I’ve researched omega-6 some more and the evidence that it is harmful is much less clear than I previously thought.
There’s some evidence that omega-6/omega-3 ratios above some threshold (between 2.5 and 4?) are harmful, but also arguments against focusing on the ratio:
This paper reviews a variety of studies that, in the aggregate, suggest that the ratio is, both on theoretical and evidential grounds, of little value. Metrics that include the n-3 FAs alone, especially eicosapentaenoic and docosahexaenoic acids, appear to hold the greatest promise.
While omega-6 can be converted into proinflammatory molecules, it also has antiinflammatory properties.
in observational studies, higher omega-6 PUFA consumption was associated with unaltered or lower levels of inflammatory markers
in 1 study in Israel,39 where 25% of the population consumes >12% of energy as omega-6 PUFA, an inverse association was found between adipose LA [Linoleic acid] and acute myocardial infarction after controlling for other omega-6 PUFAs.
This meta-analysis seems to be the most rigorous analysis:
They looked for
all RCT that increased PUFA and reported relevant CHD outcomes
For non-fatal myocardial infarction (MI)+CHD death, the pooled risk reduction for mixed n-3/n-6 PUFA diets was 22 % (risk ratio (RR) 0·78; 95 % CI 0·65, 0·93) compared to an increased risk of 13 % for n-6 specific PUFA diets (RR 1·13; 95 % CI 0·84, 1·53).
They found 7 randomized trials that raised omega-6 levels and provided a tolerable amount of info about other fats. Three that had apparently low dietary omega-3 showed that higher omega-6 combined with lower trans-fats produced more heart disease/death (i.e. above some high omega-6/omega-3 ratio, omega-6 can cause more harm than trans-fats), while four where the higher omega-6 was combined with arguably adequate omega-3 and lower trans-fats had less heart disease/death with that combination (i.e. omega-6 might have helped or might have caused harm that was small compared to trans-fats).
High intakes of linoleic acid were not associated with excess risk of AMI [acute myocardial infarction]
…
arachidonic acid, the long chain n?6 derivative of linoleic acid, was positively associated with AMI
(Arachidonic acid is found mainly in eggs, meat, and seafood).
Other sources suggest arachidonic acid is safe, mentioning studies of biomarkers that show no harm from arachidonic acid, but biomarkers are less convincing than actual disease. There are some reports that ratio of arachidonic acid to eicosapentaenoic acid (AA:EPA) in blood plasma is a useful measure of cardiovascular problems.
In my previous post I expressed doubts about high omega-3 diets, but this paper on hunter-gatherer diets gives the following estimates for fat intake:
- Omega-3 9.6g/day
- Omega-6 14.2g/day
- Saturated 18g/day
- Monunsaturated 44.3g/day
I don’t know how typical this is of our ancestors, but at least it’s likely within the range of diets that we’re adapted to. This suggests it’s hard to get too much omega-3 or too much fat from seafood (I’d get too much protein if I ate much more than 2 pounds of salmon, but not too much of any type of fat). It also suggests that it’s reasonable to aim for high enough omega-3 that I can’t easily get an omega-6/omega-3 ratio that is high enough to be of much concern.
More from that paper:
Compiled ethnographic studies of 229 hunter-gatherer societies, as well as quantitative studies of hunter-gatherers have demonstrated that animal foods contributed slightly more than half (55-65%) of the daily energy, whereas plant foods would have made up the remainder (35-45%) of the average daily caloric intake. Of the energy obtained from animal foods, historically-studied hunter-gatherers typically derived half of their energy from aquatic animals and the other half from terrestrial animals. Animal food intake would have also been constrained by the physiologic protein ceiling, which has been shown to occur between 30 to 41% of total energy.
There is an increasing body of evidence that suggests high protein diets may improve blood lipid profiles and thereby lessen the risk for cardiovascular disease (CVD).
[I plan to check this evidence later.]