So nobody's decided to try to cash in on my offer of an e-high-five, eh? I'm probably too impatient. Anyways, here's what I missed.
If you look at this table, the only oil listed that has more long-chain omega-6 than long-chain omega-3 is cottonseed. A naive reading of this would indicate that if, as Sears argues, we're trying to avoid having an AA:EPA ratio that's too slanted towards AA, all we have to do is avoid cottonseed oil.
This, however, is not necessarily correct. Consider my reference to ALA as a potentially-pertinent substance - it's a short-chain omega-3 that, it appears, is partially converted to our relevant long-chain omega-3s. Could it be that there are analogous short-chain omega-6 fatty acids that the body can convert to arachidonic acid?
-Salem et al., "In vivo conversion of linoleic acid to arachidonic acid in human adults."Quote:
Human adults are shown to be capable of conversion of linoleic acid (LA, 18:2 n-6) to arachidonic acid (AA, 20:4 n-6) in vivo. It is confirmed that they can also convert alpha-linolenic acid (LNA, 18:3 n-3) to eicosapentaenoic acid (EPA, 20:5 n-3) and to docosahexaenoic acid (DHA, 22:6 n-3) in vivo.
I'm going to try to get a copy of that study to look at. Sears is pessimistic about rates of conversion of ALA to EPA/DHA, perhaps overly so. Here's what I found when I looked into it myself:
-Muir and Westcott, Flax (p. 156)Quote:
Hence, there is no assurance that increasing ALA intake will increase tissue levels of EPA or DHA; in fact, some studies show that DHA remains unchanged or even decreases after ALA supplementation
Said book also makes reference to competition between LA (linoleic acid, 18:2 n-6) and ALA for conversion into AA and EPA/DHA, respectively.
The plot thickens.
Edit: And to confuse things even more, there are reports of gender-based differences.
-Burdge et al., "Conversion of alpha-linolenic acid to longer-chain polyunsaturated fatty acids in human adults." (full paper here)Quote:
Increasing alphaLNA intake for a period of weeks to months results in an increase in the proportion of eicosapentaenoic acid (EPA; 20:5n-3) in plasma lipids, in erythrocytes, leukocytes, platelets and in breast milk but there is no increase in docosahexaenoic acid (DHA; 22:6n-3), which may even decline in some pools at high alphaLNA intakes. Stable isotope tracer studies indicate that conversion of alphaLNA to EPA occurs but is limited in men and that further transformation to DHA is very low. The fractional conversion of alphaLNA to the longer chain n-3 PUFA is greater in women which may be due to a regulatory effect of oestrogen. A lower proportion of alphaLNA is used for beta-oxidation in women compared with men.