Grizzly bears gain weight before hibernating for the winter and seem to enter a state of “healthy” obesity without showing problems of diabetes. Their responsiveness to insulin switches from augmenting insulin sensitivity, so they are not diabetic, to a state of insulin resistance, according to whether they are hibernating, in an awakening state or are fully active.
Grizzly bears may not be the easiest subjects to study in a research experiment, but Kevin Corbit, a scientist with Amgen, which is a biotechnology company, took on the task to learn about these bears and their metabolic functions and reported the results in a recent study. While obesity is known to be a risk factor for the development of diabetes in humans, it does not seem to create the same problem in grizzly bears. The study was reported in the journal Cell Metabolism.
Each year, grizzly bears eat their way into an obese state in order to prepare for the long state of hibernation they will experience in winter. During the wintertime, the grizzly bears are not active and do not eat. The stored nutrients the bears have eaten are used by them while they are in a basically low-level metabolic state. In springtime, the grizzly bears wake up again and begin to enjoy life again. They eat during this period to both supply their nutritional needs and to restore their bodies.
The interesting fact about grizzly bear metabolism is that when they become obese before winter, their metabolism switches so that they have augmented insulin sensitivity. This insulin sensitivity ensures that they don not become diabetic.
The purpose of producing insulin is to break down the glucose in the blood so that the energy from the glucose can be used by the body. It is important that insulin is produced in the correct quantity so that the right amount of glucose is broken down.
If there is not enough insulin produced, then blood glucose levels will be too high and a diabetic state will occur. When the bears switch to augmented insulin sensitivity, they do not have systemic inflammation or hyperinsulinemia. These are two conditions which are thought to be factors in creating diabetes.
During winter, the metabolism of grizzly bears switches again. At the end of hibernation, they actually become diabetic for a while. Then, when they wake up fully and are active again, the diabetes is gone. The results of the study on grizzly bears showed that blood glucose levels and insulin levels remained very stable during each time of the year.
This was surprising to the researchers and also exciting. It means that grizzly bears have a mechanism that allows their bodies to adjust metabolic states. This prevents them from becoming diabetic. The hope is that this mechanism can be understood so that the information can be applied to diabetic treatment and prevention in humans.
The PTEN protein was also studied in the grizzly bears. This protein and the PTEN gene were linked to healthy obesity in humans in previous studies. It has been reported that humans who are missing a copy of the PTEN gene have a tendency to be obese but are metabolically healthy.
In the study on grizzly bears, the results showed that PTEN protein levels were reduced in the fat cells of obese grizzly bears but not in other tissues. It seems that grizzly bears can shut down the production of PTEN in fat tissues specifically, which has a beneficial effect. Shutting down the activity of PTEN in fat cells makes the grizzly bears more insulin sensitive and, therefore, not diabetic.
Obesity, it appears from the results of this study, may sometimes promote the risk for the development of diabetes in some animals and maybe in some humans. However, obesity, alone, may not lead a person to be at a higher risk of getting diabetes. Certainly, the study of grizzly bears may provide important insight into obesity being a factor in the development of diabetes.
By Margaret Lutze