52 Days Without Carbohydrates – Fat Oxidation in Action
Yesterday’s 60-kilometer run provided a very clear picture of where my body currently stands in fat-based metabolism—and it also showed that metabolic stability doesn’t mean pace choice comes without a cost.
Heart rate remained in a highly controlled range throughout. For long hours I ran around 130–140 bpm, which is classically the zone where fat oxidation dominates, nervous-system stress is lower, and performance can be sustained over extended periods. From a metabolic perspective, this now clearly indicates an adapted state: no runaway heart rate, no energy-level fluctuations, no “wall.”
At the same time, the pace I chose at the start was faster than what would be ideal for such a long distance. Although the heart rate still appeared low even then, the mechanical and muscular load was already elevated from the beginning. This came back to bite in the second half: increasing fatigue, slowing kilometers, and progressively higher cost on the climbs.
This is a typical phenomenon in fat-based running.
Energy doesn’t run out—but muscular and nervous-system fatigue accumulates if the pace starts too aggressively.
The climbs highlighted this particularly well. While I remained metabolically stable, muscular effort increased significantly, which naturally led to slowing over the long term. This isn’t failure; it’s valuable feedback: the optimal ultra pace is not only a question of heart rate, but also of mechanical load.
Fueling – Simple and Effective Throughout the entire run I drank only 1.7 liters of salted water, supplemented with salt tablets. No sugar, no gels, no calorie intake of any kind.
Despite this, the run was characterized by stable energy sensation, clear head, and controlled heart rate from start to finish. This is one of the greatest advantages of keto-adapted state: fueling is no longer about energy replenishment, but about keeping the system stable.
Pre- and Post-Workout I started the day with bulletproof coffee with glycine in the morning—zero carbs, no classic “carb loading.”
My first real meal came only at 6:30 p.m.
This alone shows how dramatically metabolism has changed: after a six-hour effort there was no ravenous hunger, no blood-sugar crash, no collapse. The body worked entirely from its own reserves throughout.
Where Adaptation Stands Now After 52 days without carbohydrates, I’m well into the true keto-adaptation phase. At this stage:
- Heart rate remains stable over long durations
- Fat oxidation dominates under load
- Recovery is faster
- The compulsion to refuel disappears
Yesterday’s run data confirmed all of this in practice.
What will continue to refine over the coming months is pace tolerance at these low heart rates. As the musculature and nervous system further adapt to fat-based metabolism, the same heart-rate range will allow progressively more economical movement—with less mechanical fatigue.
What to Expect in the Coming Period From both experiential and physiological perspectives, the following is typical next:
- Further stabilization of heart rate over long distances
- Better pace at the same heart-rate range
- Even faster recovery
- Reduced muscle fatigue at the end of long runs
The system is already working. Now comes the phase where it becomes truly efficient.
Summary Yesterday’s 60 km wasn’t about pushing my limits—it was a highly instructive training run.
Metabolically: stable, controlled, predictable. In terms of pace choice: precise feedback on where the optimal ultra zone lies.
In fat-based metabolism, energy isn’t what depletes first—the poorly chosen effort is what exacts its revenge.
And that makes an enormous difference.