Does Dietary Fat Influence Insulin Action?a

: What is clear from the research thus far is that dietary fat intake does influence insulin action. However, whether the effect is good, bad, or indifferent is strongly related to the fatty acid profile of that dietary fat. The evidence has taken many forms, including in vitro evidence of differences in insulin binding and glucose transport in cells grown with different types of fat in the incubation medium, in vivo results in animals fed different fats, relationships demonstrated between the membrane structural lipid fatty acid profile and insulin resistance in humans, and finally epidemiological evidence linking particularly high saturated fat intake with hyperinsulinemia and increased risk of diabetes. This contrasts with the lack of relationship, or even possible protective effect, of polyunsaturated fats. In particular, habitual increased n-3 polyunsaturated dietary fat intake (as fish fats) would appear to be protective against the development of glucose intolerance. It is reassuring that the patterns of dietary fatty acids that appear beneficial for insulin action and energy balance are also the patterns that would seem appropriate in the fight against thrombosis and cardiovascular disease. Mechanisms, though, still need to be defined. However, there are strong indicators that defining the ways in which changes in the fatty acid profile of membrane structural lipids are achieved, and in turn influence relevant transport events, plus understanding the processes that control accumulation and availability of storage lipid in muscle may be fruitful avenues for future research. One of the problems of moving the knowledge gained from research at the cellular level through to the individual and on to populations is the need for more accommodating research designs. In vitro studies may provide in-depth insights into intricate mechanisms, but they do not give the "big picture" for practical recommendations. On the other hand, correlational studies tend to be fairly blunt instruments, requiring large numbers that are very often not feasible if a greater depth of understanding of the biological processes is to be incorporated. There may be benefit in turning to the clinical case study as a framework for a more comprehensive analysis of the links between dietary fats and insulin action. The real challenge is to keep the depth of analysis rigorous enough to be able to explain and accommodate individual variation (i.e., the diversity of both environmental and genetic backgrounds) while at the same time satisfying the cultural need to provide appropriate overall dietary guidelines. Finally, David Kritchevsky brought to our attention a delightful quote from Mark Twain: "There is something fascinating about science. One gets such a wholesale return of conjecture for such a trifling investment of fact." In the field of dietary fats and the Metabolic Syndrome, this quotation is, unfortunately, apt. Much more research is necessary to define how dietary fats really work to affect insulin action. Well designed, long-term studies in "free range" humans must be undertaken if dietary guidelines for the Metabolic Syndrome are to be based on anything more than a "trifling" amount of "fact.