One thing is clear – you need energy to fuel your workouts. Ultimately, muscles use ATP to generate muscle contractions but the ATP ultimately comes from the macronutrients you eat. The two primary macronutrients that fuel exercise are fat and carbohydrates. Protein serves as a fuel source less than 5% of the time under normal conditions, although that can change when you’re restricting calories or carbs or exercising for long periods of time. So, fat and carbohydrates are primarily what “feeds” your muscles during exercise – but how does your body decide to use one fuel source or another at any given time? Here are four factors that impact whether your body primarily uses fat or carbohydrates during exercise.
How Long and Hard Your Workout Is
When you’re sitting in a chair or doing light, low-intensity exercise, around 85% of the fuel your body burns is fat. Only about 3% comes from protein and the rest carbohydrates. So, when you’re lounging around or doing light exercise, fat is the primary fuel source you use. The source of this fat is triglycerides stored in muscle tissue and fatty acids from the breakdown of stored body fat. That’s exactly what you want, right?
So, you’re ambling along, burning mostly fat, but what happens if you pick up the pace and intensity of your workout? At around 65% of your V02 max (aerobic capacity) where you’re starting to huff and puff, your body starts to burn more carbohydrates. That’s because fatty acids can’t be released quickly enough from fat cells to meet your body’s growing energy demands. At this point, muscle cells start to depend more on carbohydrates for fuel and that typically comes from muscle glycogen as well as the breakdown of adipose tissue.
As the intensity increases further and you’re breathing faster and faster to deliver oxygen to your muscles, oxygen ultimately becomes a limiting factor. For fatty acids to be completely oxidized to form ATP aerobically requires an abundant supply of oxygen. If oxygen delivery lags because your cardiovascular system can’t deliver it quickly enough, cells become more dependent on carbohydrates. Once you hit an intensity of around 85% of VO2 max, for example, during a high-intensity workout, muscle cells are mostly dependent on muscle glycogen and blood glucose as a fuel source to make ATP. The breakdown of liver glycogen is another source of energy as the demand for energy increases.
At some point, lactic acid starts to build up in the blood since carbohydrates can’t be completely metabolized aerobically due to inadequate delivery of oxygen and the fact that it takes more time to metabolize macronutrients through aerobic pathways. Instead, carbohydrates are converted to lactate and lactate builds up in the blood. Along with the buildup of lactate, the pH of your blood becomes more acidic, your muscles burn, and you “hit the wall” and have to stop and recover.
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