Only two essential fatty acids are known to be essential for humans: alpha-linolenic acid (an omegaâÂÂ3 fatty acid) and linoleic acid (an omegaâÂÂ6 fatty acid). Closely related, these fatty acids act as competing substrates for the same enzymes. The biological effects of the ÃÂâÂÂ3 and ÃÂâÂÂ6 fatty acids are largely mediated by essential fatty acid interactions. The proportion of omegaâÂÂ3 to omegaâÂÂ6 fatty acids in a diet may have metabolic consequences. Unlike omegaâÂÂ3 fatty acids and omegaâÂÂ6 fatty acids, omegaâÂÂ9 fatty acids are not classed as essential fatty acids because they can be created by the human body from monounsaturated and saturated fatty acids, and are therefore not essential in the diet.
It has been claimed that among hunter-gatherer populations, omegaâÂÂ6 fats and omegaâÂÂ3 fats are typically consumed in roughly a 1:1 ratio. At one extreme of the spectrum of hunter-gatherer diets, the Greenland Inuit, before the late 20th century, consumed a diet with twice as much omegaâÂÂ3 as omegaâÂÂ6, thanks to a diet rich in cold-water fish (a rich source of omegaâÂÂ3s) and completely devoid of omegaâÂÂ6-rich seed oils.
To date, "no one knows what the optimal ratio in the diet is for these two families of fats." Science writer Susan Allport writes that the current ratio in Japan is associated with a very low incidence of heart and other diseases. A dietary ratio of 4:1 produces almost a 1:1 ratio of highly unsaturated fatty acids (HUFAs) in cell membranes."
In a study with rats, a dietary ratio of 4:1 showed significant favorable effects on learning performance and pain tolerance compared to 6:1.
Andrew Stoll, M.D., Director of the Psycho-pharmacology Research Laboratory at Harvard's McLean Hospital, who advocates the consumption of the two fats in a 1:1 ratio, states, "Once in the body, omega-3 and omega-6 fatty acids follow parallel pathways, continually competing with each other for chemical conversion to various structures and molecules inside and outside the cells. Given this mechanism, it makes sense that the two fats might be required in approximately equal amounts."
Both Stoll and Allport assert that present-day diets in the developed world have departed dramatically from this ratio. It has been estimated that in developed countries, the ratio of omegaâÂÂ6s to omegaâÂÂ3s is closer to 15:1 Another estimate is that "[t]he diet consumed by the typical American tends to contain 14âÂÂ25 times more omega-6 fatty acids than omega-3 fatty acids."
According to a 2009 review by the American Heart Association, instead of avoiding ÃÂâÂÂ6 fats, the ÃÂâÂÂ6:ÃÂâÂÂ3 ratio should be decreased by consuming more ÃÂâÂÂ3 fats. The conversion rate of linoleic acid (LA) into arachidonic acid is very low with a diet high in linolenic acid.
The maximum ÃÂâÂÂ6:ÃÂâÂÂ3 ratio allowed in dog food by the AAFCO is 30:1.