Figure 8. Schematic diagram of successive activation of defensive mechanisms against decreasing environmental cold.

Under mild cold such as 4°C exposure, activation of the BAT-nonshivering thermogenesis and modest browning of s.c. WAT are sufficient to maintain core body temperature. However, visceral WAT remains thermogenically inactive. Further decreased environmental temperature — to –10°C, for example — enhances browning of s.c. WAT and triggers modest browning of visceral fat to generate nonshivering heat. With extreme cold such as –10°C/–20°C, browning of visceral fat markedly contributes to nonshivering thermogenesis to main core body temperature. Additionally, browning of visceral fat increases insulin sensitivity and improves liver steatosis in obese mice.