Arachidonic acid is essential. Apart from the fact that your brain is largely made out of it (I exaggerate here slightly) and it has a roll as a precursor to a myriad signaling molecules, there is a definite deficiency syndrome in the skin. The deficiency syndrome is a bit dubious in humans, especially adults, but pretty clear cut in lab animals, especially during growth. Chris Masterjohn covers this pretty thoroughly and it seems quite plausible.
In most adult humans there is enough linoleic acid stored in fatty tissue to act as a reservoir for the synthesis of arachidonic acid for a long time. Even after years on a PUFA free diet it is unlikely you could precipitate an arachidonic acid deficiency. Bear in mind that a completely PUFA free diet must be synthetic or largely so. No food, just a fat free protein source, sucrose, glucose and some multivits should cover it! Otherwise enough arachidonic acid would sneak in from meat and eggs to supply an adult's needs. Or maybe some linoleic acid from lettuce!
I suppose the flip side of this is that if you drop your PUFA intake deliberately it may well be quite a long time before you lower your blood linoleate levels. Obviously this depends on how much vegetable oil you've squirreled away in your fatty tissue, how much fat you (and your gut microbiota) actually own and if your hormone sensitive lipase ever actually gets working... And of course how low you think you need to get your linoleate level before a deficiency syndrome appears. No healthy adult has managed so far in the literature.
So arachidonic acid deficiency in an adult human seems to be very, very unlikely. Especially if they occasionally eat items of food.
What about DHA? Amongst the many things which control elongation and desaturation of fatty acids I posted about recently, the parent molecules, linoleic acid and alpha linolenic acids, are effective depressors of the system. That is, if you eat a diet absolutely loaded with linoleic acid there is a down regulation of its conversion to arachidonic acid. That's logical. No one would want an unlimited supply of arachidonic acid. It does a lot of things to you, all of which would want to be done under careful control. It doesn't seem to be possible to down regulate to the point of arachidonic acid deficiency though, some will get through even under very high linoleate intake. Not so DHA.
Exactly the same elongase desaturase system used to manufacture arachidonic acid converts alpha linolenic acid to EPA, heading for DHA. Down regulating the system with linoleic acid, as above, will down regulate DHA production too.
Anyone on a "healthy" oils diet may well need to be getting preformed DHA. As DHA is only available from animal (or algal) sources, the problem person here looks to be the high PUFA eating vegan. That's fair enough, that particular diet choice comes with its own special consequences!
The quote below, which comes from this paper, seems to sum up the current state of unknowledge as to whether this problem is real:
"individuals who adhere to vegetarian and vegan diets do synthesize small amounts of DHA, and evidence to indicate deficits in brain development among vegetarians has not been published"
Of course there is plenty of evidence that being on total intravenous nutrition (TPN), based on linoleic acid as your sole source of lipid, means that you will require more than an absolute minimum of alpha linolenic acid before you actually get any DHA produced at all.
Under these conditions there could easily be an absolute DHA deficiency. There often is.
I think a strong admonition to avoid abdominal gunshot wounds, or anything else requiring years on intravenous feeding, would (if heeded) improve your DHA status. If you must do the iv feeding thing, adding some ALA to the lipid emulsion might be a good idea. Getting pregnant under these conditions might not be the best idea either, even if you felt that way inclined and were adult...
If you are a pregnant mother who avoids fish because of the mercury concern, avoids liver due to the vitamin A concern, avoids animal fats due to the cholesterol concern then your child's eyes and brain might be marginal on the DHA front. Pregnancy and growth seems to be when the body needs bulk DHA. Again, working through this review will fill in the background. Being DHA deficient in utero and during early post natal development seems to be bad news, not fully correctable by later DHA supplementation.
Of course, once you have started eating food as opposed to junk, dumped your heart healthy oils and are eating a little liver occasionally, the chance of developing any sort of PUFA deficiency seems rather small. Occasional fish or a decent amount of ruminant meat would eliminate the risk.
The problem is, how many people eat food and how many live on junk? If everyone were to switch to eating food, would DHA supplements be needed? If the intake of linoleic acid is low enough no doubt even the alpha linolenic acid from green leaf vegetables might end up as DHA.
So it looks as if there are certain circumstances, especially with those junk food based vegetarian diets easily achieved today, when DHA may not be available to meet foetal and neonatal needs. Taking a little DHA supplement may well be needed at this time or, better still, eat some food. Food's a good idea.
That seems to sum up the risk of deficiency to me, as far as I can see. We need some EFAs, more during pregnancy and nursing. Arachidonic acid is not a problem, DHA looks to be the weakest link. TPN is bad news (in case you want more bad news than whatever the reason is for your being on intravenous feeding).
So should we all be taking fish oil supplements, all of the time? Do we need to do anything except to avoid frank DHA deficiency as a foetus or newborn baby?
Maybe, maybe not. That's another post.