Some types of training burn through glucose stored in the muscle tissue (glycogen) much faster than others on a set by set basis. That glycogen can only sustain you for so long under certain intensities. The longer the sets (time under tension) and the greater the intensity of the contractions the faster you’ll burn through it.
If that glycogen is not being replenished sufficiently between training sessions not only will performance drop off, but you will start creating an excess oxidative stress if you continue to attempt a metabolic training stimulus. Now, that is not to say oxidative stress is evil or to be avoided at all costs. There is a time and place where we may choose to create it, but we want to create it at the highest level of performance possible, which means you still need glycogen and carbohydrates.
The issue that arises with metabolic work on insufficient carbohydrates that results in excess oxidative stress is when it goes on for too long. Oxidative stress is inflammatory. When your muscle tissue becomes too inflamed it inhibits the uptake of glucose, which makes it much harder to replenish glycogen. It also can lead to a host of other issues like poor sleep, digestive issues, looking bloated or puffy, retaining fluid, etc.
Moral of the story, don’t do metabolic training without sufficient carbohydrates. Keep in mind, if your goal is body composition, you will still need to be in an energy deficit.
The precise amount of carbohydrates will depend on the type of metabolic stimulus and the individual, which is why we created the nutrition calculators for the N1 Training Programs specifically for each program along with guidelines for individualization.