Hint: intermittent fasting helps, and you don’t need to give up carbs
You may have heard the term metabolic flexibility recently. The concept isn’t new, but the idea has gained more traction on popular websites, as being metabolically flexible is becoming a key marker of health.
Metabolic flexibility’s formal definition is the ability of an organism to respond or adapt according to changes in metabolic or energy demand, as well as the prevailing conditions or activity. That’s a lot of scientific speak to say metabolic flexibility is how well your body can switch from using carbohydrates to fats for energy depending on what you’re asking from it at the moment (i.e., working, exercising, sleeping).
Every time you eat something, your body will do one of two things with those calories. It will either burn them for energy or store them for later use. Excess carbs are stored as glycogen (which your brain and muscles use for quick energy) or fat if your glycogen stores are full. Excess fat is stored as fat tissue.
People with a high degree of metabolic flexibility can easily switch back and forth between using carbs and fat depending on what their body needs most at the time.
When someone is metabolically inflexible, they have a hard time burning stored body fat and can become insulin resistant. In fact, metabolic inflexibility plays a role in various diseases. I’ll explain why this happens and what you can do to teach your body to burn more stored body fat throughout your day.
The Key is Understanding Insulin’s Role
Insulin is the body’s main fuel selector switch and the key to understanding what happens when metabolic flexibility goes south. Insulin is a hormone that promotes the absorption of glucose from the blood into the liver, fat, and skeletal muscle cells.
Think about it this way. When you eat, insulin levels go up. When you’re fasted — meaning a period without eating (sleeping counts), your insulin levels go down. Here are the key points to remember:
- When insulin is high, your body uses carbs for energy.
- When insulin is low, your body uses fats for energy.
We can eventually become insulin resistant when this series of events happens:
- A lot of blood sugar enters the bloodstream through food.
- Your pancreas pumps more insulin to get the blood sugar out of your bloodstream and into cells.
- Insulin is knocking at the door, but eventually, the cells stop letting it in. Then, insulin has to knock louder and louder (meaning the pancreas makes more insulin to get to the cells to respond).
- Eventually, blood sugar rises to an unhealthy level.
When insulin is knocking at the door, and we stop letting it through, that’s how we become insulin resistant. At this point, we are metabolically inflexible.
Of course, not everyone gets this far. Regardless, we can use periods of not eating to promote our body's ability to burn more fat. Remember — not eating allows insulin levels to drop. When insulin is low, we burn fat.
As a society, we’ve grown uncomfortable with hunger. We freak out. When the pantry is loaded with snacks, and it’s only a short walk to the kitchen, we find ourselves eating all day long.
If you find you can’t go more than a few hours without eating before you start to feel awful, your body isn’t good at functioning on low insulin or burning body fat for energy. You may have to work to increase your time between meals slowly. You can start by cutting out snacks or extending the amount of time before eating breakfast in the morning.
When the Body Prefers Carbs and When it Prefers Fat
A concept known as the crossover effect, shown in the graph below, demonstrates that our bodies switch from burning a higher percentage of fat to carbs at a certain level of intensity. Aerobic power is simply a measure of how hard your body is working. Zero is when you’re sitting or lying still; 100 is an all-out effort like a sprint.
Metabolic flexibility is how well you can move back and forth between the two fuel sources. The good news is you can use the crossover effect to teach your body to burn more fat. Here’s how:
- If you don’t exercise, start. Exercise is the number one way to boost your metabolism and promote fat-burning. To continue the analogy, when you use your muscles during exercise, insulin doesn’t have to knock as loud for muscle cells to let it in. Exercise doesn’t have to be complicated. See #2. Also, add some components of resistance training a few days a week.
- Embrace daily walking, especially first thing in the morning. Do this before eating while your insulin is at its lowest. Low insulin = more fat burning.
- Even for moderate to advanced athletes, training at low to moderate intensity can help push the crossover point further to the right — meaning you’re able to burn fat longer before your body switches energy sources. If you’re not sure what low intensity means, keep your heart rate around 50–70% of your max heart rate. Your max heart rate is generally around 220, minus your age.
- Continue to eat carbs (or add them back) for your high-intensity exercise. You’ll feel and perform better with healthy carbs on board. You can also consider eating most carbs before and after your workout, as this is when your body is most primed to use them. Have a sweet tooth? Eat your candy before your workout.
Fasted Low-Intensity Cardio
A meta-analysis of 27 studies concluded that aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state.
Translation: fasted cardio pushes your body to burn stored body fat. Low insulin + low intensity exercise = more fat burn.
I don’t recommend high-intensity work on an empty stomach. You may get away with a few times, but mostly you’ll feel like dog poo — no energy, and you can’t push as hard. I also don’t recommend lifting weights fasted. Protein before a workout will increase muscle protein synthesis — the process by which our bodies repair muscle tissue torn by exercise. This is how our muscles grow.
What About the Keto Diet
Despite ranking as the worst out of 35 diets by U.S. News and World Reports, the low to no-carb approach remains incredibly popular.
Why so popular? Because most people lose weight — and fast. Each gram of glycogen — the stored form of carbohydrates in the muscles and liver — is accompanied by three to four grams of water. Drive your muscle glycogen super low without replacing it, and you’ll drop water weight.
I’ve seen clients lose up to eight pounds in a week. That’s motivating! Forget that giving up carbs (keto diets require 15–30g of net carbs per day) long-term is incredibly hard and that it rules out healthy foods like fruits, whole grains, and starchy vegetables.
There is also truth to the claim that your body will burn more fat by eating mostly fat and protein. The keto diet is about 70–75% fat. The problem is few people stick to it long-term. It turns out giving up carbs forever is hard.
Also, by definition, you are training your body to be metabolically inflexible. When you give up carbs for a period, your body becomes less efficient at using them whenever you add them back. We evolved to eat both fat and carbs for fuel, so why not train your body to do both?
I get it. That’s a lot of scientific information. If you want to cut to the chase and start pushing your body to promote more fat, here’s what to do:
- Exercise to improve your metabolism and grow muscle
- Slowly extend the amount of time you can go between meals. There are many ways to try intermittent fasting. I prefer to stop eating at around 7 p.m. and eat breakfast at around 10 a.m. I also do a 22–24 hour fast once per week. Work up to it, or it will be crazy difficult in the beginning.
- Mix up your exercise to include both low to moderate-intensity and high-intensity work.
- Try some form of fasted cardio. The easiest is a daily morning walk.
That’s it. If you’ve tried many different ways to lose body fat and none have worked, give these tips a shot. It won’t hurt, and you may find yourself dropping that stubborn body fat after all!