Obesity-associated deterioration of the hippocampus is partially restored after weight loss

Obesity is a multidimensional condition that is treatable by the restoration of a lean phenotype; however, some obesity-related outcomes may persist after weight normalization. Among the organs of the human body, the brain possesses a relatively low regenerative capacity and could retain perturbations established as a result of developmental obesity. Calorie restriction (CR) or a restricted ketogenic diet (KD) are successfully used as weight loss approaches, but their impact on obesity-related effects in the brain have not been previously evaluated.

We performed a series of experiments in a rat model of developmental obesity induced by a 12-week cafeteria diet, followed by CR to implement weight loss. First, we assessed the impact of obesity on neurogenesis (BrdU incorporation into the hippocampus), cognitive function (water maze), and concomitant changes in hippocampal protein expression (GC/MS-MS, western blot). Next, we repeated these experiments in a rat model of weight loss induced by CR. We also measured mitochondrial enzyme activity in rats after weight loss during the fed or fasting state. This study was extended by additional experiments with restricted KD used as a weight loss approach in order to compare the efficacy of two different nutritional interventions used in the treatment of obesity on hippocampal functions. By using a modified version of the water maze we evaluated cognitive abilities in rats subjected to weight loss by CR or a restricted KD.

In this study, obesity affected metabolic processes, upregulated hippocampal NF-κB, and induced proteomic differences which were associated with impaired cognition and neurogenesis. Weight loss improved neurogenesis and enhanced cognition. While the expression pattern of some proteins persisted after weight loss, most of the changes appeared de novo revealing metabolic adjustment by overactivation of citrate synthase and downregulation of ATP synthase. As a consequence of fasting, the activity of these enzymes indicated hippocampal adaptation to negative energy balance during the weight loss phase of CR. Moreover, the effects on cognitive abilities measured after weight loss were negatively correlated with the animal weight measured at the final stage of weight gain. This was alleviated by KD, which improved cognition when used as a weight loss approach.

The study shows that cognition and mitochondrial metabolism in the hippocampus are affected by CR- or KD-induced weight loss.