Choose wisely -- foods close to nature, mostly plants -- and you will avoid a host of ills, from the wrong kinds of fat, to excesses of sugar, salt, starch and calories. By choosing wholesome foods, you construct a wholesome diet -- with a good chance of adding both years to your life and life to your years . Vegetables, fruits, whole grains, walnuts, almonds, lentils, beans, seeds, olives, avocados and fish are all among the foods most decisively recommended for health promotion and all are low in saturated fat. That is by no means their only virtue, but it is among them.
Studies on the cell membranes of mammals and reptiles discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids ( DHA , omega-3 fatty acid ) than reptiles  Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega-3 fatty acids as compared to omega-6 for a given body size.  This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions (H+ & Na+), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitant warm-bloodedness of mammals and birds.  However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In fish increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lower temperatures .