Yesterday I ran another 60 kilometers. This was already the third run of this length in recent times, yet it felt completely different from the previous two. Those were faster and smoother, while yesterday’s started out as a conversational, relaxed training session. The goal wasn’t pace, but rather a long-duration effort during which I could observe my body’s reactions.
Before the run, my fasting blood ketone level was 0.4 mmol/L. In the morning I didn’t eat anything, just drank a butter coffee with glycine about an hour before starting. This is a typical low-insulin starting state in which the body’s energy supply relies predominantly on fatty acids and ketone bodies. One of the purposes of these kinds of runs for me is precisely to observe how fat-based energy supply functions during long, multi-hour efforts.
The first thirty kilometers had to be run into a strong headwind. Many runners tend to underestimate the effect of wind, yet from a biomechanical point of view it’s a serious factor. In a headwind the body leans slightly forward, trunk stabilization increases, steps become less economical, and the muscles have to perform continuously greater work. This particularly taxes the quadriceps and the hip stabilizers.
Around 35–40 kilometers the first more serious fatigue appeared in the thighs. This didn’t manifest as an energy deficit, but rather as that well-known ultra runner feeling when the legs simply start to get heavy after tens of thousands of steps. At such times the cardiovascular system and energy supply often remain stable, and the real limiter of performance becomes the mechanical loading of the muscles. The quadriceps and hip stabilizing muscles perform continuous eccentric work, which over time leads to micro-damage and localized muscle fatigue. Nevertheless, in the later stages of the run I managed to partially bring the pace back up, which showed that energy supply had remained stable throughout.
During the run I drank a total of 3.5 liters of water, with 2–3 grams of salt per liter. I took in zero calories. This approach is one of the basic principles of fat-based endurance functioning: energy supply is not based on continuous carbohydrate intake, but on the body’s own energy stores.
The heart rate data became unreliable in the second half of the run because after about 30 kilometers the chest strap began giving erroneous readings due to sweat. This is not uncommon on long runs, as changes in the contact between sweat, salt crystals, and the electrodes can interfere with the measurement.
After the run my ketone level was 1.6 mmol/L. At first glance this might seem surprising – after all, if ketone bodies are being used as an energy source, one would expect the level to drop. However, during prolonged aerobic exercise the opposite often happens. Fatty acid mobilization increases, the liver starts producing ketone bodies more actively, and if production outpaces immediate utilization, the measurable blood ketone level can actually rise.
My first real meal came only around 5 p.m.: sausage and beef shoulder steak. This well demonstrates that a metabolically adapted body is capable of maintaining stable energy supply for many long hours without external carbohydrate intake.
The most important lesson from the run for me once again was that in long-distance running, energy supply is very often not the real limiting factor. The cardiovascular system and metabolism would still be capable of sustaining the effort, but the muscles eventually fatigue from the tens of thousands of repetitive movements. That’s why progress doesn’t depend solely on accumulating more kilometers. Developing muscular endurance is just as important – through strength training, hill running, stabilization exercises, or kettlebell work, for example. Long runs are therefore not only about energy supply, but also about teaching the muscles to tolerate prolonged mechanical stress.
Fat-based functioning doesn’t make ultrarunning easy, but it reliably provides one crucial thing: the body can supply energy for many hours without external carbohydrate intake. And once that system is working well, the next challenge increasingly becomes muscular endurance.