A Mysterious Ingredient in Bear Blood May Boost Human Muscle Growth

Researchers from the University of Hiroshima have found that the blood of hibernating black bears contains a component that allows the creature to hibernate without any muscle loss. However, the component remains a mystery to researchers.

Haven’t you always wondered if there was something that could keep your body in shape, even if you took a well-deserved break from your gym routines? A week is fine, but stay away for a little longer than two or three weeks and you’ll be starting over. Your body simply forgets the effort you put in and loses the muscles you used previously.

If this is something that continues to bother you, you’ll be happy to know that you’re not alone. Associate Professor at the University of Hiroshima, Mitsunori Miyazaki, began his research journey with this singular goal of finding out exactly why muscles deteriorate so rapidly and what can possibly be done to stop it.

The destruction mechanism

Muscle mass in the body is the balance between muscle synthesis and the destruction process, both of which are at stake. However, when muscles are not used for a period of time, the impact of the destruction mechanism is more pronounced. Previous research studies have shown that enzymes like Muscle RING-finger protein-1 or affectionately called MuRF-1 have a role to play in reducing muscle mass.

On the other hand, insulin-like growth factor-1 (IGF-1) has been associated with maintaining muscle mass in hibernating animals like bears. Miyazaki and his team were looking at this factor, which previously varied seasonally. However, during their study, the researchers discovered that the variation may be the result of other factors and not a means of maintaining muscle mass in bears.

In search of the secret ingredient

To learn more about what helps hibernating animals maintain their shape, Miyazaki and his research team cultivated human skeleton muscle cells in serum extracted from the blood of hibernating Japanese black bears. The researchers found that the serum promoted the growth of the protein component of the cells within 24 hours of treatment. However, this growth was not observed when the cells were infused with the serum collected from the bears during the active summer months.

The researchers concluded that the ingredient that allows muscles to grow during hibernation is present in the blood during winters, not summers. The only problem is that they don’t know exactly what it is.

“By identifying this ‘factor’ in the serum of hibernating bears and clarifying the unexplored mechanism behind ‘muscles that do not weaken even without use’ in hibernating animals, it is possible to develop rehabilitation strategies effective in humans and to avoid becoming bedridden in the future,” Miyazaki said in the press release.

The research results have been published in the journal PLOS One.


Hibernating bears remain in their dens for 5-7 months during the winter and survive without eating or drinking while remaining inactive. However, they maintain their physical functions with minimal skeletal muscle atrophy and metabolic dysfunction. In bears, resistance to skeletal muscle atrophy during hibernation is likely mediated by seasonally altered systemic factors that are independent of neuromuscular activity. To determine if there are compounds in bear serum that regulate protein and energy metabolism, differentiated human skeletal muscle cells were treated with bear serum (5% in DMEM/Ham’s F -12, 24 h) collected during active summer (July) and hibernating winter. (February). Serum samples were collected from the same individual bears (Ursus thibetanus japonicus, n = 7 in each season). The total protein content of cultured skeletal muscle cells increased significantly after 24 h treatment with hibernating bear serum. Although protein synthesis rate was not altered, expression of MuRF1 protein, a muscle-specific E3 ubiquitin ligase, was significantly downregulated with concomitant activation of Akt/FOXO3a signaling. Increased levels of insulin-like growth factor (IGF-1) have also been observed in the serum of hibernating bears. These observations suggest that protein metabolism in cultured human myotubes may be altered when incubated with hibernating bear serum, with a significant increase in serum IGF-1 and a decrease in the expression of MuRF1, a potential target of Akt/FOXO3a signaling. A protein-sparing phenotype in muscle cells cultured by treatment with hibernating bear serum offers potential for the development of methods to prevent human muscle atrophy and associated disorders.

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