What Is DHA?
Docosahexaenoic acid (DHA) is an essential omega-3 fatty acid that is abundant in the brain, being crucial in brain structure. As such DHA is a key component of neuronal membranes together with arachidonic acid  (a major opposing omega-6 fatty acid), making up 15-20% of the brain’s dry mass.
This polyunsaturated fatty acid is obtained from fish sources of food.
In healthy human volunteers, positron emission tomography (PET) has shown that the normal human brain consumes 4.6 mg/day of DHA.1 
DHA is particularly concentrated in highly active membranes such as nerve synapses (junctions) and photoreceptors in the eye (retina).
Q: How much DHA is in the retina?
A: DHA makes up more than 30% of the retina.2 
In other roles, DHA is an important building material for the eicosanoids, a large group of highly bioactive hormone-like substances including prostaglandins, leukotrienes, and thromboxanes that are involved in blood clotting, inflammation, and vasoconstriction.
DHA has been shown to increase insulin sensitivity as opposed to the opposite problem of insulin resistance, to improve muscle mass, and protect against non-alcoholic fatty liver disease.3 
Egert et al. in a study of people aged 19 to 43 years with normal cholesterol showed that DHA intake significantly increased serum HDL (good) cholesterol. Also, DHA significantly decreased fasting serum triglycerides.4 
What Is DHA Deficiency In Celiac Disease and/or Gluten Sensitivity?
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Rapoport SI. Brain arachidonic and docosahexaenoic acid cascades are selectively altered by drugs, diet and disease. Prostaglandins Leukot Essent Fatty Acids. 2008 Sep-Nov;79(3-5):153-6. Epub 2008 Oct 29. ↩ 
Richardson AJ. The importance of omega-3 fatty acids for behavior, cognition, and mood. Scandinavian Journal of Nutrition. 2003;47(2):92-8 ↩ 
Espinosa A, Valenzuela R, González-Mañán D, D’Espessailles A, Guillermo Gormaz J, Barrera C, Tapia G. Prevention of liver steatosis through fish oil supplementation: correlation of oxidative stress with insulin resistance and liver fatty acid content. Arch Latinoam Nutr. 2013 Mar;63(1):29-36. ↩ 
Egert S, Kannenberg F, Somoza V, et al. Dietary alpha-linolenic acid , EPA , and DHA have differential effects on LDL fatty acid composition but similar effects on serum lipid profiles in normolipidemic humans. J Nutr. 2009;139:861–868. doi: 10.3945/jn.108.103861 ↩