There is a big difference! Plant-based omega-3s (alpha-linolenic acid) are shorter-chained. Fish-based omega-3s (docosahexanoic acid and eicosapentanoic acid) are long-chained. The plant-based omega-3s must be converted to long-chained in the body, but the conversion in humans is limited in its efficiency. The majority of the studies showing cardiovascular benefit are from those using fish oil omega-3s.
In short – nutritionally, fish oils (omega3-DHA & EPA) do not supply the overall daily requirements and quantity of essential fatty acids (omega 3 and omega 6); they also are – some more some less – processed oils. Though, even with plant oils one ought be careful to choose properly pressed versions, as regular pressing methods especially of small and hard seeds will involve high pressure and friction, thus creating heat up to 150 degrees Celsius; technically, such an oil can still be labelled ‘cold-pressed’ (even delicately in-stable flax oil may be pressed in this manner and sold as cold-pressed ! It is more efficient that way for commercial interests – so ask some questions before buying to find one that is pressed with health in mind ! )
There are numerous studies in the context of vascular health showing benefit from plant omega 3, too, but there seems less profit to be made from plant oils, compared to the fish oil industry.
To be sure, I fully agree that there is a therapeutic place for fish oil benefits; in actual daily practice, however, many people will begin taking fish oils exclusively (with the majority of the marketing messages being related to fish oils), ignoring the requirements of the body for daily supplies of omega 3 ALA and omega 6 LA.
As for conversions of ALA (plant omega 3) to EPA and DHA – at the average rates of
2 to 9 % DHA from ALA (women naturally convert more in their bodies than men, as women can get pregnant and pass on most of their DHA to the infant – pregnancy and nursing ), even at minimum conversion rates of 2%, with 2 – 3 tablespoons (minimum daily average for average size adult) of plant omega3 (flax, chia, or a Omega 3-6-9 Blend), one still converts up to between 200 – 300mg of DHA from those 2 – 3 tablespoons of plant omega 3 –
The few true medical conditions in which conversion is indeed compromised can certainly be *supplemented* with a direct DHA supplement (even then, it may be worthwhile to consider algae oil based DHA, instead of fish oil based DHA).
Important, after all this, is to keep in mind that before supplementing with specific DHA product, one may wish to make sure the nutritional foundation is taken care of first – providing ample amounts of daily ALA and LA as the actual *essential* fatty acids for overall healthy physiological function, then *adding* the therapeutically indicated specific nutrient/s case by case.