Food has a big role in culture, society and quality of life — obviously, it's delicious! — but, ultimately, food is fuel. The carbs and fat in food both serve as sources of fuel for your cells and tissues, and help you power through your day-to-day activities, including tough workouts. Your body burns fat and carbs slightly differently — especially during exercise, with carbs burned more during higher-intensity work. But it's the total calories you burn that matters the most for weight control — not whether those calories came from carbs or fat.
Burning Fat vs. Burning Carbs
Both carbohydrates and fat can serve as sources of energy — and they're both preferred as fuel over protein, which you can get from your diet or by breaking down muscle tissue. You'll get carbs for energy from blood glucose, a simple sugar, or stored glycogen — a large carb molecule made of hundreds of glucose units arranged in branched chains.
Your muscles and liver keep a store of glycogen for almost-immediate energy and start using glycogen for fuel as your muscles work hard — for example, during a workout. Your cells can also pull sugar from your bloodstream and convert it to usable energy.
You can get energy from fat as well. After you eat a fatty meal, fat gets broken down into fatty acids, which get absorbed into your bloodstream and can be used for energy. Stored fat — like your body fat ― also serves as a source of fuel. When you need more energy than you get from your food, your fat cells start to break down and release fatty acids, which your other tissues use. As your stored fat cells release more and more fat, they get smaller — so you'll lose weight and look leaner.
Energy for Exercise
You can burn both fat and carbs during a workout — and you'll likely end up burning a combination of both. Blood glucose and stored glycogen offer "fast" energy — they convert to fuel quickly. Because carbs only have 4 calories' worth of energy per gram, however, your glycogen stores won't last forever. As your workout continues, you'll start burning fat. While it takes longer to convert fat into usable energy than it does carbs, fat has 9 calories per gram, so it serves as a more concentrated source of energy.
The carbs and fat you burn during exercise mean you're torching calories. For example, a 150-pound person will burn 260 calories in 30 minutes of moderate cycling on a stationary bike and 391 calories in 30 minutes of vigorous cycling. Your individual calorie burn may vary depending on your weight — the heavier you are, the more calories you'll burn — and how many calories come from carbs vs. fats during an activity may vary from person to person as well.
The "Fat-Burning Zone" Myth
What proportion of carbs vs. fat you'll burn during a workout depends how hard you're working. Higher-intensity workouts require faster energy, so you'll use a greater proportion of carbs for fuel. Lower-intensity exercise doesn't need as much "fast" energy, so you'll burn a greater proportion of fatty acids while you work out.
That's the principle behind your treadmill's "fat-burning zone" — the zone usually indicated on the cardio machine's heart rate chart. Working out in the fat-burning zone means you're keeping your intensity relatively low — so you have a relatively low heart rate — which means that, of the calories you burn, a greater proportion will come from fat.
Sounds great, right? Not really. While you'll burn a greater proportion of your calories from fat at a lower intensity, you'll burn more calories overall working at a higher intensity. That means you'll lose more weight working out at a higher intensity — and burning more calories — than staying in the so-called "fat-burning zone."
The Bottom Line
Ultimately, figuring out whether you're burning carbs or fat for energy isn't essential for weight loss — what matters is burning more calories than you eat, period. Each pound of fat stores roughly 3,500 calories, so you'll need to burn an extra 500 calories a day, on average, to shed a pound per week. Your body taps into stored fat during that time — to make up for the energy "deficit" — which means you'll start to slim down.
Instead of worrying about the proportion of fat vs. carbs you're burning, focus on living an active lifestyle and following a calorie-controlled diet filled with minimally processed, nutritious foods — think whole grains, lean protein, beans, nuts, veggies and fruits. You'll not only achieve or maintain a healthy weight but boost your health and overall well-being.
- Clackamas Community College: "Carbohydrates vs Fats"
- University of Michigan: "Weight Reduction"
- Harvard Medical School: "Calories Burned in 30 Minutes for People of Three Different Weights"
- American Council on Exericse: "Does Your Heart Rate Really Matter?"
- American Academy of Family Physicians: "What It Takes to Lose Weight"