Mice are nocturnal animals, extremely active in dim light conditions, but the obese mice moved less than normal during nighttime. Their tissue composition gradually changed, more of their bodies turning to fat, less to muscle. A CT scan showed that the fat accumulated primarily in the abdomen – similar to the way fat tends to accumulate in older people – and the individual fat droplets inside cells grew significantly. The researchers also found that mice lacking the SARAF gene had higher levels of white fat, the type related to obesity, which accumulated at the expense of brown fat, the type that is beneficial to healthy metabolism and heat production.
By the time the mice turned one (older-middle-aged for a mouse), white fat had also accumulated in their livers, causing fatty liver disease. This disease increases the risk of liver cirrhosis and cancer. These mice also suffered from hypothyroidism, which slows down metabolism and contributes to obesity. The genetically engineered mice underwent all of these changes despite eating the same diet and maintaining the same blood sugar levels as the control mice.