A: From Renee Eastman: If that were true, wouldn’t the slowest cyclist or runner you know also be the thinnest? The reason athletes who work harder and go faster tend to be leaner is – at least partly – because working harder and going faster burns more total calories per minute and per hour than going slower. And when you burn more total calories, you also burn more total fat. This myth about the “fat burning zone” persists because people forgot middle-school math, namely fractions and percentages.

The truth is you are always burning some combination of fat and carbohydrate, no matter how hard or easy you are exercising. When you go slower, a higher percentage of your energy comes from burning fat, but a higher percentage of a low number is still a low number. Going slow simply burns too few calories to help many people reach their weight management goals. This is especially true when athletes have only 60-90 minutes, three times a week to devote to aerobic exercise. If you had unlimited training time, riding at a low intensity would eventually take off all the weight you want to lose, and more. But you don’t have that kind of time, so you have to go faster to burn more calories.

What happens when you exercise at a harder, but still sustainable intensity? Well, you’re still burning a combination of fat and carbohydrate, but now the percentage of energy coming from fat is lower (40-60% instead of 70-80%). But a lower percentage of a higher overall caloric expenditure can still be quite a high number.

Let’s take a cyclist who is riding at the lower end of his endurance range, about 75% of his lactate threshold power output. At that pace he may be burning 430 calories per hour with 75% of those calories, or 327, from fat.

If he increased his effort to 85% of his lactate threshold power, he could increase calorie expenditure to maybe 700 calories per hour, a total increase of 270. At that intensity the contribution of fat would drop, maybe to 60% instead of 75%, but 60% of 700 is 420 calories, 93 more than he burned by riding slower for the same amount of time.

Furthermore, it’s important to note that in this example, the athlete was still well below his lactate threshold power, meaning the effort level was sustainable. The impact of intensity on fat burning is even greater when you add intervals at or above lactate threshold power outputs.

What often gets lost in the argument over low-intensity, “fat-burning” workouts vs. harder efforts is the influence these intensities have on your fitness. If you want to get faster and stronger as well as leaner, you need to provide enough training stimulus to make your body adapt. Cruising along at a leisurely pace hoping to burn more fat is only going to stimulate your body to give up and find an easy chair and pants with an elastic waistband. You want to be faster, stronger, and leaner? Ramp up the intensity, increase your caloric burn rate, and challenge your body with demanding workouts that force it to adapt.