Malonyldialdehyde (MDA) is a small molecule formed by the random oxidation of a polyunsaturated fatty acid. The exact chemistry seems quite complex but needs, as an absolute minimum, two double bonds in the fat molecule, three omega numbers apart. But a feature of organic chemistry makes the free radical attack much more successful if there is a third double bond, located three omega numbers away from that bare minimum pair. So linoleic acid, that good old omega 6 fatty acid, can form some MDA because it has a double bond at the omega 9 position and at the omega 6 position, but it struggles to do it. Alpha linolenic acid, with its third double bond down at the omega 3 position, really allows MDA production to get going. That's chemistry. There's the briefest of summaries here.
So omega three fatty acid supplementation will increase MDA production. Adding vitamin E will largely eliminate this effect in the short term and the theory is that the vitamin E protects the omega 3 fatty acids in the chylomicrons en route to the liver. So more undamaged PUFA reach the liver, which has a greater impact on fasting triglycerides (VLDLs). This much comes from this paper.
But what I found really interesting is what happens within the liver itself. This paper comes up with some answers. The VLDL particles are manufactured as per normal, but if there is enough lipid peroxidation, the particle is degraded and never released. The liver appears to use iron to generate MDA from PUFA as a decider as to whether to release the VLDL particle or degrade it. The more MDA generated within the liver cell, the lower the plasma VLDL levels drop.
Why should that be? The liver makes a VLDL particle, tests how stable it is in terms of lipid peroxidation, and refuses to release any VLDLs deemed too unstable. This peroxidation is what omega 3 fats do, far better than omega 6 fats do it. The message I get from this is that our liver does not want lipid peroxidation prone VLDLs being released in to the circulation. So, if we accept that VLDLs from carbohydrate are stable (palmitic acid based) lots can be safely released. Render then unstable with fish oil and they, and their components, stay in the liver. Is this good or bad?
Well, taking fish oil makes your fasting triglyceride value look like it belongs to a LC eating person, even though you may not be a LC eating person. Does your cardiac risk belong to the the LC person or the mixed diet person? Draw the comparison with torcetrapib. Fantastic lipids, increased death risk. Now look at atrovasatain, quite "good" lipids and permission to trade in your heart attack death certificate for a cancer one, with the same date. Then look at LC eating and wonder about the blank cause of death and date.
Where do fish oils fit in to this spectrum? Dropping your triglycerides is treating a number. Fine for cardiologists. Eating like a Greenland eskimo requires strict LC in addition to 15g/d of EPA+DHA. This is a double triglyceride lowering approach but one which, in addition, dramatically minimises the glycation of apoB containing lipoproteins too. Is it the LC, the low trigs or the changes in tissue lipids which helps reduce CV risk? The mixed diet eating person with fish oil induced LC style triglycerides may well be munching lots of fruit as healthy low fat snacks to stave off hunger pangs between mixed meals. VLDL gycation?
If you have already lived your way to a heart attack, just "doing" EPA+DHA at 3.5g/d is as effective as 4g/d of corn oil for prevention of that second heart attack within the next 12 months! It's okay, unless you are one of the 25% of heart attack victims in each group re infarcting. Of course the fasting triglycerides were MUCH lower in the omega 3 group...
So do I think fish oils are useless? Not at all, but I think that using them as a tool to manipulate a number is, well, not a good idea. I do know that low dose EPA+DHA seems to benefit me, at around 1g/day. Whether this is "treating" the amount of omega 6 fatty acids I get from chicken and pork, I wouldn't like to say. But there's a lot more to omega 3 supplements than meets the eye.
What does seem lacking to me is convincing evidence of toxicity. The Greenlanders (back in the 1950s) were at low CV risk on high omega three intakes and I think it's reasonable to assume they were at the same low cancer risk as the Inuit described by Stefansson, see Stephan's post here. So I don't rate omega 3 as coming with the same toxicity as omega 6s.
There's another post on hepatic VLDL stability testing, but I'll call it a day on this rambling entry...