How Essential are Omega-6 Fatty Acids + How Much do We Actually Need? Study Says: Requirements May Have Been Significantly Overestimated Due to Methodological Flaws

I have to admit: Given the high linoleic (N6) acid content of most of the commonly used plant oils in the Western diet, deficiencies are probably rare occur (if at all) mostly in clinical settings.

I am quite sure all of you will have read something about the essential fatty acid requirements of mammals before. Unfortunately, I am also quite sure that you will have found recommendations ranging from the "peatarian" zero (as in "beware of essential fats, the are toxic") to the "Robb Wolff fish oil calculator bullocks" of the early days which recommended copious amounts of omega-3s and allowed for at best zero omega-6s in your diet.

One of the reasons that the opinions on human essential fatty acid requirement diverge in the aforementioned ways is that scientific evidence to form an educated unbiased opinion on how much of these precious fats we actually need is scarce.
What's even worse, though, is the fact that a recent paper by scientists from the INRA and the Université Paris-Sud XI in Paris suggests that we may have overestimated the omega-6 requirements significantly.

Why? Well, the reason lies in the absurd design of corresponding rodent studies - and I am not using the bullshit killer argument that "mice are no little men" (go and conduct a human study if that's all you have to offer), but rather the mislead, albeit downright scientific desire to eliminate all confounding factors and design a "flawless", and thus eventually unrealistic experiment.

Figure 1: Graphical overview of the study design | the numbers indicate the content of LA(N6) and ALA (N3) of the test diets the scientists used to identify the basal requirements (Choque. 2015).

If we put faith in the results Benjamin Choque, Daniel Catheline, Bernadette Delplanque, Philippe Guesnet, and Philippe Legrand, the lack of omega-3s in the test diets of said experiments may have mislead us to significantly overestimate the actual omega-6 requirements in mammals, of which previous studies suggested that it would be 2% of the total energy intake (for the standard 2,000kal/day diet that's 40kcal/day and thus ~4.4-4.5g of linoleic acid aka "omega-6s" per day. Or, as Choque et al. (2015) put it:

Table 1: The biological activities related to linoleic (N6) or linolenic (N3) have long been established (Holman. 1971).

"The first dose–response studies of LA requirements in rats have been corrupted by an unintentional exclusion of ALA (Holman. 1971), leading to a significant overestimation of the dietary requirement for dietary LA (Cunnane. 2003). The n-3 and n-6 families have been known to be nutritionally different, with different biological functions; nevertheless, dietary ALA deficiency could affect physiological parameters similar to LA deficiency.

For instance, several studies on rats have shown that the dietary intake of 0.5 en% from ALA could prevent the biological (growth, reproduction and early development) and biochemical functions with a LA intake as low as 0.3 en% (Greenberg. 1950; Bourre. 1990)" (Choque. 2015).

Furthermore, more recent results show that the mere lack of omega-6s in the diet won't suffice to induce a complete depletion of the abundant linoleic acid stores in the adipose and other organ tissue of mammals (Guesnet. 2011). Against that background it was necessary to revisit the basic mammalian omega-6 requirement in a non-omega-3 deficient scenario where the stored linoleic acid would not mess with the study outcomes.
Accordingly, the present study was especially designed to create a specific and strong deficiency in linoleic acid (LA) by depriving not just the test animals, but also the maternal diet of LA.  In that, the main purpose of the present study was to carry out a dose-response study on a male rat model in order to determine the minimal intake of dietary LA required to avoid n-6 deficiency symptoms, when ALA is included in the diet. The procedure involved two basic steps:

1. This first step determined whether the n-6, n-3 and n-9 fatty acid compositions in the tissue of the growing male rats could be affected by the nutritional status of their mothers, and it helped to obtain a powerful deficiency model. Moreover, the scientists studied whether an adequate intake of 0·5 en% from ALA in growing male rats could partly prevent or not growth and other markers of LA deficiency in the present model. 
2. The second step was performed using the optimised conditions of deficiency, in order to determine the dietary LA requirement in the presence of ALA in growing male rats.

The results which were quite unambiguous are summarized in the tabular overview below. In short, the first step revealed that the best LA deficiency model was obtained using rats fed the 0LA/0ALA diet, born from female rats fed the 0LA/0·5ALA diet.

Table 2: Summary of the different diet effects on physiological and biochemical criteria (Choque. 2015).

The second step demonstrates that in growing rats, LA deficiency was corrected with an intake of 1–1·5 en% from LA and 0·5 en% from ALA. As the scientists point out, "[t]hese data suggest that the requirements in humans should be revisited, considering the presence of ALA to set up the recommendation for LA" (Choque. 2015). Unfortunately, this means that future studies are required before I can tell you, not your rodents or pregnant wife, how much linoleic acid actually need.
References:

• Bourre, J. M., et al. "Dietary linoleic acid and polyunsaturated fatty acids in rat brain and other organs. Minimal requirements of linoleic acid." Lipids 25.8 (1990): 465-472.
• Choque et al. "Dietary linoleic acid requirements in the presence of a-linolenic acid are lower than the historical 2 % of energy intake value, study in rats." British Journal of Nutrition (2015): Published Online Ahead of Print.
• Cunnane, Stephen C. "Problems with essential fatty acids: time for a new paradigm?." Progress in lipid research 42.6 (2003): 544-568.
• Greenberg, Samuel M., and Harry J. Deuel. "The protective effect of high fat diets on immature rats fed thyroid." The Journal of nutrition 42.2 (1950): 279-284.
• Guesnet, P., et al. "α-Linolenate reduces the dietary requirement for linoleate in the growing rat." Prostaglandins, Leukotrienes and Essential Fatty Acids 85.6 (2011): 353-360.
• Holman, Ralph T. "Biological activities of and requirements for polyunsaturated acids." Progress in the Chemistry of Fats and other Lipids 9 (1971): 607-682.