Metabolic flexibility

Metabolic flexibility is how readily the body changes fuel source to match availability and demand — burning predominantly fat at rest and at easy intensities, shifting towards carbohydrate as intensity rises or after a carbohydrate-rich meal.

It matters because inflexibility travels with metabolic disease. People with insulin resistance tend to be stuck partway between fuels, oxidising less fat at rest and failing to increase glucose oxidation appropriately after eating. It is measured in a lab through the respiratory exchange ratio.

In practice, aerobic training is the most reliable way to improve it — endurance work increases the mitochondrial and enzymatic capacity to oxidise fat, which widens the range over which either fuel can be used well. The concept is sound; be wary of supplements and diets that claim to deliver it directly.