designed to test the effect of finasteride on prostate cancer occurrence, the "Prostate Cancer Prevention Trial", found that DHA measured in blood serum was associated with an increase in high-grade prostate cancer risk. In addition to DHA's possible anticancer effect, it may also be used as a non-toxic adjuvant to increase the efficacy of chemotherapy.
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. DHA has recently gained attention as a supplement for pregnant women, noting studies of improved attention and visual acuity. Given the recently gained attention, the majority of pregnant women in the U.S. fail to get the recommended amount of DHA in their diets. 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.
One study suggests that women who take DHA supplements during pregnancy give their babies some degree of added protection against getting common colds.
DHA concentrations in breast milk range from 0.07% to greater than 1.0% of total fatty acids, with a mean of about 0.34%.
Although most studies demonstrate positive effects of dietary DHA on human health, some studies show little or no effect. For example, 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.
Additional studies confirmed DHA benefits for other nervous system functions, cardiovascular health, and potentially other organs. In one study, men who took DHA supplements for 6–12 weeks decreased the concentrations of several inflammatory markers in their blood by approximately 20%. Higher intakes of DHA and EPA may reduce the risk of cardiovascular diseases. A new study found that higher intake of DHA was associated with slower rates of telomere shortening, which is a basic DNA-level marker of aging. Preliminary studies showed that a high intake of DHA was associated with reduced risk for developing Alzheimer's disease and Parkinson's disease consistent with DHA being the most abundant omega-3 fatty acid in the brain. Neuroprotective effects of dietary DHA have been described in animals models of Parkinson's disease. It is now considered so important to brain and eye development that DHA is included in most infant formulas. Lastly, in preliminary research, it was found that a diet rich in DHA might protect stroke victims from brain damage and disability and aid in a speedier recovery. Accordingly, dietary administration of DHA reduces stroke-induced neuroinflammation in animal models.
According to a new study, DHA is very likely important in the formation of the acrosome, an arc-like structure on the top of sperm, which is critical in fertilization because it houses a variety of enzymes that sperm use to penetrate an egg. Because humans and other mammals are able to make their own DHA from other fatty acids, DHA deficiency is not common. But, if that DHA-synthesizing enzyme is defective, it could lead to problems with infertility.
Researchers in Norway are testing a treatment for psoriasis with a synthesised molecule based on DHA.
Ordinary types of cooked salmon contain 500–1500 mg DHA and 300–1000 mg EPA per 100 grams. Additional top fish sources of DHA include tuna, bluefish, mackerel, swordfish, anchovies, herring, sardines, and caviar. However, greater fish consumption's benefits in providing the recommended dosage of DHA are negated by the brain damage caused by toxic pollutants such as mercury.
Discovery of algae-based DHA
In the early 1980s, NASA sponsored scientific research in search of a plant-based food source that could generate oxygen and nutrition on long-duration space flights. The researchers discovered that certain species of marine algae produced rich nutrients. This research led to the development of an algae-based, vegetable-like oil that contains two essential polyunsaturated fatty acids, DHA and ARA (arachidonic acid), which can now be found in 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, and by 2007 DHA-fortified dairy items (milk, yogurt, cooking oil) started to appear in grocery stores.
DHA is believed to be helpful to people with a history of heart disease, for premature infants, and to support healthy brain development especially in young children along with supporting retinal development. 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. A supplemental DHA, available in algae-derived oils or capsules, has been shown to increase DHA levels. While there is little evidence of adverse health or cognitive effects due to DHA deficiency in adult vegetarians or vegans, fetal and breast milk levels remain a concern.
DHA and EPA in fish oils
Fish oil is widely sold in gelatin capsules containing a mixture of omega-3 fatty acids including EPA and smaller quantities of DHA. One study found that fish oil higher in EPA than DHA lowered inflammatory cytokines, such as IL-6 and IL-1β, associated with neurodegenerative and autoimmune diseases. They note the brain normally contains DHA, but not EPA, though both DHA and EPA plasma concentrations increased significantly for participants.
Oxidation levels of available fish oils
A study published in January 2015 revealed that "Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA". In another study "Oxidation of Marine Omega-3 Supplements and Human Health", the authors note: "It is currently impossible to determine how oxidation affects the efficacy or potential harms of marine oil. This makes interpretation of the clinical trial literature problematic. If the oxidative state of marine oils may affect efficacy or harm, then physicians should recommend, and consumers select, a supplement with the same oxidative state as the oils used in clinical trials that have shown benefit and safety."
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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