There are few studies on PUFA deficiency in which the rats consume a good amount of food, most studies keep the nutrients below the requirement and the rats degenerate.
I saw a study in which rats developed the symptoms of PUFA deficiency when placed on a fat-free diet ad libitum, the symptoms were cured spontaneously as long as the diet continued ad libitum, and it was even theorized that the rats produced the essential fatty acids. There's another study in which they didn't even develop symptoms even with a high amount of Mead's acid, but the diet was ad libitum from the start.
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Without him talking about the symptoms, we can't be sure. The skin problems of PUFA deficiency seem to be caused by the replacement of linoleic acid(18:2n-6) by oleic acid(18:1n-9). You need a degree of unsaturation to control water permeability, there's a study that rats don't suffer many symptoms of deficiency because they kept the humidity high, it seems to take a while for mead acid(20:3n-9) to take over
WHE = 90% humidty
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Bearing in mind that saturated and hydrogenated fats accelerate the depletion of DHA, I believe that the deficiency of DHA is real and there is no other that can replace its function.
In rats, 0.26% of calories in ALA was enough to maintain the maximum concentration of DHA(in organs such as the brain) in adult rats, 0.4% in young rats(which means it's difficult to become deficient in DHA). I haven't looked to see if it can be translated to humans literally, as a few oysters would do the job.
Tokelauans(max 7g PUFA/day) of the past seemed relatively depleted of omega-6 and were relatively free of the diseases of today, I don't remember reading about any particular problem
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