Is it better to power your workouts with carbs or fats? For well-rounded athletes, the answer may be surprising: both.
It all has to do with something called metabolic flexibility, which is the body's ability to switch back and forth between using carbohydrates (glucose) for fuel and fat for fuel, Emily Field, a registered dietitian, explains. This means that it doesn’t matter whether you eat a carb-heavy or fat-heavy meal prior to training, your body will know what to do with it. In fact, you can even do a fasted workout. And you won’t feel excessively hungry, because your body can use stored fat to power you through.
It’s important to note that metabolic flexibility exists on a spectrum. Most people — especially those who are active and eat relatively well — are pretty flexible. But that doesn’t mean there isn’t room for improvement that could boost health and performance.
Ideally, everyone would be flexible to a high degree, Field says. But in our modern world, this isn’t the case.
Why Inflexibility Happens
“People can become less metabolically flexible when they develop insulin resistance, are sedentary, gain excess weight, have chronic inflammation, or eat a high-sugar, high-carbohydrate diet,” Brigid Titgemeier, a functional medicine registered dietitian, says.
For an otherwise-healthy population, the cause is often that they are emphasizing carbohydrates in their diet (for instance, by “carb-loading” before a race or training session) and not balancing them out with protein and fat.
Signs you might have room to improve include getting sleepy after eating carbs, having a midday crash in energy, or struggling to find that “go switch” or “next gear” when you want to push harder in a training session.
The Benefits of Metabolic Flexibility
For one, you won’t feel like you have to eat a certain way or at a certain frequency in order to get through the day, Field says. You’ll have wiggle room to eat a fat-heavy or carb-heavy meal without feeling exhausted or seeing your weight spike. In other words, you won’t get a cheat meal hangover.
Furthermore, you can improve your overall metabolic health and decrease risk of developing type 2 diabetes and heart disease, while also improving your energy, mood, GI health, and the overall functionality and efficiency of your body, Titgemeier says.
There are also performance perks, of course.
“Better metabolic flexibility for a Spartan athlete means they can move through longer periods of aerobic activity with stable energy, while also being competitive in obstacles that require powerful plyometric movements, for example,” Field explains.
This is because your body is accustomed to switching fuel sources when needed, helping you feel energized longer.
How to Increase Metabolic Flexibility
1. Vary Your Training
Your body can get very efficient at using either fat or carbohydrates for fuel — and inefficient with the other — if you only do one type of exercise. For instance, endurance runners may get fat-adapted because they spend so much time in the “low and slow” aerobic heart rate zone. For this reason, Field recommends mixing lower-intensity aerobic training with higher-intensity work to ensure that carbohydrates are being utilized for fuel, too. Learn how to vary your training here.
2. Start Carb Cycling
Carb cycling sounds complicated, but it’s not actually that tricky.
“I give my clients two sets of macros to help them become more metabolically flexible,” Field says.
The higher-carbohydrate set is for highly active days, when you need more carbohydrates to power you through your workouts. The lower-carb set is for days when you’re resting or doing lower-intensity workouts to encourage using fat for fuel.
3. Use Intermittent Fasting
Titgemeier says that time restricted eating, or intermittent fasting, may improve metabolic regulators. She recommends doing a 14- to 16-hour overnight fast three or more nights per week.
"Avoid going down the rabbit hole of ‘the more, the better’ mentality when it comes to fasting and restriction," she says. "There is a point of diminishing return, and pushing harder or longer is not always better.”