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Now we are around the 80-second mark of this intense hike, but you start to slow down dramatically because of a few factors: your muscles are burning, you have run out of PCr, and you have reached the maximum capacity for glycolysis. At this point, the energy system that helps you keep going is known as oxidative phosphorylation. This system has more oxygen available to it and yields the most ATP, and it can do so for a very long time (it is the central energy system during endurance sports). This process uses two systems inside the mitochondria (energy factories inside cells) to generate ATP: the Krebs cycle (a process that generates most of the ATP during oxidative phosphorylation) and the electron transport chain (a system of rapidly moving electrons that creates kinetic energy to generate ATP). It’s important to note that the two key enzymes that start the Krebs cycle are pyruvate (from glycolysis and amino acids) and acetyl-CoA (from fatty acids). Some supplements that assist in this more aerobic environment are sodium phosphate, cordyceps mushrooms, beet juice, and green tea extract (see chapter 12, Supplements for Mountain Athletes).

During exercise, all of these energy systems are at work, but one or more are amplified depending on the intensity of your workout and the availability of oxygen (see table 6.1 for more on this). For these processes to work efficiently, especially for mountain athletes, nutrition has to be optimal—not just the food, but also the vitamins and minerals. They play a massive part in the chemical steps that lead to regenerating ATP. If you don’t get sufficient nutrition, these processes can be altered, ultimately compromising your athletic performance and even your health.

One example of how essential vitamins play a big part in chemical reactions during oxidative phosphorylation involves vitamins B2 and B3. Molecules that attract hydrogen ions such as NAD+ are actually derived from these vitamins. A lack of these vitamins, or any other mineral or vitamin, can significantly affect energy levels and body composition, and can make whatever mountain sport you’re into more difficult. That is why it’s so important to be intentional about what you eat and potentially supplement if you are aware of deficiencies.