Loss of muscle mass directly causes insulin resistance in humans, meaning the body's cells become less responsive to insulin, leading to higher blood sugar levels.

Appendicular Skeletal Muscle Mass and Insulin Resistance in an Elderly Korean Population: The Korean Social Life, Health and Aging Project-Health Examination Cohort

·Cross-Sectional Study·Human·2015

This study found that older people with less muscle mass (relative to their body weight) tend to have higher insulin resistance, which matches the claim that less muscle leads to insulin resistance, but it can't prove that one causes the other.

Association between insulin resistance and low relative appendicular skeletal muscle mass: evidence from a cohort study in community-dwelling older men and women participants.

·Cohort Study·Human·2014

This study shows that having insulin resistance makes you more likely to lose muscle later, not that losing muscle makes you insulin resistant. So it goes against the claim.

The impact of immobilisation and inflammation on the regulation of muscle mass and insulin resistance: different routes to similar end‐points

·Narrative Review·Review·2019

The study shows that losing muscle and developing insulin resistance often happen together, but they can be caused by different things, so losing muscle itself may not be what causes insulin resistance.