Lifting a weight, breaking into a sprint or even taking a leisurely walk all increase your body’s need for fuel. As you might have guessed, sprinting places a greater energy demand on your body than taking a slow walk. The “energy currency” your muscles use to fuel exercise is called adenosine triphosphate, or ATP, a molecule with high-energy bonds. Breaking the high-energy bonds that hold the ATP molecule together releases the energy that powers muscles contraction.
ATP is in every cell in your body, but muscle cells have a particularly high requirement for ATP during intense exercise. Without a constant supply of fuel to make more ATP, your body couldn’t sustain movement. Where does this fuel come from?
During high-intensity exercise, carbohydrates are the main source of fuel your body taps into. High-intensity exercise, whether it’s HIIT training or heavy resistance exercise, primarily uses carbohydrates for fuel. What is the source of these carbohydrates? Primarily muscle glycogen. Muscle glycogen is the main fuel supply for intense exercise and is a major fuel source during the first hour of exercise.
During periods of low-intensity exercise, such as taking a leisurely walk, blood glucose along with fat stores is mainly used to make ATP. Some of this glucose comes from the breakdown of glycogen by the liver. The two main organs in your body that store glycogen are muscle and liver. During low-intensity exercise and during long periods of exercise when muscle glycogen stores start to fall, liver glycogen breakdown becomes an overriding fuel supplier. Once your liver breaks down glycogen, it releases it into the bloodstream as glucose so that all the cells in your body, including muscle cells, can use it.
After a workout, your body needs dietary carbohydrates to help rebuild muscle and liver glycogen stores. If your muscles are depleted of glycogen, you’ll feel weak and tired, and when liver glycogen stores are drained, you may experience symptoms of low blood sugar – fatigue, lightheadedness, dizziness and hunger.
Two Main Sources of Carbohydrates
During low-intensity and with long periods of steady-state exercise, fat is a dominant fuel source. The stored fat in your body is in the form of triglycerides. During exercise, your body breaks down stored fat (triglycerides) to glycerol and free fatty acids. Cells can use free fatty acids to make ATP. Your muscles also store triglyceride molecules, made up of carbon, hydrogen, and oxygen. Stored triglycerides can be broken down during exercise to make ATP and used to drive muscle contractions. So, intramuscular triglycerides and adipose tissue triglycerides both supply fatty acids that power ATP production and muscle contraction.
You use both carbohydrates and fat as fuel during exercise, but the one that predominates depends on the intensity of the exercise you’re doing. As exercise intensity increases, carbohydrate use increases and fat utilization goes down.
Two Main Sources of Fat (Triglycerides)
Adipose tissue (stored fat)
Protein isn’t a major fuel source during exercise unless your body runs low on glycogen, as could happen during prolonged exercise. Normally, protein makes up only about 2% of the fuel your body uses during a workout, but when glycogen is depleted, protein utilization can rise to as high as 10%. Unfortunately, a portion of that protein comes from muscle tissue – not what you want. That’s why it’s important to keep your muscle and liver glycogen stores from dwindling during a prolonged workout.
Most people think of lactate in a negative sense because of its link with muscle fatigue, but lactate can also serve as a source of fuel for muscles, including your heart muscle. The lactate that builds up in muscle tissue during intense exercise can travel to the liver and be converted to glucose through a process called gluconeogenesis. The newly made glucose can then return to the muscle cells that need it and be used to make ATP.
How Does Your Body Select Its Fuel Sources?
Exercise intensity and duration are two major factors in fuel selection. As mentioned, high-intensity exercise (above 70% of V02 max) is mainly fueled by carbohydrates in the form of muscle and liver glycogen. Low-intensity exercise (below 30% of V02max) is largely fueled by intramuscular and adipose tissue triglycerides or fats. As more fast-twitch muscle fibers are called into play during periods of high-intensity or heavy resistance training, carbohydrate use rises and fat utilization falls.
The exercise intensity at which your body shifts from using fat as the main fuel source to carbs is called the “crossover point.” The point at where crossover occurs depends upon your level of training but is usually between 30 to 40% V02 max.
In summary, long periods of exercise, depending on the intensity, uses a mixture of fats and carbohydrates, although protein can be a secondary fuel source when glycogen stores are low. Lactate, which builds up when you exercise above your lactate threshold, is a “bonus” source of fuel that your liver can turn into glucose and send back to your energy-starved muscles.
Improving Your Exercise Performance
If you want to perform your best, whether it is HIIT training, weight training or any other form of moderate to intense exercise, make sure you’re replenishing your glycogen stores with carbohydrates after a workout. A drop in muscle glycogen leads to fatigue and a reduction in performance. You can’t grab a carbohydrate-rich snack an hour before a workout and expect it to supply your glycogen needs. It takes 24 hours or so to rebuild glycogen stores once they’re depleted. Don’t forget that the effects of glycogen depletion can be addictive, especially if you’re doing daily workouts. Research also suggests there’s a two-hour window after exercise where your muscles readily suck up glycogen. Don’t forget that post-workout snack!
The Bottom Line?
Your body can use more than one fuel source to make ATP and the one it uses predominantly depends on several factors, including exercise intensity and duration. No matter what type of workout you do, supplying your body with the proper macronutrients is essential. Make sure you’re giving the body the support it needs!
The Sport Journal. “Glycogen Replenishment after Exhaustive Exercise”
Exercise Physiology: Theory and Application to Fitness and Performance. by Scott Powers, Edward Howley. (2011)
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