There's no reason why insulin should not be sufficiently "sensitive" in the first place.  My guess is that they are assuming (without even knowing it) that the people who come to their offices with diabetic issues are having problems with AA metabolites, and their sense of that is something along the lines of "the body is just breaking down with age," even when the patients are young.  Since the fish oil inhibits AA metabolization, it will look "beneficial" in short-term studies.  The idea that insulin loses its "sensitivity" for no reason is just ridiculous, especially for people who are middle-aged or younger.  I tell people to eat only three meals a day, spaced out at least 3 hours apart, and to eat nothing with calories in between meals.  In that way, the insulin will remain "senstive," so long as the diet is low in unsaturated fatty acids and sufficient in high-quality protein.  There is no need for a "supplement" to prevent insulin from losing its "sensitivity," so long as you know what and how to eat, from my reading of the evidence.