Muscle typologyIn general, a relationship between muscle fibre type andobesity exists and it is characterized by fewer type I (oxi-dative) and/or more type IIb (glycolytic) muscle fibres inobese subjects than in lean individuals (18,19,22).A key factor related to heredity and lifestyle that mayinfluence fibre type is hyperinsulinaemia. Obese individualstypically exhibit elevated fasting and/or postprandialplasma insulin concentrations (23). In rodents, the induc-tion of hyperinsulinaemia with insulin infusion resultedin an increased percentage of type IIB muscle fibres atthe expense of type I fibres (24). The skeletal musclefrom obese African–American women has been shown tocontain a lower percentage of type I muscle fibres thancomparably obese Caucasians (19). This observation isconsistent with the higher incidence of obesity and greaterweight gain reported in African–Americans as well as withthe observations of insulin resistance and reduced lipidoxidation in this ethnic group. Interestingly, type I fibresare known to be more sensitive to insulin than type IIBfibres, which could explain the changes in muscle typologydue to obesity-mediated insulin resistance (18). In addition,this predominance of type II fibres with severe obesity issuggestive of a general phenotype in their muscle in favourof a low capacity for lipid oxidation and insulin resistance.Other researches have reported a negative relationshipbetween adiposity and the proportion of type I muscle fibres (25,26). In turn, changes in muscle typology couldhave potential consequences on metabolic capacity of skel-etal muscle in obesity. Indeed, skeletal muscle from obeseindividuals shows markedly lower oxidative capacity andmitochondrial content (20,27). These modifications areassociated with a decrease in lipid oxidation (26,28) andthus an increase in fat storage within skeletal musclebut also in adipose tissues, explaining the associationbetween adiposity and typological features related toobesity (22,25,26). To illustrate this hypothesis, rodentsthat gained the most weight with high-fat feeding possessedsignificantly fewer type I fibres than littermates that gainedlittle to no weight (29). This is indirectly suggested by thedata in humans, indicating that subjects with higher respi-ratory quotient (i.e. lower fat oxidation) display a morerapid weight gain (30).
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