TLR4 and obesity due to melanocortin 4 receptor deficiency: Recent studies also suggest that various types of nutrients/metabolites and molecules from the gut (e.g., long-chain fatty acids, ceramides, and lipopolysaccharides via leaky gut permeability) can easily reach the brain and induce cellular stress/inflammatory responses, mainly via Toll-like receptor 4 (TLR4) during obesity development [17,18,19,20].