Fasting induces an anti-inflammatory effect on the neuroimmune system which a high-fat diet prevents.

The neuroimmunological and behavioral consequences of a high-fat diet (HFD) are not well delineated. This is especially true when short term (24 h) fasting is used as a physiologic stressor. In this study, we examined the impact of a HFD on learning and memory and depressive-like behaviors to understand how fasting impacts neuroimmunity and whether obesity modulates the response. Mice were fed diets containing either 10% (low-fat diet (LFD) mice) or 60% (HFD mice) calories from fat for 10-12 weeks. Gene transcripts for 26 pro-/anti-inflammatory cytokines and markers of macrophage activation were examined in adipose tissue and whole brain. Mouse learning and memory (spontaneous alternation, novel object) and depressive-like behaviors (saccharin preference, burrowing, forced swim) were studied in the fed and fasted state as were gene transcripts for F4/80, CD11b, interleukin-1α (IL-1α), IL-1β, IL-1R1, IL-1R2, IL-1RA, IL-6 and tumor necrosis factor-α in cortex, hippocampus and hypothalamus. In the fed state, HFD mice compared to LFD mice had reduced locomotor activity, and were adverse to saccharin and burrowed less. After fasting, LFD mice vs. HFD mice lost 18 vs. 5% of their body weight, respectively. In addition, HFD mice failed to downregulate gene transcripts for the myeloid-cell associated proteins F4/80, CD11b and IL-1α in the brain, failed to appropriately explore a novel object, failed to reduce locomotor activity and had increased saccharin consumption and burrowing. These data indicate that fasting induces an anti-inflammatory effect on the neuroimmune system which a HFD prevents. This breakdown appears linked to the IL-1 system because of the association of this cytokine with memory and learning.