Mild stress stimulates the activity and heat production by brown fat associated with raised cortisol, according to a study. Brown adipose tissue (BAT), also known as brown fat, is one of two types of fat found in humans and other mammals. Initially only attributed to babies and hibernating mammals, it was discovered in recent years that adults can have brown fat too. Its main function is to generate body heat by burning calories (opposed to white fat, which is a result of storing excess calories. People with a lower body mass index (BMI) therefore have a higher amount of brown fat.