A lot of people talk about metabolic flexibility, but don't understand it. They may know it's the ability of the body (or specific tissues, to be more precise) to use either fat or glucose, but they don't understand what regulates this shift. Insulin is the determining factor. When insulin is high, cells will rely more on glucose (the high glucose is what spiked the insulin anyway) and, at the same time, availability of fats will be reduced (via inhibition of lipolysis). Furthermore, insulin inhibits fat burning by inhibiting essentially every enzyme involved in the process. In contrast, when insulin is low, glucose use drops (and glucose levels should normally be lower) while fats are mobilized from fat tissue. Moreover, the low insulin removes the inhibition of burning that fat for fuel. So, metabolic flexibility is simply another way to gauge the levels of insulin. Of course, in people with chronically elevated insulin, the body becomes "stuck" in glucose-burning mode. This is why people with insulin resistance have metabolic inflexibility.