Fat content in mice affected by environment

A recent study found that mice living in socially enriching environments converted a greater portion of their energy-storing white fat to energy-burning brown fat, losing weight despite their increased caloric intake. These findings reinforce how social and physical environments can impact animal metabolisms.

Adipose tissue, commonly known as fat, comes in two types: white and brown. White fat, which constitutes as much as 20 percent of the body weight in men and 25 percent of the body weight in women, serves to store excess energy, cushion organs and maintain body temperature. Brown fat, which is especially abundant in newborns and hibernating mammals, expends energy to generate heat. Increases in white fat and brown fat concentrations are associated with weight gain and loss, respectively.

Functional differences between white and brown fat are predicted by differences in structure. White adipocytes contain a single, large lipid droplet that occupies most of the cell volume. In contrast, brown adipocytes contain numerous small droplets, more iron-containing mitochondria, which explains the characteristic brown color, and more capillaries to deliver greater amounts of oxygen.

While brown fat is best known for its role in insulating infants, scientists found that active brown fat is also present in adults. In addition, cold exposure and activation of the sympathetic nervous system drives the production of more brown fat. The new study, whose results are published in the September issue of Cell Metabolism, suggests that an engaging environment can provide another, perhaps more effective means to increase brown fat.

According to Lei Cao of The Ohio State University, the typical laboratory mouse leads what might be considered a "couch potato" lifestyle. Lab mice are given free access to food and water, as well as a few potential playmates. However, they are not given much else to do.

In the enriched environment, mice live in groups of 15 to 20. They are given more space as well as exercise wheels, mazes and toys. After four weeks in the enriched environment, the mice's abdominal fat decreased by 50 percent...