Omega-6 / omega-3 ratio
The balance between omega-6 and omega-3 fatty acid intakes, heavily skewed in Western diets, and discussed as a factor in chronic inflammation.
Omega-6 and omega-3 are two families of essential fatty acids: the body cannot make them. They use the same enzymes to be converted into long-chain derivatives, which puts them in direct competition.
This competition is what gives the ratio its meaning: an excess of omega-6 saturates the enzymes and slows the conversion of plant-based ALA into EPA and DHA, which is already inefficient. The derivatives of the two families also have different effects on inflammatory processes.
The ratio is estimated at between 10:1 and 20:1 in Western diets, against a figure of around 4:1 often cited as desirable. This shift comes from the place taken by sunflower, corn and soya oils, which are very rich in omega-6, and by the processed products that contain them.
Two caveats are needed, however. Omega-6 is not harmful: these are essential fatty acids, and cutting them back is not a goal in itself. And the exact value of the ideal ratio is debated, with several studies judging the absolute intake of EPA and DHA to matter more than the ratio itself. In practice, the conclusion is the same: increase omega-3 (oily fish, rapeseed, flax, walnuts) rather than hunting down omega-6.