Muscle Protein and Strength Retention by Bears During Winter Fasting and Starvation



While maintaining the body temperature at only 5°C below normal, black bears in the winter undergo 5 months when they do not eat, drink, urinate, or defecate. They have a high hypothalamic set point regulating fall hyperphagia and a large fat reserve followed by complete anorexia with a 50% reduction in energy demands while in their den. Cardiac and skeletal muscle is preserved, perhaps at the expense of smooth muscle and labile protein reserves. Winter protein demands are reduced by (1) no arousal bouts, (2) delayed implantation by pregnant females, and (3) urea hydrolysis to avoid urinary nitrogen and water loss. Bears recycle almost 100% of their urea due to urea transporters (UTB) in the bladder and intestines which salvage urea for microbial hydrolysis in the intestine with hepatic reammination of ammonia nitrogen into new amino acids and muscle protein. Bears exhibit subtle EMG patterns and respiratory sinus arrhythmia throughout the denning period. As a result of protein conservation and muscle contraction patterns, bears during 150 days of inactivity and complete food deprivation show no cardiac left ventricular atrophy. In addition, skeletal muscle exhibits no or only marginal loss of: protein, fiber number, size, or conversion of slow oxidative (MHC1) to fast glycolytic (MHC2x) fiber composition with a concomitant retention of strength. Bears are truly adapted to long periods of food deprivation and immobility that can afford a model for the study of humans in space flight and hospital confinement.


Ground Squirrel Respiratory Sinus Arrhythmia Biceps Femoris Black Bear Skeletal Muscle Protein 
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© Springer-Verlag Berlin Heidelberg 2012

Authors and Affiliations

  1. 1.Department of Zoology and PhysiologyUniversity of WyomingLaramieUSA

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