DHA is a long chain omega-3 polyunsaturated fatty acid derived from the tissues of oily fish and marine mammals. As far as dietary sources are concerned, the location of highest concentration seems to be within the the livers of fish. Otherwise, DHA is widely available through the ingestion of various nutritional supplements.
DHA is a potent inhibitor of the many inflammatory processes taking place regularly in the body; competing with arachidonic acid in the cyclooxygenase and lipoxygenase inflammatory pathways.  These enzymatic pathways are chiefly responsible for the many adverse symptoms resulting from inflammation. In fact, these two pathways are targeted by several prescription drugs. Drugs like aspirin and the newer Cox-inhibitors work to eliminate the symptoms associated with these inflammatory processes.
DHA is readily converted into eicosapentaenoic acid, another essential polyunsaturated fatty acid in humans.  The term ‘essential fatty acid’ relates to the fact that these are a grouping of fats that are necessary for human health; their synthesis is minimal in the body and, therefore, must be obtained from diet to insure proper biochemical functioning. DHA and other long chain polyunsaturated fatty acids comprise nearly one-third of the fatty material in brain tissue. DHA has been shown to not only be a major component of brain tissue, but affects neurologic and eye development to such an extent that is has recently been included in commercial baby formulas.  Receiving inadequate amounts of this nutrient has been linked to impaired cognitive function, visual disturbances, and attention-deficit hyperactivity disorder (ADHD) in children.
Although widely available in purified form as a supplement, DHA is found in nearly all forms of seafood, most notably cold water fish. Popular cold water fishes include; salmon, herring, sardines, halibut, bluefish, tuna, and mackerel. In fact, the colder the environment from which the fish is found, the higher its content of DHA. This is important to keep in mind as farm raised fish (including those listed above) are typically very low in DHA and other omega-3 fatty acids. They are not feed algae, nor do they live in an extremely cold environments like wild fish. Sea algae provides some of the precursor material for fatty acid development in fish.
Other animal sources of DHA include eggs and organ meats. While these foods do contain some DHA, they are considered a less than optimal source, especially when comparing relative quantities to those found in fish sources.
Vegetable sources of DHA include flax seed, soy, and canola oils. Although other oils (e.g. borage and evening primrose) contain only minimal amounts of DHA that are ineffective at fulfilling daily requirements, they do still provide a healthy source of essential fatty acids. Seeds and nuts contain another type of fatty acid, known as alpha-linolenic acid, and the body can convert this fatty acid into DHA if needed.
DHA, included with eicosapentaenoic acid (EPA), comprise what are known as essential fatty acids (EFAs). DHA is incorporated into the human brain quite rapidly during the third trimester of pregnancy and during the course of early postnatal development (0-6 months).  DHA plays a highly important role in brain function and is thought to contribute to proper structural development, neurologic development, and synaptic membrane development. 
Additional research shows that DHA exerts a positive effect on retinal function and development, visual function, learning ability, and memory.  Further evidence for the importance of DHA in human development is its presence in breast milk; children that are fed solitarily with DHA-devoid formulas have lower levels of DHA in the brain and throughout the body. Clinical studies are ongoing to determine what effect this may have on one’s neurologic development and future health outcome.
DHA has beneficial effects in cardiovascular disease; supplementation with DHA results in elevated HDL (the “good” cholesterol) and reduces serum triglyderides.  It also, however, elevates LDL cholesterol (the “bad” cholesterol) and particle size, but does not appear to elevate total cholesterol.  DHA exerts slight effects on blood viscosity by increasing the ability of red blood cells to deform, which allows them to travel with greater ease through narrow spaces (thus decreasing potential clots in narrowed arteries). 
DHA as a supplement has been shown to decrease stress-related aggression.  This is being studied in people with developmental disorders. When given to premature infants, DHA was shown to improve visual function, and decrease the severity of movement disorders in children with dyspraxia, a specific type of physical movement disabilty. [11, 12] Supplementation with DHA seems to improve the development of neurologic structures, as this fatty acid is incorporated into the grey matter of the brain - 1/3 of which is comprised of long chain polyunsaturated fatty acids. 
Diets rich in DHA may decrease mortality in people with coronary artery disease as well.  This is thought to be due to DHA’s positive effects on blood lipid profiles.
There is no established RDA for DHA. It is typically supplied in combination with other polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA). The majority of research suggests 1 - 3 grams of DHA per day; most fish oil supplements contain roughly 12% DHA (in combination with EPA). 5 grams of supplemental fish oil will yield approximately 72 - 312 milligrams of DHA - 0.32%. DHA is also a common ingredient in infant formulas. 
Diets that contain no cold water marine animal sources may be low in DHA. Likewise, diets low in animal foods and seeds or nuts may be further depleted. Researchers believe that pregnancy may deplete DHA stores in women, setting them up for deficiency toward the end of pregnancy.  As mentioned, nfants that are not breast fed have lower amounts of DHA in their bodies than those that are. This may have a negative effect on neurological development.
DHA Side Effects:
Taken in large amounts, DHA may cause nausea and gas. Side effects specific to DHA are extremely rare, but often occur after consumption of combination fish oils (EPA and DHA). Side effects reported include, increased bleeding time and risk of bleeding, nose bleeding, loose stools, and fishy taste when combination products are ingested. [17-19]
DHA is not thought to cause interaction with any foods. Although DHA itself does not affect blood clotting status, when taken in combination with other fish oils (i.e. EPA) a concern arises. Caution should also be used when supplementing with herbs that affect platelet function. [20, 21] These include, but are not limited to; garlic, ginger, ginkgo, red clover, angelica, dong quai, and turmeric.
DHA Drug Interactions:
Combined with EPA, DHA may increase the risk of blood thinning when taken with an anticoagulant or antiplatelet drugs, including aspirin. In addition, because DHA may theoretically lower blood pressure, caution should be used when combining it with high blood pressure medicines; as it may have an additive effect. [22, 23]
Lab Test Interactions:
DHA may change lipid profiles. HDL may be elevated, and LDL concentration and particle size may be increased in patients with already high cholesterol levels. 
DHA can also raise triglyceride levels in people with elevated cholesterol levels as well.
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