Inter-relationships among diet, obesity and hippocampal-dependent cognitive function.

Intake of a Western diet (WD), which is high in saturated fat and sugar, is associated with deficits in hippocampal-dependent learning and memory processes as well as with markers of hippocampal pathology. In the present study, rats were trained to asymptote on hippocampal-dependent serial feature negative (FN) and hippocampal-independent simple discrimination problems. Performance was then assessed following 7 days on ad libitum chow and after 10, 24, 40, 60, and 90 days of maintenance on WD, on ketogenic (KETO) diet, which is high in saturated fat and low in sugar and other carbohydrates, or continued maintenance on chow (CHOW). Confirming and extending previous findings, diet-induced obese (DIO) rats fed WD showed impaired FN performance, increased blood-brain barrier (BBB) permeability, and increased fasting blood glucose levels compared to CHOW controls and to diet-resistant (DR) rats that did not become obese when maintained on WD. For rats fed the KETO diet, FN performance and BBB integrity were more closely associated with level of circulating ketone bodies than with obesity phenotype (DR or DIO), with higher levels of ketones appearing to provide a protective effect. The evidence also indicated that FN deficits preceded and predicted increased body weight and adiposity. This research (a) further substantiates previous findings of WD-induced deficits in hippocampal-dependent FN discriminations, (b) suggests that ketones may be protective against diet-induced cognitive impairment, and (c) provides evidence that diet-induced cognitive impairment precedes weight gain and obesity.