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.

Matthew During, also from Ohio State, says that the percentage of fat reduction in these mice is greater than the results a treadmill would yield. The enriched housing situation is more energetically demanding because it forces mice to deal with each other and a more complex environment. While stress is usually thought of as negative, certain kinds of stress can benefit one's health.

The study follows one conducted by the same research team last year, which showed that more complex housing had beneficial effects on cancer. The researchers also showed that an enriched environment led to enhanced learning and memory, greater resistance of the brain to insults, and improved cerebral health as defined by increased levels of neurogenesis, or the production of new neurons.

These cases pointed to an increase in the brain's production of a growth factor, known as brain-derived neurotrophic factor (BDNF). Interestingly, animals made to produce more BDNF also showed weight loss and increases in brown fat.

Cao and During also noted previously that the mice in the enriched condition showed changes in their fat tissue and became leaner than mice living under standard conditions. They attributed the leaner build to increases in brown fat.

During says that the switch from white to brown fat is usually hard to induce. Even with months of exposure to cold conditions, the degree of induction pales in comparison to what is achieved through mild changes in physical and social environments.

These results reinforce studies that show a link between loneliness and ill health, according to Cao. In addition to a sedentary lifestyle and consumption of high-calorie foods, lack of social engagement can contribute to being overweight and developing obesity.

While the reasons behind this are not yet clear, the study also showed that compared to other organs, fat is very sensitive to environmental changes. These results give some insight on the obesity epidemic, as more and more people make extensive use of online networking and social media.