When we think about muscle repair, growth and performance, protein intake and training usually take centre stage. Yet one of the most overlooked drivers of muscle health is metabolic function, specifically insulin sensitivity and metabolic flexibility.
Insulin is a key anabolic hormone. It helps shuttle glucose and amino acids into muscle cells, supporting glycogen replenishment, protein synthesis and recovery. When insulin sensitivity is high, muscle tissue responds efficiently to both nutrients and training stimulus. However, in states of insulin resistance, this process becomes impaired.
Insulin resistance reduces glucose uptake into muscle and blunts the muscle protein response to both food and exercise. Research shows that insulin resistant individuals experience reduced activation of key anabolic pathways such as mTOR, alongside increased muscle protein breakdown. Over time, this contributes to poorer muscle repair, slower recovery, reduced strength gains and accelerated loss of lean mass, particularly with ageing.
Metabolic flexibility refers to the body's ability to switch between fuel sources, primarily fat and carbohydrate, depending on availability and demand. A metabolically flexible person can efficiently burn fat at rest and during low intensity activity, then transition to glycogen use during higher intensity exercise. Poor metabolic flexibility, often seen alongside insulin resistance, limits this adaptability.
When metabolic flexibility is compromised, muscles rely more heavily on glucose, fatigue earlier and struggle to sustain performance. Glycogen replenishment is impaired, mitochondrial efficiency declines, and inflammation increases. This creates an internal environment that is less favourable for muscle maintenance and growth, even when training volume and protein intake appear adequate.
From a body composition perspective, insulin resistance and poor metabolic flexibility promote fat storage while simultaneously impairing lean tissue preservation. Chronically elevated insulin levels drive lipogenesis (conversion of excess energy into fat), while muscles remain metabolically under-fuelled and less responsive to anabolic signals. This explains why many people struggle to lose fat or build muscle despite exercising regularly and eating what appears to be a healthy diet.
Addressing insulin sensitivity and restoring metabolic flexibility is therefore foundational to improving body composition. Strategies such as resistance training, appropriate carbohydrate intake, sufficient protein, improved sleep, stress management and periods of low insulin demand all improve insulin signalling at the muscle cell. Over time, this enhances nutrient partitioning, directing energy toward muscle repair and growth rather than fat storage.
Ultimately, strong, resilient muscle is not just built in the gym. It is built through metabolic health. By prioritising insulin sensitivity and metabolic flexibility, we create the internal conditions that allow muscle to adapt, perform and endure across the lifespan.
