Our bodies will tap into different forms of stored and circulating energy depending on the intensity, type and duration of a session. Understanding how and when the body uses various energy sources allows us to fuel the body properly, and can help maximize performance.
Fats are the body’s preferred source of fuel at rest and during low intensity activity while carbohydrates (stored as glycogen in the body) are essential for high intensity exercise. Your ability to switch between and efficiently use both of these forms of energy is termed ‘metabolic flexibility’. It’s a common misconception that becoming a better fat burner is for those athletes who want to lose body fat. The aim of an endurance training program is always to increase the efficiency of both our fat and glycolytic engines. While this may lead to a reduction in body fat (if this is the goal) you are actually creating the adaptations required to increase performance.
Let’s talk more about those fuel types.
Fat Stores
Your stored fat gives you the largest bang for your buck in terms of energy. You can exercise for hours at low intensities (<60% VO2 Max) since your body has a huge store of calories stored as fat, making fat a great source of fuel for long sustained efforts. You’ll be able to travel a far distance, however, you won’t be able to travel it at max power.
Glycogen Stores
Think of fuel source number 2, stored glycogen, like your reserve tanks. Your muscles store glycogen, which can be broken down and used to contract skeletal muscle when you need it. Your liver’s glycogen “reserve tank” of stored glucose can be used to maintain adequate levels of glucose in the blood. While your glycogen is a great source of quick fuel for sprints or a few short high-intensity efforts, these reserve tanks can only hold a certain amount of fuel. The average male (70 kg) can store between 1600 and 2400 kcals of glycogen based energy in the muscle and liver combined. This amount of fuel will be significantly depleted after only about 90 min of high-intensity exercise.
If you are an athlete in a sport with repeated sprints, interval efforts, or sustained high-intensity exercise lasting longer than 45 minutes, then ingesting carbohydrates during exercise will help to spare liver glycogen stores and delay the onset of fatigue. The quantity of carbohydrates needed is largely dictated by duration and intensity of exercise.
How efficient are you?
There are two ways in which you can tell how efficient your Fat and Glycogen burning engines are.
- Continuous glucose monitoring technology (CGM)
This technology gives visibility into how the body is using energy before, during, and after an athletic event, and can take some of the guesswork out of choosing how and when to re-fuel. The ability to see how your body respond to different meals throughout the rest of the day will go a long way in explaining the peaks and troughs in sugar cravings, mood etc.
What do you want to see from a GCM?
Low intensity training (<60% VO2 Max)
Your blood sugar should not rise. In fact, you may see a brief dip in blood glucose at the onset of exercise while your body uses the readily available glucose already in circulation (it’s a quick and easy fuel source) in combination with stored fat. Since the intensity of exercise is not high enough to cue your liver to mobilize stored glycogen, after you burn through the small amounts of glucose in circulation, your blood sugar should be steady as you primarily use fat for energy.
High intensity training (>80% VO2 Max)
At high intensities, you are going to primarily be using glucose for energy. You want to make sure that you are not starting out with extremely low blood sugar; if the training starts out at a high intensity then you will be depleting your glycogen stores quickly, leaving you to rely on feeding for any subsequent high intensity efforts.
- Metabolic Profiling
Metabolic profiling will show you exactly what fuel source is powering you watts. The following is the type of graph output from an INSCYD test. As you can see when exercise intensity increases, percentage of energy obtained from carbohydrates increases (red line), while percentage of energy obtained from fat decreases (green line)
There are specific training sessions that you can integrate into your program to improve both your glycolitic and fat burning engines. There are also nutritional strategies that will help your body become more metabolically efficient. At Athlete Nutrition Coach we teach the athlete to periodize their fuel in accordance with their training duration and intensity – in other words, fuel for the work required.
The Athlete Nutrition Coach programs outline fueling strategies for varying intensities ranging from 30 minutes to 5 hours plus. These evidence based fueling strategies will improve your ability to complete the session with energy left in the tank. And our nutrition principles will enhance your metabolic flexibility to ensure you are making the correct adaptation to those sessions.