Metabolic flexibility

Metabolic flexibility is your body's ability to switch fuel sources smoothly. The two main fuels are glucose and fatty acids. You switch between them as you eat, fast, and move. This skill relies on three things: healthy mitochondria, good insulin sensitivity, and intact hormone signaling. When you lose it, you see two signs. You burn fat poorly while fasting. And you handle blood sugar clumsily after meals. That loss is a hallmark of insulin resistance, obesity, and aging. It is also a key target of fasting and exercise.

Last reviewed: June 22, 2026

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