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Comment by gumby | Hacker News Reader

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?

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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.

This is an excellent, concise, clear explanation, thank you.

> fish oil is only sold by EPA/DHA concentration. Nobody will buy natural fish oil because the EPA/DHA levels are too low.

So only free fatty acids are allowed to be listed on the label as EPA/DHA? They're not allowed to count any esterified fatty acids?

I don't know the details of labeling laws, but I believe fish oil is most commonly sold as ethyl esters, and this is called "fish oil concentrate". Free fatty acids are considered a defect in edible oils. They taste rancid and make the oil decay faster. Triglycerides and ethyl esters both get digested in the small intestine by pancreatic lipase enzymes, releasing free fatty acids where you can't taste them. Both raise blood levels of free fatty acids, and the quantity of ethanol released from the ethyl esters is small enough that I think it's very unlikely to matter.

But as the article points out, phospolipids get digested differently, and phospolipids are converted to ethyl esters in the transesterification processing too. By focusing only on blood levels of fatty acids, people overlooked a potentially important difference between natural and processed fish oil.

Is this the same for bottled flax oil? I am under the impression that flax oil has similar properties to fish oil.

I've never heard of concentrated flax oil, and all flax oil I've seen has the ALA levels I'd expect of natural flax oil. However, as discussed elsewhere in the comments, humans are not good at converting ALA to EPA and DHA, so flax oil cannot be considered a full substitute for fish oil.

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.

But eg sardines should not contain much of any heavy metal? Salmon is also pretty low in mercury.

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.

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