Long duration, low intensity exercise is the best way to burn fat, right? Look at the workout categories on your treadmill or elliptical. Where is the “fat burning zone”?
It’s true that our bodies prefer to use fat for energy whenever it can. At rest, our bodies are working almost exclusively through fat utilization, while some organs like our brains sip on a small amount of carbohydrates in the form of glucose.
During exercise, we still use fat for fuel. But as we increase intensity, our muscles start to use more and more carbohydrates (and relatively less fat) to produce the energy needed to move. This is because carbohydrates are easier and faster to break down at higher intensities, and our muscle fibers that produce more power really love carbs. It has nothing to do with “efficiency” or oxygen availability – our metabolic efficiency doesn’t really change, and oxygen is always available on planet earth!
At rest about 99% of our energy comes from breaking down fat; at max exercise it’s 99% from carbs. Given this information, it seems pretty obvious that we burn the most fat when we are moving at lower intensities. However, that’s just not the case.
Here’s why: you burn more calories when you’re exercising at higher intensities. And although the relative amount of fat burning decreases as we increase exercise intensity, it’s a smaller percentage of a much bigger number.
Confused? Let’s do the math: While you’re burning around 4 calories per minute at low intensities (20-30% max, or zone 1), you’re using around 15 calories per minute at higher intensities (75% max, zones 3-4). So even though almost all 4 of those calories at low intensity are coming from fat-burning, a little less than half of those 15 calories at the higher intensity are still coming from fat. 6 is greater than 4. So the real “fat burning zone” actually happens at around 60% of your max effort. Not at those low intensities like your elliptical says.
Here’s some more cool facts: higher intensity exercise actually use a little more fat after exercise while you’re recovering. And the more endurance-trained you become, the more fat you can access and utilize for energy, even at higher intensities. This is one of the main training adaptations that keeps endurance athletes moving harder for longer periods of time: fat has more energy, and we can store a ton of it.
So to lose weight (and keep it off) should you get to as many high intensity workout classes you can?
Probably not. If your goal is to lose weight, things become a bit more complicated. We can discuss that in another post – or ten. But consider this: if you do a high intensity blaster on Monday, then need to lay on the couch Tuesday and Wednesday to recover, how many calories did you really burn? The best research suggests that successful weight loss involves long-term caloric deficits and consistent movement throughout the day, every day; for the rest of your life.
So what exercise intensity is best to burn fat? It depends on your goals! If you’re an endurance
competitor, push yourself at higher intensities to get those metabolic training adaptations. But if you’re trying to maintain your health and fitness, and maybe lose a few pounds, hit those higher intensities when you’re feeling good, and move as much as you can every day!
Author – Kevin Fasing is an Exercise Physiologist in Denver CO. An athlete, coach, scientist, and avid dog-walker and bike commuter, he believes everyone can train better through personal data collection and the information to interpret it.