What is the difference between DHA from fish oil vs. flaxseed?

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Dear M,

All fish oils contain omega-3 fatty acids including some DHA in varying amounts depending upon the specific source/type. Most fish oils contain very little to trace amounts of the plant-based omega-3 fatty acid known as alpha-linolenic acid (LNA). In contrast, flaxseed and flaxseed oil contains approximately 50-60% of the total fatty acids as LNA while being devoid of DHA. Some varieties of flaxseed oil contain up to approximately 75% of the total fatty acid content as LNA (again without the concomitant presence of DHA omega-3). During the pre-natal period/development, there is very strong evidence that an ample supply of nutrients which includes DHA omega-3 fatty acid is very important for the provision of DHA for brain development and optimal functioning. DHA omega-3 is a physiologically-essential nutrient for the brain. In view of the very limited metabolic conversion of dietary/nutritional sources of LNA to DHA in the human body (including infants), a supply of pre-formed DHA is of great importance for ensuring an adequate supply of this important nutrient for the brain. DHA omega-3 is needed at considerably high concentrations in the brain (neuronal membranes, elsewhere) for optimal development and cognitive functioning. Regardless of the level of intake of LNA omega-3, it is present at very low concentrations in brain tissue in direct contrast to the high levels of DHA omega-3 which are present therein.