It seems like a very long time ago (only last year!) that George Henderson posted links in comments to the blog* about the absolutely crucial work done by Gibson and colleagues, documented in this paper
*Ooooh look, I just noticed how to link to comments. I'm so tech savvy!
Docosahexaenoic acid synthesis from alpha-linolenic acid is inhibited by diets high in polyunsaturated fatty acids
Another aside: Paywalled. If anyone has a few pence looking for a home Alexandra Elbakyan might be a good destination. I didn't say that. End aside.
It is impossible to say how good this work is. It's very good.
I'm no hyper-enthusiast for DHA. It's a tool. It does a job. Saturating yourself with the stuff is very likely to be a Bad Thing. This is perhaps best exemplified by the fierce negative feedback exerted by all of dietary C18 VLCPUFA precursors (omega 6 and omega 3s) on its synthesis (I would assume the same happens for arachidonic acid as well). The conversion of alpha linolenic acid to DHA is, for rats at least (and I would go with for humans too), very, very easily achieved by simply getting close to eliminating linoleic acid from the diet and also keeping ALA low, under 3% of calories. Here's my favourite figure from the paper, already tweeted and blogged by George:
These are the DHA levels in phospholipids, presumably LDL and HDL secreted by the liver, extracted from plasma after three weeks of dietary intervention in Hooded Wistar rats.
"We conclude it is possible to enhance the DHA status of rats fed diets containing ALA as the only source of n-3 fatty acids but only when the level of dietary PUFA [ie all combined PUFA*] is low (less than 3% of energy)."
*My insert for emphasis.
Does anyone begin to recognise a pattern to PUFA requirements here?
Random aside. Rats. Have they been scavengers of the small amounts of edible tissue left on mammoth carcasses after humans had finished with them? Are rats evolved to be opportunist high fat, low PUFA adapted facultative carnivores? Now that's an interesting and useless thought but might help explain why they behave exactly as humans do on Surwit diets compared to low PUFA Surwit-like derivatives. Well, the idea entertains me. But then I like rodent studies...