Fitness
Fat vs. carbs: What are you really burning during exercise?
By Gary Scheiner MS, CDCES
Published: Sep. 2, 2025
2 min read
The content in this article should not be taken as medical advice. Please consult with your healthcare provider regarding your individual health needs.
If you use a glucose biosensor to track your glucose during exercise, you’ve probably asked yourself: Am I burning fat or carbs? And which is better?
The answer isn’t either-or. Your body uses a mix of both, depending on the type of activity you’re doing. But understanding how your body chooses fuel can help you train smarter, improve your metabolic health, and make better sense of your glucose patterns and trends.
The science of fuel: How your body processes food for energy
When you eat, your body breaks the food down into its basic components carbohydrates, fats, and proteins.
Macronutrient | What happens in your body | How it’s used |
Carbohydrates | Broken down into glucose |
|
Fats | Stored in fat cells |
|
Proteins | Supports tissue repair |
|
How your body decides which fuel to use
Your body is always burning a combination of fat and carbohydrates (in the form of glucose), but the ratio shifts based on exercise intensity and duration.
- Think of fat as your long-lasting, slow-burning fuel. It’s ideal for low-intensity activities like walking, yoga, or light cycling—when your energy demands are steady and your body has time to break fat down.
- Carbohydrates, on the other hand, are your quick-access fuel. Your body leans on them more heavily during higher-intensity workouts like sprinting, strength training, or HIIT. Glucose can be burned quickly for fast energy, especially when your muscles need immediate power.
So the idea that you’re either burning fat or carbs? That’s a myth. You’re always using both, just in different proportions depending on the activity.
Is burning fat better?
It depends on your goal.
If you’re focused on weight loss, it might seem logical to prioritize workouts where fat is the primary fuel. But it’s not quite that simple. Ultimately, your body uses a mix of fuels depending on intensity, duration, and your nutritional state. For example, a low-intensity walk may burn a higher percentage of fat, but a high-intensity run might burn more total calories (and stored fat) in the long run.
If your goal is performance, the priority shifts to metabolic flexibility: your body’s ability to seamlessly switch between using fat and glucose stores based on what’s needed. A metabolically healthy system can tap into carbs for bursts of energy and rely on fat for endurance and recovery.
What Stelo can tell you
While a glucose biosensor like Stelo doesn’t tell you exactly which fuel you’re burning, it does track your glucose levels 24/7, which can offer helpful clues about how your body is responding to movement.
Here’s what to watch for:
1. Stable glucose levels
If your glucose is steady during physical activity, this often means your body is relying more on fat for fuel. This is common during lower-intensity cardio.
2. A noticeable glucose drop
This might signal that your body is using up glucose rapidly, especially during moderate to high-intensity cardio workouts.
3. A glucose spike
If your glucose spikes during exercise, it typically means your liver is releasing stored glucose into the bloodstream in response to stress hormones like adrenaline. This is common during high-intensity, short-duration efforts like sprints or heavy lifting.
As your fitness improves, you might notice more stable glucose trends across different types of workouts. That’s a good sign: your body is getting more efficient at using both fat and glucose/glycogen as needed.
So, which fuel is better?
There’s no one right answer. Energy derived from burning stored fat, stored glucose (glycogen) or recently digested carbohydrates all play important roles, and none is “better” than another.
The real goal is flexibility: a body that can efficiently switch between energy sources based on the task at hand. That’s what supports performance, recovery, and long-term metabolic health.
What’s more important is that your body can access both efficiently. That’s where tools like Stelo become valuable in helping you understand how your body responds so that you can adjust your movement, meals, or timing accordingly. And when it comes to weight loss, the calories burned, and not the source of energy, is the most important consideration.
The production of this article was sponsored by Stelo by Dexcom.
Author profile
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Gary Scheiner MS, CDCES
Gary Scheiner is the owner and Clinical Director of Integrated Diabetes Services, a practice specializing in intensive insulin therapy and advanced education for children and adults throughout the world.