Algae based DHA has been found to be bio-equivalent to DHA in fish oil and seems to provide the same benefits without the contaminants (even in distilled fish oil), which are present even in organic farmed salmon. Fish can contain mercury, but blue green algae and spirulina supplements have been found to contain neurotoxins and hepatotoxins. The algae-based EPA and DHA are procured from toxin-free algae. Chlorella is similarly safe.
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