It should come as no surprise that every cell in your body needs energy. The form of energy cells produce and use is called ATP, the energy currency of all cells. Mitochondria inside cells make ATP from the food that you eat, specifically the three macronutrients, carbohydrates, fat, and protein. Under normal circumstances, your cells use carbohydrates and fat as a fuel source. Only when you’re in a fasted state, does protein become a significant source of fuel. However, you need protein to preserve and build muscle tissue.
You may have heard the term “metabolic flexibility” used and wondered what it means. Metabolic flexibility, simply put, is the ability of cells to switch between using carbohydrates and fat as a fuel source. If you have a high level of metabolic flexibility, you can make the transition from using carbohydrates as fuel to fat and vice versa easily.
Why is it important to be metabolically flexible? If you participate in any kind of endurance exercise or events, you can go longer without “hitting the wall” if your body can easily tap into stored fat as a fuel. By using fat as the primary fuel source during a sustained period of exercise, you don’t tap into glycogen stores as much and don’t get fatigued as quickly. Your performance is better and this applies to any type of moderate-intensity exercise – a step class, spin class, aerobic etc. In contrast, someone who’s less metabolically efficient, can’t make the switch to burning fat as rapidly and will burn more glycogen and fatigue faster.
Being metabolically efficient also has consequences for weight control and health. If you’re metabolically flexible, when you eat a high-fat meal, you can more easily burn the fat rather than storing it. Likewise, when you eat a high carbohydrate meal, your body can burn the glucose rather than it staying in your bloodstream and increasing your blood glucose level. Metabolic flexibility means you can quickly switch to burning the macronutrient source that’s available.
Let’s suppose you fasted for a time. What should happen is your body taps into fat stores to supply cells with energy. However, if you’re not metabolically flexible, your body isn’t as adept at using those fat stores. As a result, your body demands glucose, which you’re not supplying it with. In response, you feel fatigued and your hunger level skyrockets as your body demands more glucose. Because glucose needs aren’t being met and your body senses starvation, your resting metabolic rate slows. Even short periods of fasting may be difficult if your body can’t easily switch between using glucose and fat as fuel.
How Do You Know if You’re Metabolically Flexible?
Are you wondering how metabolically flexible YOU are? You can measure metabolic flexibility in a lab setting using indirect calorimetry. This test monitors how much oxygen you inhale versus how much carbon dioxide you exhale. Using this information, you can calculate a respiratory exchange ratio, the ratio of how