Browning of white adipose tissue may be an appropriate adaptive response to critical illness

Both the baseline amount of brown adipose tissue (BAT) and the capacity to stimulate browning of white adipose tissue (WAT) may provide a protective effect to the patient in a critical care setting. Critical illness is associated with reduced mitochondrial volume and function resulting in the increased production of reactive oxygen species, greater demand for adenosine triphosphate, a switch to uncoupled fat metabolism, and hibernation of the organelle, which all contribute to multiple organ failure. Increasing insulin resistance, decreasing fatty acid oxidation, and dependence on carbohydrate metabolism result. Browning of WAT may oppose many of these adverse effects. The presence of BAT and the changes associated with browning may help dissipate oxidative stress, increase consumption and utilization of metabolites, and reduce pro-inflammatory actions. The number of mitochondria increases, and there is greater infiltration of macrophages into adipose tissue. A shift occurs in macrophage expression from the M1 to M2 phenotype, an effect which further dampens inflammation, increases insulin sensitivity, and improves tissue healing and remodeling. Any benefit from these responses may be lost in the disease states of chronic hypermetabolism (such as burns or cancer cachexia) in which the persistence of these physiologic effects may become detrimental, contributing to excessive weight loss, adipose wasting, and loss of lean body mass. This paper discusses the plasticity of adipose tissue and whether shifts in its physiology provide clinical advantages in the intensive care unit.