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, aerobics 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 much carbohydrates versus fat you’re burning. A value at the higher end means you’re burning more carbohydrates relative to fat and a lower ratio indicates fat burning predominates.
You might not have the time to make a trip to a lab to measure your respiratory exchange ratio. A rough indicator of how metabolically efficient you are is how well you handle exercise in a fasted state. If you wake up and do a moderate-intensity cardio workout, say jog at 40-50% of your V02 max, how do you feel? If doing this wipes you out quickly, you’re probably not very metabolically efficient and your body is having problems switching to fat as an energy source. You’re dependent on a steady supply of glucose even for moderate-intensity exercise.
As you may know, you use a higher proportion of fat as fuel during low to moderate-intensity exercise. As you increase the intensity, the ratio of carbohydrates to fat you burn goes up in proportion to how hard you’re exercising. If you can’t make the transition quickly, your performance will suffer. Now you know why metabolic flexibility affects exercise performance and if you not very metabolically flexible, you won’t perform well unless you eat carbohydrates beforehand.
Metabolic Flexibility and Health
As mentioned, metabolic inflexibility may be one driving force behind insulin resistance and metabolic syndrome as well as obesity. Research shows obese people display reduced metabolic flexibility after eating a high-fat meal. Their body can’t successfully make the switch to burning the extra dietary fat they’re taking in. In contrast, lean, metabolically flexible people can.
Here’s the good news. Exercise increases metabolic flexibility in people who are overweight or obese by restoring the ability to burn fat as fuel, assuming you exercise consistently. That’s partially because exercise improves insulin sensitivity. Insulin can be a factor when your body tries to switch energy systems to be more adaptable. When your insulin level is high, your body has a hard time using fat as fuel.
Becoming More Metabolically Flexible
Anything you can do to improve insulin sensitivity can enhance metabolic flexibility. If you’re overweight, losing weight will work in your favor. On the other hand, it’s harder to lose weight when you’re metabolically inflexible. That’s one reason exercise is such an important part of a weight loss program. It improves metabolic flexibility, so you can lose weight and, in turn, become even more metabolically flexible.
Are you a morning exerciser? Doing moderate-intensity cardio in a fasted state, first thing in the morning without breakfast forces your body to tap into fat stores and become more capable of doing so. That means your metabolic flexibility improves. However, don’t underestimate the power of strength training and weights. Weight training improves insulin sensitivity, which, in turn, can improve metabolic flexibility.
Some experts point out that periods of fasting improve metabolic flexibility since you force your body to tap into fat stores. A one-day fast might be too extreme for the average person but you may also benefit by eating an early dinner and not eating again until the next morning. Frequent snacking, especially carby snacks, means your body always has glucose available it can turn to for fuel. You want to force it to tap into fat and become better at doing that.
So, now you know why metabolic flexibility is so important to your health and fitness.
References:
J Cardiometab Syndr. 1(1):47-52, 2006.
Am J Physiol Endocrinol Metab. 2012 Dec 15; 303(12): E1440–E1445.Published online 2012 Oct 9. doi: 10.1152/ajpendo.00355.2012.
Am J Physiol Endocrinol Metab. 2008 Nov;295(5):E1009-17. doi: 10.1152/ajpendo.90558.2008. Epub 2008 Sep 2.
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