In particular, studies on TLR2-knockout mice placed emphasis on the fact that TLR2 inexpression is associated with better glucose tolerance and insulin sensitivity (as well as reduced levels of leptin and tumor necrosis factor) compared to normal mice in a germ-free environment and whilst intaking a high-fat diet, but in a standard (non-germ-free) environment, TLR2-knockout mice have higher levels of Bacteroidetes and firmicutes in gut microbiota than the control group and have increased lipopolysaccharide absorption, glucose intolerance, insulin resistance, and obesity. Here, TNF is linked to obesity due to melanocortin 4 receptor deficiency.