Diets high in saturated fat and added sugars—such as those common in Western countries like the U.S.—may impair memory and lower ability to control appetite. A small study conducted in Australia and published recently in Royal Society Open Science fed 102 healthy, lean adults breakfast for four days. The experimental group consumed more calories, total fat, saturated fat, total carbohydrates, and sugar, but less protein, at breakfast then those eating the control meal.
After just four days, the intervention group had significantly poorer scores, relative to the control group, on a specific memory test related to function of a brain structure called the hippocampus. Additionally, the experimental group became significantly less sensitive to the satiating effects of the breakfast: when asked to rate their hunger after the meal, the high fat, high sugar group required more calories to shift hunger and fullness ratings an equivalent amount on day four relative to day one. The authors suggest, based on prior studies in rats, that either changes in blood sugar levels after breakfast or the overall higher intake of saturated fat could be responsible for the observed impact on the hippocampus.
While this study is very preliminary, the known negative health effects of diets high in saturated fat and added sugars (rather than unsaturated fat and whole grains) is enough of a reason to limit these dietary components.