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.
Your muscle fiber composition influences how well your body handles insulin. Slow-twitch fibers are more efficient at glucose uptake. While you cannot completely rewrite your genetics, endurance training can stimulate molecular pathways (like PGC-1 and PPAR-α) that enhance the oxidative capacity of your muscles, effectively making them more 'slow-twitch' in function and improving insulin sensitivity. This is particularly relevant if you are at risk for type 2 diabetes or obesity.
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
Based on multiple cited human studies (Lillioja 1987, Henriksen 1990, Song 1999, Daugaard 2000) showing consistent correlations, though mostly observational/correlational rather than interventional in humans.
Source
Skeletal Muscle Fiber Type: Influence on Contractile and Metabolic Properties
Juleen R. Zierath et al. · PLoS Biology · 2004
DOI 10.1371/journal.pbio.0020348
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