Bears are a typical hibernation animal.
For bears, the year is divided into three main seasons: the hedonistic season from spring to autumn, the eating season from autumn to winter, and the sleeping season from winter to spring.
Grizzly bear before hibernation
（图片来源：National Park Service, via Reuters）
Do you hibernate together?
During hibernation, bears have a slowed metabolism and their body temperature drops to 1°C to 9°C. There will be a significant decrease in all bodily functions: breathing is weak, the heartbeat slows, and sensitivity to external stimuli decreases.
In addition to this, it is also very important that during this time, it will not poop or pee. Urine and urine can excrete nitrogenous compounds, because nitrogenous compounds cannot be excreted properly, and the amount of non-protein nitrogen in the bear's blood rises dramatically. Oddly enough, this does not damage the bear's kidneys or liver.
It is important to know that if the non-protein nitrogen content in human blood exceeds the normal value, a series of health problems will occur. The normal nitrogen content of blood in the human body is 20~25 mg/100 ml (whole blood). Urea contains the most nitrogen, and anything above this value indicates that there may be a problem with your kidneys. The measurement of non-protein nitrogen content in the blood is an important indicator of kidney function in clinical practice, and when the kidney fails, the non-protein nitrogen compounds in the blood will increase. This is one of the reasons why humans can't hibernate, because the metabolic rate is too slow, resulting in excessive accumulation of harmful substances in the body.
Xiong: I'm fat and I'm healthy, can you?
And, there is a magical place for bears.
As mentioned earlier, before the start of hibernation, the bear will eat a lot and become a big fat man. Alaskas Katmai National Park in the United States even held a "Fat Bear Week" event for bears to see who eats the most, so that netizens around the world can vote for who is the fattest cub. The winner of 2019 was Bear No. 435 Holly, which is a sister paper.
（图片来源：ABC News go.com）
If humans want to hibernate, they have to eat and drink to accumulate fat, but if people want to eat a big fat person like Holly, something will happen. When humans gain a large amount of weight, fat, liver, and muscle tissue cells become less sensitive to insulin, and insulin levels rise. The increase in insulin, in turn, prevents the breakdown of fat cells, creating a vicious cycle that eventually develops into insulin resistance. High levels of insulin and sugar in plasma caused by insulin resistance often lead to metabolic syndrome, gout, and type 2 diabetes.
The mechanism of type 2 diabetes (Image source: Frontiers)
At this point, the bear is very enviable, jealous and hateful.
Scientists have observed blood sugar levels, insulin levels, body weight, and other metabolic markers in bears before, during, and after hibernation. It was found that after gaining more than about 45 kilograms, their cells remained insulin-sensitive, and their blood sugar and insulin levels remained stable. Although after hibernation begins, bears experience a temporary, seasonal period of insulin resistance. But when spring came, everything was back to normal.
So every spring when the bear wakes up, it doesn't have health problems because of hibernation, and although it is a little groggy at first, it is generally healthy. In simple terms, they are a healthy fat person although they are in the middle of hibernation. What an angry SKR person!
Finally, after the bear woke up from hibernation in the spring, except that the fat was almost gone, it looked thinner, and the muscles basically did not change, and it was able to recover again by eating. However, if a person lies in bed for too long, the problem of muscle wasting will occur. If a person is lying still all the time, the muscles will decline at a rate of 10-15% per week. Of course, this reduction will not last forever until it is at a par with your energy gain for the day.
Comparison of before and after weight gain for the same bear
(Image source: Washingtonpost.com)
Why do bears hibernate without problems? It's all about genes
So why don't bears have these hibernation problems?
A team of researchers at Washington State University published a study in the journal Communications Biology in September 2019 explaining what happens to the cells of hibernating grizzly bears.
The university is the only W.S.U. Bear Center in the United States, with a total of 11 bears. The grizzly bears were either kept in captivity or transferred after being identified in the wild as bears with questionable survival.
灰熊（图片来源：WSU Bear Center）
The researchers took samples from the liver, fat and muscle tissue of six captive grizzly bears three times a year. Then in the lab, the researchers analyzed the DNA of the samples to see how the cells changed over the course of the year. Studies have found that hibernation affects the cells of each tissue differently.
Among them, the gene expression difference in adipose tissue of bears was greatest during hibernation, while muscle tissue was almost unchanged.
The difference in fat expression in bears is much greater than that between muscle and liver
(Image source: Document 1)
It was also found that even in the fat cells of the same bear, most of the genes in the fat cells changed their expression levels at different times of the year. Keep in mind that when researchers previously observed Madagascar dwarf lemurs hibernating, only a few hundred genes in their fat cells seasonally changed their expression levels. In other words, if we want to get bear-like hibernation through gene editing, it's very difficult because you can't modify one or two genes......
In these genes, the expression of functional genes related to insulin signaling, muscle protein degradation, and urea production decreases, while the expression of genes related to muscle protein synthesis increases. This explains why grizzly bears keep their livers and kidneys healthy during hibernation, and their muscles do not atrophy. After all, it's still genes!
Although humans may use the ability to hibernate in the future, such as interstellar travel or something, it is a matter of the future, and it is still difficult to perfectly control the metabolism of human cells and maintain a relatively healthy level at this stage.
Source: Compound of Science