There are two types of fat in the human body: brown fat and white fat. White fat has the job of storing excess calories. Brown fat generates body heat by burning calories.
A study published in Experimental Physiology found that stress stimulates the activity and heat production by brown fat associated with raised cortisol.
Researchers gave a math test to five healthy lean women in the first experiment, and then a relaxation video in the second run. Their stress responses were monitored through cortisol saliva tests and their brown fat activity was assessed through thermography. Although the actual math tests did not elicit an acute stress response, the anticipation of being tested did, and led to raised cortisol and warmer brown fat. Both were positively correlated, with higher cortisol linked with more fat activity and thus more potential heat production.
Prof Michael E Symonds from The School of Medicine, University of Nottingham and Co-author of the study comments,
‘Our research indicates that the variation in brown fat activity between individuals may be explained by differences in their response to psychological stress. This is important as brown fat has a unique capacity to rapidly generate heat and metabolise glucose.
‘Most adults only have between 50-100 g of brown fat but because its capacity to generate heat is 300 times greater (per unit mass) than any other tissue, brown fat has the potential to rapidly metabolise glucose and lipids. There is an inverse relationship between the amount of brown fat and BMI, and whether this is a direct consequence of having more active fat remains to be fully established.
‘A better understanding of the main factors controlling brown fat activity, which include diet and activity, therefore has the potential to introduce sustainable interventions designed to prevent obesity and diabetes. In future, new techniques to induce mild stress to promote brown fat activity could be incorporated alongside dietary and/or environmental interventions. This is likely to contrast with the negative effects of chronic and more severe stress that can contribute to poor metabolic health.’