Your body uses more than one substrate for energy (ex., fat and carbohydrates). Carbohydrate use will increase as exercise intensity increases. Fat use also increases with exercise intensity, but unlike carbohydrates, fat use will decrease again at high intensities. However, fitter athletes have been shown to use more fat at higher intensities. Which substrate you use doesn't act like an off switch; instead, think of it as a dimmer that changes with exercise intensity. You're always using both fuels. I speak about this during my podcast with Erik here.
When you're training above threshold (going HARD!), your fat utilization is much lower, especially if you are not well-trained. You may have heard the term "Fat Max" before. It's essentially an intensity of V02 max when you're burning the highest % of fat in relation to carbohydrates—also known as the maximal rate of fat oxidation (use) or MFO. It's NOT the absolute amount of fat used or carbohydrate used. (See the diagram above)
The test shows that between 40-65% of V02 max, you will use 50%/50% carbohydrate/fat. This test was developed as a short-graded test. Fat Max fails to consider that as exercise duration increases, so does the % of fat used relative to carbohydrates. We all know that we can only ride very hard for so long. Imagine holding your 15km time trial pace for over 2 + hours!?
Essentially as the duration of exercise increases (think 2+ hours on the bike), the ratio starts to flip to higher fat use relative to carbohydrate. The message here isn't that you no longer need optimal carbohydrate intake. We know that sports of longer duration with bursts of high intensity require good glycogen stores and regular carbohydrate intake for optimal performance. Cycling, for example, is an aerobic sport with intensity riddled throughout and right until the finish sprint. But if we look at an ultra, 15+ hours, the reality is the predominant fuel used is fat because the intensity has to be significantly lower with that duration.
The misconceptions in mass media are that if you exercise at a lower intensity, you will burn more fat from body stores and lose more fat. And if you train fasted, when your blood sugar is low and free fatty acids are more available to use as energy, you will burn more fat and lose more body fat.
It's true that if you've fasted, you'll have more fatty acids available in your blood to use as fuel and lower blood sugar since you haven't recently consumed carbohydrates. What's most important, though, is whether you'll burn more CALORIES in a fasted or non-fasted ride. And if you'll burn more calories in an easier or harder effort. Caloric expenditure will determine your fat loss, which depends on the workout intensity and duration.
I could give ten examples of how this shakes out, but let's use a time trial.
2 Scenarios: Which will help you lose body fat?
Scenario 1: Let's say you decide to ride a 15km TT course in training, but you choose to do it at a lower intensity to burn a higher % of fat relative to carbohydrates, closer to 65-75%%, thinking "I'll burn more fat and lose more body fat this way." You ride at a pace where you're overall calories burned from fat and carbohydrates is 500 calories per hour.
Scenario 2: You decide to ride a 15km TT course in training, but you do it at a higher intensity, closer to the race pace. So now, on the fat max chart, your % of fat burned relative to carbs will be lower, closer to 50-55%, BUT your overall calories burned from fat and carbohydrates goes up, and you burn 850 calories per hour.
If you want to lose body fat, you want the option that burns more calories, not the higher % of fat relative to carbohydrates.
Fitness is a significant consideration in these situations because some athletes (for example, cyclists) can ride at a relatively high % of their V02 max (pretty hard pace) and still be sub-threshold (imagine the Fat Max at 70-75%), therefor burning a relatively high percentage of body fat relative to carbohydrates and a lot of calories. There are many nuances related to fitness, dietary intake, and environmental factors to consider in these 'scenarios.'
If we jump back to being fasted, there is something else to consider. If you're planning on doing a 2-hour ride with intervals fasted, you may very well bonk and be unable to reach the interval intensity desired. In this scenario, you may burn fewer calories than if you had just been fueled appropriately. And you could have a subpar workout. There's a lot to consider.
Generally, I don't recommend training fasted for fat loss. There are other ways you can lean out while being fueled. Fueling optimally for your training and recovery will likely give you greater returns on your training and help you avoid Relative Energy Deficiency in Sport, recover properly and support your immune system.
If you have another reason for fasted training besides fat loss, this is a different topic altogether. I'm not dogmatic about not training fasted since there is some interesting science around the adaptive response to training fasted related to metabolism. Training with low glycogen has been shown to alter your training adaptations; however, the science isn't there to show it will improve your performance. Lastly, we're talking about training here. Remember, when we talk about performance (race day!), your first choice should always be arriving at that line adequately fueled. Listen to what Erik had to say on this topic here.
Bottom line, when it comes to fat loss, burning calories matters. Don't confuse a higher % of fat burned relative to carbohydrates with total fat calorie burn and total calorie expenditure. Also, just because you are burning a higher % of fat relative to carbohydrates fasted, it doesn't mean you will lose more body fat, and you may be undermining your training. Think about your priorities.
