Whether you just started to work out or you are a professional athlete, there is one thing that puts a damper on your routine: Fatigue! There is nothing more frustrating than feeling at your physiological limit during the last meters of your run, or starting to feel dizzy half throughout your training at the gym. What in your body leads to fatigue? Your glycogen stores are depleted! Read on to learn what happens in your body during exercise that leads to fatigue and how to avoid it during high or low intensity training.
Let’s start with some background knowledge…
Generally speaking, the fuel for your muscle is coming from carbohydrate, stored as glycogen in muscle, and fat, stored as triglycerides within adipose tissue and muscle fibers. However, depending on the intensity of your workout, your body will either use your fat or carbohydrate stores as predominant fuel.
During high-intensity exercises, such as sprints, weightlifting and push-ups, your body needs to produce energy rapidly and under anaerobic conditions, since your body has hard times to deliver oxygen to your muscles fast enough. Since your muscle glycogen can rapidly be burned with a lack of oxygen, it is the main fuel source during exercise.
During low to moderate intensity workouts, such as long-distance running, cycling and yoga, you are burning a greater percentage of fat compared to carbohydrates (which doesn’t mean that you automatically lose more weight, since it is still the calorie net deficit that counts – but that’s another topic).
In a nutshell: The higher the intensity, the more are your glycogen stores used for fuel compared to your fat stores; and the lower the intensity, the more are your fat stores used in relation to your glycogen stores.
Fatigue is the result of depleted glycogen stores…
What happens if you run out of your glycogen stores while exercising? Your performance will be impaired1! That is because your body will lack its supply of carbohydrates from its stores, leading to low blood glucose levels (hypoglycaemia) and you may experience extreme fatigue, and even dizziness.
This basically means that the glycogen content in your muscle determines how long you can perform heavy exercise2. And filling these glycogen stores is solely possible through sufficient carbohydrate intake.
Your daily food should fuel your glycogen stores ….
So filled glycogen stores are crucial for optimal performance, especially for high-intensity and prolonged exercises3. In fact, when our body runs out of glycogen, it might start to provide energy by converting your muscle protein into glucose, which results in a decrease in muscle mass – and that’s the least you want! Therefore, make sure you ingest enough carbohydrates to have sufficient stores of glycogen before your workouts.
But how much is enough? An average 68 kg person can store 1400 kcal of carbohydrates in the muscle4, which should be enough energy supply for minimum 90 minutes at the gym. But if your carbohydrate intake is low, these stores are low as well and if you then hit the gym, you might not see the results you desire. According to Nancy Clark’s Sports Nutrition Guidebook4, 55-65 % carbohydrate intake or 9 grams carbohydrate per kilogram body weight daily prevents chronic glycogen depletion and will make you train at your best.
Pre-workout: What about timing?
Research shows that consuming sugar less than 1 hour before exercise can lead to a drop in blood glucose right at the start of your training, which can impair your performance5. Therefore, eat carbohydrates 2-3 hours prior exercise and choose foods with low glycaemic index, such as complex, fiber-rich carbohydrates (whole wheat bread, oatmeal, rice, barley, legumes, lentils, fruits and vegetables), to avoid unnecessary cravings that will make you eat again before your workout.
Post-workout: Protein for muscle recovery and carbs for energy replenishment…
After a proper workout – and proper means you were boosting your heart rate for an hour or longer – your glycogen stores are low or even depleted and your muscle fibers are damaged. Carbs replenish the depleted stores while protein is needed to repair and build muscle.
In fact, research shows that after exercise your body is building up muscle to a much greater extent than at rest6, implying that protein intake immediately after exercise may be more effective than when ingested at a later time point. So if your aim is to build up muscle quickly, consider eating a protein snack sooner than later after finishing your training.
- Hermansen L, Hultman E, Saltin B. Muscle glycogen during prolonged severe exercise. Acta Physiol Scand. 1967 Oct;71(2):129-39.
- Bergström J, Hermansen L, Hultman E, Saltin B. Diet, Muscle Glycogen and Physical Performance. Acta physiologica. 1967 Oct; 71(2): 140-150.
- Michael J. Ormsbee, Christopher W. Bach and Daniel A. Baur. Pre-Exercise Nutrition: The Role of Macronutrients, Modified Starches and Supplements on Metabolism and Endurance Performance. Nutrients. 2014 May; 6(5): 1782–1808.
- Clark N. Sports Nutrition Guidebook. 3rd edition. 2003.
- Ormsbee MJ, Bach CW and Baur DA. Pre-Exercise Nutrition: The Role of Macronutrients, Modified Starches and Supplements on Metabolism and Endurance Performance. Nutrients. 2014 May; 6(5): 1782–1808.
- Biolo G, Tipton KD, Klein S, Wolfe RR. An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Am J Physiol. 1997 Jul;273(1 Pt 1):122-129.