Metabolic Fuel Switching in Hibernating Grizzly Bears
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Bears utilize fat during hibernation as their sole source of energy. However, as many bears in zoos are fed during the winter months we tested the hypothesis that a single macronutrient, carbohydrate, could serve as a enough fuel for bears during hibernation and thus cause them to stop using stored fats as fuel. The fed bears (N=7) received dextrose for 10 days at 53% (Jan. 2018) and 100% (Jan. 2019) of their expected daily energy cost. Unfed bears (N=4) serve as controls. Blood samples were collected from both groups of bears several days before the feeding trial began and on the day after the feeding ended. Bear tissues can use free-fatty acids (FFA) and ketones as energy sources in addition to glucose. Hibernating grizzly bears do not normally eat for 4-5 months and thus exhibit a type of starvation relying on stored fat for energy. The energy from fat is derived via a process called lipolysis. Lipolysis which takes place mostly in fat tissue (adipose) is very efficient and yields ATP (energy), glycerol, and water, in addition to FFA. However, the liver can also produce fuel ketones, through a process known as ketogenesis. In this study, I compared FFA, glycerol, and ketone concentrations in blood between the fed and unfed bears over two hibernation periods and two energy levels. Glycerol concentrations decreased with increasing energy intake reflecting a decrease in lipolysis. Concentrations increased over the same time period in unfed bears. Ketone (beta hydroxybutyrate) concentrations also decreased in proportion to the amount of energy being replaced and increased in unfed bears. FFAs also decreased but not as dramatically, potentially indicating that FFAs may have been produced by adipose. This could reflect increased adipogenesis as a result of increased insulin sensitivity following feeding. The results of this study reveal that bears can utilize carbohydrates during hibernation as an alternative fuel source during hibernation. These results may have implications for zoos when considering the feeding of bears during hibernation.