Beef Extract Supplementation Increases Leg Muscle Mass and Modifies Skeletal Muscle Fiber Types in Rats

Abstract

SummaryThe objective of this research was to investigate the effects of beef extract on fat metabolism, muscle mass and muscle fiber types in rats. We also investigated the synergetic effect of endurance exercise. Twenty-four male rats weighing about 270 g were assigned to two diets containing 0 or 6% beef extract (BE). Half the rats fed each diet were subjected to compulsory exercise (CE) for 30 min every other day. After 4 weeks feeding, the blood was collected and various organs were dissected. The muscle fiber type of the soleus and extensor digitorum longus (EDL) muscles were evaluated by histochemical and electrophoretical analyses. Rats supplemented with BE showed a decrease in fat content in liver and abdomen and an increase in the activity of carnitine palmitoyl transferase II in liver. BE as well as exercise increased the relative weights of both soleus and EDL. BE alone and BE plus CE did not affect the distribution of muscle fiber types in soleus. BE without exercise decreased in type IIb of EDL from 54% to 44% with compensatory increase in type IIa from 41% to 49% and type I from 5% to 7% compared with the nonsupplemented, nonexercised control group. No synergetic effect on a fast to slow fiber conversion due to the combination of BE and CE was detected. Thus, BE supplement increased muscle mass and slow type fiber in EDL. The effects of BE supplement on muscle characteristics were similar to those of exercise. Key Words beef extract, fat metabolism, muscle fiber type, muscle mass, L -carnitine A great deal of research has been done to elucidate the general nutritional value of beef on the growth of human subjects and the effects of protein quality and exercise on the growth and protein metabolism of animals and human beings ( 1-3 ). However, existing information about the nutritional effects of beef extract on muscle metabolism is not sufficient.Skeletal muscle is composed of at least three fiber types on the basis of their contractile and energy metabolic properties; type I fibers exhibit an aerobic energy metabolism (slow-twitch fiber), type IIb fibers are predominantly glycolytic (fast-twitch fiber) and type IIa fibers are the intermediate type exhibiting both oxidative and glycolytic metabolisms. Lipids are easily accumulated in type I fibers ( 4 ), while glycogen depletion occurs primarily in the type II ( 5 ). It is well known that cross-reinnervation, electrical stimulation or exercise evokes the fiber type conversion, although the molecular mechanisms mediating the transformation are not clearly elucidated ( 6 , 7 ). This means that skeletal muscle cells possess a capacity to respond and adapt to metabolic changes imposed by physical stimulation or neural stimulation. Concretely, endurance exercise requiring the consumption of oxygen stimulates a shift to the slow twitch, oxidative muscle fibers ( 8 ). The fiber types of adult skeletal muscle are closely related to energy metabolic characteristics as described above. Nutrition-induced regulation in mRNA levels for a number of metabolic gene...

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