Metabolic flexibility is the body's ability to fluently switch between substrates. Glucose, fat, ketones. Based on availability and demand.
Mechanism: Mitochondrial enzymes adapt to whichever fuel is most available. A metabolically flexible person burns fat at rest, glucose during high intensity, and ketones during fasting. Without symptoms or performance drop.
Use case: Build it via varied dietary patterns, fasted training, ketone exposure, and aerobic base work (Zone 2). Test via RER (respiratory exchange ratio) across exercise intensities.
Caveats: Inflexibility is a hallmark of metabolic disease. Most modern diets (constant carb feeding) actively erode this capacity.