Docosahexaenoic acid

Docosahexaenoic acid (commonly known as DHA; 22:6(ω-3), all-cis-docosa-4,7,10,13,16,19-hexaenoic acid; trivial name cervonic acid) is an omega-3 essential fatty acid. In chemical structure, DHA is a carboxylic acid with a 22-carbon chain and six cis double bonds; the first double bond is located at the third carbon from the omega end.

DHA is most often found in fish oil. Most of the DHA in fish and other more complex organisms originates in microalgae of the genus Schizochytrium, and concentrates in organisms as it moves up the food chain. DHA is also commercially manufactured from Crypthecodinium cohnii.^[1] Most animals make very little DHA through metabolism; however small amounts are manufactured internally through the consumption of α-linolenic acid, an omega-3 fatty acid found in chia, flax, and many other seeds and nuts.

DHA is metabolized to form the docosanoids—several families of potent hormones. DHA is a major fatty acid in sperm and brain phospholipids, and especially in the retina. Dietary DHA can reduce the level of blood triglycerides in humans, which may reduce the risk of heart disease. Low levels of DHA result in reduction of brain serotonin levels^[2] and have been associated with ADHD, Alzheimer's disease, and depression, among other diseases, and there is mounting evidence that DHA supplementation may be effective in combating such diseases (see external links at the end of this article).

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Central nervous system constituent

DHA is the most abundant essential fatty acid (polyunsaturated fatty acids, PUFAs) in the brain and retina. It comprises 40% of the PUFAs in the brain and 60% of the PUFAs in the retina. 50% of the weight of the neuronal membrane is composed of DHA.^[3]

Of all the fatty acids, DHA has the largest effect on brain PUFA composition.^[4] DHA is found in three phospholipids: phosphatidylethanolamine, ethanolamine plasmalogens, and phosphatidylserine (PS). It 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 phospholipid vesicles, among many other functions.^[5]

DHA deficiency is associated with cognitive decline.^[6] PS controls apoptosis, and low DHA levels lower neural cell PS and increase neural cell death.^[7] DHA is depleted in the cerebral cortex of severely depressed patients.^[8][9]

Metabolic synthesis

In the human body, DHA is either present in the diet or it is derived from eicosapentaenoic acid (EPA, 20:5, ω-3) via docosapentaenoic acid (DPA, 22:5 ω-3) as an intermediate. This is done with an elongation step followed by the action of Δ4-desaturase. Another pathway has also been described in peroxysomes and mitochondria. 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.^[10][11]

Role in health

Alzheimer's Disease

A large NIH drug trial is currently recruiting patients for evaluating DHA in Alzheimer's disease.^[12] This is the first human trial of DHA. Animal studies in TG3 transgenic mouse model of Alzheimer's disease have shown decrease in amyloid plaques and tau by DHA. Animal studies also show that when combined with arachidonic acid (another fatty acid present in fish oil) the effectiveness of DHA in preventing plaques was lesser than when used alone.

Cancer

Researchers at the University of Nevada conducted a study on the effects of DHA supplementation on the growth of human colon adeno carcinoma in mice. The animals received one of four specialized diets: low fat with corn oil (omega-6 linoleic acid), high fat with corn oil (omega-6 linoleic acid), high fat with fish oil (omega-3 EPA and DHA), and high fat with algae-derived DHA (omega-3 DHA). At completion, the mice receiving a diet supplemented with algae-derived DHA had tumors that were smaller than those of mice receiving diets supplemented with omega-3 from fish oil as well as those fed both the high and low fat corn oil (omega-6) diets. These results indicate that algae-derived DHA possesses greater tumor suppressing properties than do corn or fish oil.^[13]

Pregnancy and lactation

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%. DHA levels in breast milk are higher if a mother's diet is high in fish.

DHA has recently gained attention as a supplement for pregnant women,^[14] noting studies of improved attention and visual acuity. One recent study indicates that low levels of plasma and erythrocyte DHA were associated with poor retinal development, low visual acuity, and poor cognitive development. In that same study, alpha-linoleic acid was shown as a source of fetal DHA, but that preformed DHA was is more readily accredited. A working group from the ISSFAL (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 / day of the women in the study. Other requirements are available from other sources.^[15]

DHA has been an ingredient in several brands of premium infant formula sold in North America since 2001 after Mead Johnson, the first infant formula manufacturer to add DHA and ARA (arachidonic acid) to its Enfamil Lipil product, received approval by the Food and Drug Administration and Health Canada. Both DHA and ARA are permitted in infant formula, as both are components of human breast milk. The addition of DHA at dose-effective levels has been shown to improve congitive function in both term and preterm infants.

DHA makes infant formula more like human milk than "conventional" formula containing linolenic acid and linoleic acid, which are precursors to DHA. Formula sold in North America uses lipids from microorganisms grown in bioreactors as sources of DHA.^[16][1]

Superiority of Algae Derived DHA in Infant Nutrition

A study^[17] found that preterm infants fed baby formulas fortified with DHA derived directly from algae gained weight faster than infants fed formula fortified with DHA from fish oil. In addition, there are no risks of harmful contaminants such as methyl mercury or dioxins, which may be present in fish and fish oils.^{[citation needed]} This is especially important for pregnant and nursing women and young children.

Studies of Vegans and Vegetarians

Vegans and vegetarians have markedly lower stores of DHA. Their bodily DHA levels do not rise much even with high dietary levels of linolenic acid. This, and features of the production and distribution of DHA in pregnant and lactating women, indicates that DHA per se is an essential nutrient.^[18]

Promotion as a food additive

DHA is actively promoted by manufacturers as a food additive. Until recently, sales other than to makers of infant formula have been minimal; however, in 2007, several DHA-fortified dairy items (milk, yogurt) began selling in grocery stores.

There is less DHA available in the average diet than formerly, due to cattle being taken off grass and fed grain before butchering; likewise, there is less in eggs due to factory farming. DHA is widely 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. Some manufactured DHA is a vegetarian product extracted by a patented process from algae, but costs approximately twice as much as fish oil. Both are odorless and tasteless after processing.^[19]

See also

• Chlorella
• Cyanobacteria
• List of omega-3 fatty acids
• Polyunsaturated fatty acids

References