Fuel Utilization While Training

Fuel utilization is the heart of where nutrition meets physical performance: How does your body use food to make ATP as an expendable form of energy?

When we’re training/exercising, ATP is continuously supplied to our muscles through various metabolic pathways. These pathways vary in their fuel use and their ability to produce ATP anaerobically (without the use of oxygen). Different exercises, different training modalities/techniques and different sports prioritize different muscle fiber types and different metabolic pathways. 

During a training session or competition, three energy systems are active, at least minimally. These include the ATP-PC system, Glycolysis and Aerobic or Oxidative Metabolism. Although all three systems are active, certain movements and intensities may largely prioritize one system over the others. 

The ATP-PC system is highly favored during speed/power/max-strength movements. During strength(75-85% 1RM/1 rep max) and hypertrophy training (even lower intensity but higher sets and higher reps than strength), we get two times more ATP from glycolysis than we do from aerobic metabolism. On the flip side, aerobic metabolism yields more of our energy than glycolysis during muscular endurance and aerobic activity. Why is this helpful information? Because if speed/power is part of your sport or training plan, which it is part of movements in most sports, then carbohydrate and phosphocreatine are especially important fuel sources for you. A sports/performance dietitian can help ensure you are eating an appropriate amount of carbohydrate and fat to fuel your training plan/sport.

Let’s look into these systems in more detail:

ATP-PC (aka alactic system): This is our quickest source of ATP. Think 10-15 second 100% sprint at as close as you can get to your VO2max. There are 3 possible sources of this ATP including preexisting endogenous ATP, which we deplete in about 1-2 seconds if not replaced. When ATP/ADP ratio is low(stored energy>available energy), we need to activate the enzymes adenylate kinase and creatine kinase in order to make ATP from ADP and Phosphocreatine (PCr, which yields creatine). 

Aerobic/Oxidative Pathway: During low intensity and more prolonged exercise, we make ATP using aerobic metabolism. Although this pathway is typically not favored during endurance exercise, within months to years of training, our bodies can actually adapt to better favor this pathway at a higher energy output. This helps endurance athletes, because they are better able to use fat as a fuel source and potentially postpone the intense fatigue that accompanies glycogen depletion. How is this possible? With endurance training, physiological changes occur that actually increase the metabolic capacity of our working muscles. I’ll dedicate another post to training adaptations sometime down the road, because it’s really its own topic…. and I’m digressing…

Anaerobic Pathway: We need a sufficient oxygen supply to allow aerobic metabolism to continue. When we can no longer circulate oxygen to our working muscles fast enough, we have to depend primarily on anaerobic metabolism. This pathway produces a biproduct…. LACTATE. Not lactic acid… these are two different chemicals! Lactate is a base, not an acid. You do not build up and need to “flush out” lactic acid from your muscles during a hard training session… despite what is commonly said in the fitness world… Lactate is actually an important FUEL during exercise, especially for type 1 skeletal muscle(slow twitch) and cardiac muscle. We actually like lactate. Lactate helps us when we are short on breath.

Some Key Takeaways:

·     While lipid and carbohydrate are the main sources of fuel for ATP production, the degree to which each substrate is used varies based on substrate availability, exercise intensity, exercise duration, training adaptations and other factors. 

·     With increasing intensity, there is a progressive increase in % of energy from carbohydrate and a reciprocal decline in % energy from fat. 

·     During light intensity exercise, virtually none of the expended energy in your working muscles comes from muscle glycogen (carbs stored in your muscles). Instead, it is coming primarily from circulating free fatty acids with a little coming from muscle triacyglycerol(fat stored in your muscles) and circulating glucose. 

·     Type II muscle fibers(fast twitch), which are recruited at higher intensities, rely more on carbohydrate than type 1(slow twitch). This makes sense based on what we’ve discussed about intensity and fuel preference.

·     Longer duration(which can’t be high intensity because that’s not sustainable.. so at about 60% VO2max…) results in progressively greater reliance on fat with a reciprocal reduction in reliance on carbohydrate. Glycogen supply (muscle and liver stored carb) is limited while our fat stores (in most) are virtually unlimited. Glycogen depletion is the main cause of fatigue during endurance peformance, so by shifting towards using more fat, sustained performance is possible! 

Exercise Duration’s Effect on Metabolism:

1). First, we rely on stored intramuscular carbohydrate and fat, which makes sense because it’s already “on site” in muscle and easy to get to. However, this diminishes quickly. Although there is room for about 2,050 total kcals available in the entire body in a 145lb very lean athlete, this energy is locked into the muscle. You cannot, for example, move this energy from your arm to your leg. It also takes training and eating a certain way to maximize these stores for performance. The best way to maximize these stores? Work your muscles and then eat adequate carbohydrate post-training.

2). Blood glucose from your liver’s stored glycogen is a major secondary source, which diminishes with liver glycogen depletion (about 450 kcals of storage available in a 145lb athlete). When liver glycogen is close to full depletion, our liver starts making glucose from other substrates like protein(gluconeogenesis). This is why it’s so important to maintain adequate supply of glucose from carbohydrates/sugars for long training sessions and endurance competitions.

3). Serum free fatty acids is our major long-term source of energy, which increases with duration as other energy sources are expended. This prevents energy depletion of other important substrates (carbs!). 

*Protein doesn’t contribute much to ATP production in working muscles except as a last resort (complete glycogen depletion/starvation)… but protein is still very important for other purposes!

Final thought to leave you with: Lipid oxidation rates decrease at higher intensities. In the fitness world, it is often claimed that there is a “fat burning zone” where your body burns more fat. Indeed, research has shown that at about 65% intensity (%VO2max), you’re oxidizing the most fat(meaning absolute rate, not relative to total energy expenditure). This is partly because, at higher intensities, blood flow to adipose tissue (fat stores) decreases and redistributes to working muscles. However, just because this reduces delivery of fat to the muscle, the actual amount of fat mobilized continues to increase. Additionally, high intensity exercise increases post-exercise lipid oxidation.Therefore, although our bodies do have a “fat burning zone”, staying within this zone IS NOT THE BEST WAY TO BURN FAT and build/maintain our muscle mass. Although this is a personal opinion, I think most exercise physiologists and sports dietitians would agree with me. 

If you’d like me to provide research articles to support any of my claims above, I am happy to share some with you! However, this information was just taken from my brain and notes from a bioenergetics course I took as part of my masters degree… course’s slides were written by Edward P. Weiss PhD. 

I hope you learned something new! : )