Abstract
Human muscle fiber composition is heterogeneous and mainly determined by genetic factors. A previous study reported that experimentally induced iron deficiency in rats increases the proportion of fast-twitch muscle fibers. Iron status has been reported to be affected by genetic factors. As the TMPRSS6 rs855791 T/C and HFE rs1799945 C/G polymorphisms are strongly associated with iron status in humans, we hypothesized that the genotype score (GS) based on these polymorphisms could be associated with the muscle fiber composition in humans. Herein, we examined 214 Japanese individuals, comprising of 107 men and 107 women, for possible associations of the GS for iron status with the proportion of myosin heavy chain (MHC) isoforms (I, IIa, and IIx) as markers of muscle fiber composition. No statistically significant correlations were found between the GS for iron status and the proportion of MHC isoforms in all participants. When the participants were stratified based on sex, women showed positive and negative correlations of the GS with MHC-IIa (age-adjusted p = 0.020) and MHC-IIx (age-adjusted p = 0.011), respectively. In contrast, no correlation was found in men. In women, a 1-point increase in the GS was associated with 2.42% higher MHC-IIa level and 2.72% lower MHC-IIx level. Our results suggest that the GS based on the TMPRSS6 rs855791 T/C and HFE rs1799945 C/G polymorphisms for iron status is associated with muscle fiber composition in women.
Highlights
IntroductionType II muscle fibers are further divided into type IIa and IIb/x [1,2]
Several previous studies have reported that chronic hypoxia induces a muscle fiber composition shift from slow- to fast-twitch fibers in rats [22,23,24,25]. Chronic hypoxia, such as that associated with chronic obstructive pulmonary disease (COPD) has been shown to increase the proportion of type II muscle fibers in affected individuals compared with healthy controls [26]. These results suggest that low iron status and hypoxia, which is occasionally induced by low iron status, have been implicated in an increase in the proportion of fast-twitch muscle fibers
The results of the present study revealed that the genotype score (GS) for iron status was associated with muscle fiber composition in women
Summary
Type II muscle fibers are further divided into type IIa and IIb/x [1,2]. The human skeletal muscle is an extremely heterogeneous tissue composed of different muscle fiber types. A previous study has reported that large differences in muscle fiber composition (i.e., 15−85% type I fibers, 5−77% type IIa, and 0−44% type IIb) of the vastus lateralis muscle exist in the individuals of a general population [5]. This variation in muscle fiber composition explains the marked differences in physical performance among individuals [6,7]. It is important to understand the factors that determine muscle fiber composition, which can be used for talent identification in athletes to predict the future capacity of individuals or defining the optimal training regimen for individuals, as well as for preventing metabolic diseases
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