What Is 'Metabolic Flexibility'?

Do you constantly feel hungry or feel tired when you fast? This may be related to metabolic flexibility — or a lack of it. Metabolic flexibility is essential to diet and energy, but what exactly does it mean?

The metabolism is always active, burning fuel to process energy (per Cleveland Clinic). The body needs energy to function even at rest, but depending on what you're doing, that energy requirement changes. Your metabolism can adapt to less or more calories or calories being present or not and still find a way to provide this energy. So if you have food in your system, it will use it. If you don't, it will switch over to burning stored fat as fuel. This ability to switch over effectively is metabolic flexibility (via Cell Metabolism).

Metabolic flexibility is the body's ability to adapt to changes in the energy the body requires and uses. For example, when exercising, the body uses large amounts of energy. Depending on what's available, it may have to switch from generating energy from food to burning stored energy to maintain physical output. This same mechanism also helps prevent blood sugar spikes after eating, keeping glucose available to be burned and distributed throughout the body as needed. This process is important and helps explain the cause of certain health conditions. Does metabolic flexibility occur naturally, and what can we do to support it?

Although metabolic resistance can occur naturally, it may not for some, leading to insulin resistance (via Cell Metabolism). Insulin resistance is a problem in which cells don't respond to insulin, so the body keeps increasing blood sugar. The condition is linked to obesity and type 2 diabetes, reports the Centers for Disease Control and Prevention (CDC). Insulin resistance can be reversed by increasing insulin sensitivity through physical activity.

Interestingly, a 2017 study found that high levels of physical activity led to greater metabolic flexibility and overall metabolic health. In contrast, a sedentary lifestyle with low energy use led to metabolic inflexibility. In an interview with Goop, Will Cole, a leading functional medicine expert and author of "Intuitive Fasting," explained, "When we lack it, we're more likely to experience fatigue, insatiable cravings, irritability and more not-so-fun states of being." He also asserts that "The single greatest tool for achieving metabolic flexibility is to fast intermittently."

Intermittent fasting is when you go without eating for extended periods to encourage the body to burn through fat for energy, reports Johns Hopkins Medicine. Doing this forces the body to switch between the two options for producing energy rather than just relying on a steady flow of food — which without exercise, can lead to excess sugar and weight. By definition, intermittent fasting essentially reinforces metabolic flexibility. It can improve heart health, memory, and physical performance. It may also reduce obesity and the severity of type 2 diabetes.