Brown adipose tissue is found in almost all mammals, including humans. Its cells produce heat by oxidizing sugars and fatty acids. The thermogenic protein UCP1* plays a key role in this process in order to maintain body temperature when it is cold. In the cold, another hormone is produced in the brown fat, the fibroblast growth factor 21 (FGF21)**. “The media describe brown fat as the Holy Grail in obesity and diabetes research, and FGF21 as a reinforcing hormone,” reports Dr. Martin Jastroch. He is head of the “Mitochondrial Biology” Research Unit at the Institute for Diabetes and Obesity (IDO) at the Helmholtz Zentrum München. Jastroch continues, “Our study refutes this view by showing that FGF21 has no effect on brown fat’s heat generation in cold environments.”

Thermogenesis also possible without UCP1 and FGF21
“In this study, we use new mouse models to show that in the cold, the increase of energy metabolism is possible without UCP1 and FGF21, which was previously assumed to have an essential role,” adds IDO researcher Dr. Susanne Keipert. Together with Jastroch and colleagues, she has examined their role in a number of organ systems. When there is no UCP1 in the mouse, more FGF21 was released, but even if FGF21 was deleted additionally, energy consumption still increases and heat is produced. “Neither UCP1 nor FGF21 are necessary in order to survive in cold climates and to increase the metabolism,” concludes  Jastroch. “Speculative views regarding FGF21 in brown fat have therefore been clarified.”

Jastroch is involved in a further publication in Science Advances that shows the loss of brown fat during evolution in a number of animal groups, although geological history predicts that they must have adapted to the cold.

Using extensive systems biology analyses of gene expression, the scientists found a number of genes that could explain alternative mechanisms of thermogenesis. Keipert adds, “Although obesity and diabetes treatment focuses on brown fat, UCP1 and FGF21, our study has opened up new perspectives for alternative approaches.”

Further Information
* Thermogenin (Uncoupling Protein 1, UCP1) is found in the mitochondrial membrane of brown adipose tissue. It is used to generate heat without muscular activity. This type of heat generation is very important for hibernating animals and in newborn mammals.

** FG21 (Fibroblast growth factor 21) is one of the so-called fibroblast growth factors, a group of proteins with various functions. The hormone is attributed with increased insulin sensitivity, increased energy metabolism and reduced obesity. Elevated FGF21 blood levels are associated with thermogenesis.

Original publications:
Susanne Keipert et al., Long-term cold adaptation 1 does not require FGF21 (n)or UCP1, Cell Metabolism, doi: 10.1016/j.cmet.2017.07.016

Gaudry MJ, et al., Inactivation of thermogenic UCP1 as a historical contingency in multiple placental mammal clades. Science Advances, doi: 10.1126/sciadv.1602878