Dr. Rhonda Patrick on DHA in Phospholipid Form and the Prevention of Alzheimers

When you eat oily fish you are eating the same oil that is expelled and inserted into a capsule. So how is the form changed?

This is a type of fish oil that usually comes from krill. It has a higher capacity to pass the blood-brain barrier of carriers of the APOE4 gene.

The paper is open access, so the full text is available sans paywall. :) Worth checking out if for no other reason than to appreciate the figures! https://www.fasebj.org/doi/10.1096/fj.201801412R

The natural oil in the fish is found as triglycerides and as phospholipids.

A triglyceride is an ester made from three fatty acid molecules attached to a glyerol molecule (you get esters by reacting carboxylic acids with alcohols, and glycerol is a polyol alcohol with three hydroxyl groups). A phospholipid is very similar except one of the fatty acids is replaced by some hydrophilic molecule that's attached to the glycerol via a phosphate group.

The problem from a commercial point of view is that the fatty acids are mixed, but fish oil is only sold by EPA/DHA concentration. Nobody will buy natural fish oil because the EPA/DHA levels are too low. The solution is transesterification, where the glycerols are removed and replaced with ethanol. Ethanol is an alcohol with only a single hydroxyl group, so it can only form an ester with a single fatty acid. Now that all the fatty acids are in separate molecules, you can distill off the lower boiling ethyl esters of short chain fatty acids. EPA and DHA are long chain fatty acids, so their higher boiling ethyl esters will be left behind.

Theoretically could transesterify the ethyl esters back to triglycerides again, but when you can just sell the ethyl esters as "fish oil concentrate" there's no reason to.

Chelation. When you concentrate the oil you can remove impurities there are a lot of in fish. In particular, Mercury and I think Cadmium too impede and reverse myelination, making taking fish oil for the nervous system benefits almost counterproductive.