Hormonal
A higher proportion of slow-twitch (ST) muscle fibers is associated with greater insulin sensitivity and accelerated glucose uptake, whereas a shift toward fast-twitch (FT) fibers is linked to insulin resistance.
If you are concerned about insulin resistance or metabolic health, prioritize endurance exercise. This type of training can shift your muscle phenotype toward a more insulin-sensitive state by increasing the abundance of slow-twitch fiber characteristics and oxidative enzymes. You do not need to be an elite athlete to benefit; the plasticity exists in healthy humans.
Insulin sensitivity also correlates with the proportion of ST oxidative fibers... insulin-stimulated glucose transport is greater in skeletal muscle enriched with ST muscle fibers... the ratio between glycolytic and oxidative enzyme activities in the skeletal muscle of non-insulin-dependent diabetic or obese individuals is related to insulin resistance.
Why this rating
The paper cites multiple human studies (Lillioja 1987, Henriksen 1990, Simoneau 1995) showing consistent correlations, though it is a review of observational and animal mechanistic data rather than a single RCT.
Source
Skeletal muscle fibre type: Influence on contractile and metabolic properties
Juleen R. Zierath et al. · RMIT Research Repository (RMIT University Library) · 2004
DOI 10.1371/journal.pbio.0020348
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