Skeletal muscle fiber composition may modify the effect of nutrition on body composition in young females.

BACKGROUND AND AIM: The aim of this study was to investigate the hypothesis that healthy, normal-weight females with greater proportions and sizes of the oxidative muscle fibers would also be characterized by a healthier body composition compared with individuals with increased glycolytic fibers, even if both follow similar nutritional plans. METHODS AND RESULTS: Vastus lateralis muscle fiber-type composition, body composition through dual-energy X-ray absorptiometry, and dietary intakes through questionnaire were evaluated in twenty-two young, healthy, non-obese females (age: 21.3±1.8yrs, body mass: 67.5±6.2 kg, body height: 1.66±0.05m, body mass index (BMI): 24.2±2.6  kg m-2). The participants were allocated into two groups according to their type I muscle fibers percentage [high (HI) and low (LI)]. The participants of the LI group were characterized by significantly higher body mass, fat mass, BMI, and cross-sectional and percentage cross-sectional area (%CSA) of type IIx muscle fibers compared with participants of the HI group (p < 0.021). In contrast, the HI group was characterized by higher cross-sectional and %CSA of type I muscle fibers compared with the LI group (p < 0.038). Significant correlations were observed between body fat mass, lean body mass, total energy intake, fat energy intake, and %CSAs of type I and IIx muscle fibers (r: -0.505 to 0.685; p < 0.05). CONCLUSION: In conclusion, this study suggests that muscle fiber composition is an important factor that at least partly could explain the observed differential inter-individual responses of the body composition to nutrition in female individuals. Increased %CSAs of type I muscle fibers seem to act as a protective mechanism against obesity and favor a healthier body composition, neutralizing the negative effect of increased caloric fats intake on body composition, probably because of their greater oxidative metabolic properties and fat utilization capacities. In contrast, female individuals with low type I and high type IIx %CSAs of type I seem to be more metabolically inflexible and dietinduced obesity prone, even if they consume fewer total daily calories and fats.

Different eccentric-based power training volumes improve glycemic, lipidemic profile and body composition of females in a dose-dependent manner: Associations with muscle fibres composition adaptations.

The present study aimed to investigate the effect of different volumes of fast eccentric-based training on body composition and lipidemic-glycemic profiles in females, as well as to explore the relationship between the change in glycemic-lipidemic profiles and the change in muscle fibre composition. Twenty-nine young females were assigned into three groups and performed 10 weeks (2 training sessions per week) of either 3 (LV), 6 (MV) or 9 (HV) sets/session of four fast velocity eccentric-only half-squats against 70% of concentric 1RM, followed by 3 maximum countermovement jumps (CMJ) after each set. Body composition, vastus lateralis fibre-type composition, and resting blood lipidemic and glycemic indices were evaluated 1 week before and after the training intervention. Significant changes in body composition, fasting glucose, HOMA-IR and blood lipids were found after training with MV and HV (p < 0.05; η2: 0.135-0.390). Significant correlations were found between muscle fibres' percentage cross-sectional areas (%CSA) and resting glycemic-lipid values (r:-0.543to 0.730, p < 0.05). Training-induced changes of glycemic-lipid profiles were highly correlated to those of type IIa and IIx %CSAs (r: -0.895 to 0.898, p < 0.05). Partial Correlations revealed a significant impact of the imposed training volumes on these correlations. These results suggest that six but mostly nine sets per training session of the imposed training stimuli are needed for beneficial changes in resting glycemic-lipidemic profiles, changes which are related to the training-induced changes in muscle fibre composition. However, these relationships are dictated by the imposed training volumes.Highlights Power training induces beneficial changes in body composition, glycemic and lipidemic profiles.Greater training volumes are needed for the healthier changes in glycemic-lipidemic profiles.Higher Type I, IIA and lower IIX percentage cross-sectional areas are linked with healthier body composition and glycemic-lipidemic profiles.Individuals experiencing the greatest increase in Type IIa and decrease in Type IIX muscle fibres cross-sectional areas after power training are those with the greatest beneficial changes in body composition, glycemic and lipidemic profiles.