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At lower intensities of exercise, muscles burn a higher percentage of fat than carbohydrate, but not necessarily more total fat, or more total calories, than at higher intensities. This is a subtle distinction, but it's an important one. Here's some background to help understand why.
- You get more bang for the buck when muscles burn fat compared with carbohydrate, because fat has more than twice the number of calories (nine calories vs. four calories per gram). Fat is the high-test fuel.
- Muscles need more oxygen to burn fat compared with carbohydrate, because fat is denser.
- Less oxygen reaches the muscle when you exercise hard, or fast, and get out of breath. The term sucking wind means that you are working hard to get more oxygen in. When less oxygen reaches the muscles, a situation known as oxygen debt, carbohydrate becomes the preferred fuel because it burns completely with less oxygen.
With the above information in mind, the problem with exercising slowly and at a lower heart rate to burn more fat is that you end up burning fewer total calories and less total fat. Consider the following example. Let's say you walk for one hour on a treadmill at 3.0 miles per hour (mph). If you weigh 150 pounds, then in one hour you burn approximately 300 calories (a 150-pound person burns 100 calories per mile), and if you walk at 4.0 mph for one hour, then you burn 400 calories. Now, let's say you burn 70 percent of your calories from fat at 3.0 mph. That would be .70 x 300, or 210 calories from fat. Now, let's say you burn 60 percent of your calories from fat at 4.0 mph. That would be .60 x 400, or 240 calories from fat. You can see that at 4.0 mph you burn less fat by percent but more total fat (and more total calories).
As for which form of exercise (slow or fast) will help you lose weight faster, it's important to know that to lose weight you must consume fewer calories than you burn no matter how much exercise you do. Even if you run a marathon every day, you will not lose any weight if you consume more calories than you burn. So, whether it's fast or slow exercise, you still must consume fewer calories than you burn to lose weight. And just for the record, exercise is the single best predictor of maintaining weight loss (it's pretty much guaranteed that you will regain lost weight if you remain sedentary), but the more efficient way of losing weight is to reduce your calorie intake.
With all of this said, I encourage people to maximize the calorie burn during their workouts as long as they keep it within their physical limits and don't risk injury. I suggest this because it does burn more calories and contributes some to weight loss, but even more importantly, it makes you more aerobically fit. And the more aerobically fit you are, the more efficient the muscles will be at burning fat and burning more calories overall.
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care