Your body stores much more energy in form of fat than as carbohydrate. Some athletes strive to change their metabolism so that their bodies rely more on fat to fuel exercise which in turn allows them to spare the carbohydrate that’s stored in muscle and liver and improve your endurance capacity. In theory, training when carbohydrate availability is low, may force your metabolism toward a greater reliance on fat as a fuel for exercise.
There are two approaches to creating a state of Low Carbohydrate Availability:
1. Exercising after an overnight fast. Stores of carbohydrates are used while you sleep and not replaced before a workout in the morning.
2. Exercising after a prior heavy, high intensity exercise. In this case carbohydrate is used up during an exercise bout and not replaced before the second workout.
These two approaches are different in their bio energetics and will lead to different adaptations.
At night, while we sleep the liver secretes glucose to maintain blood glucose levels and supply the body with carbohydrates. This is very important for the brain because it relies almost entirely on blood glucose to function. By morning 60 to 80 percent of the liver glycogen has been used by your body. However muscle glycogen levels remain high throughout the night and are high when you wake up for your morning run, because it isn’t secreted from or consumed by skeletal muscle cells while you sleep.
In summary, you go to sleep with high glycogen levels in both muscle and liver and you wake up with no change in muscle but with low levels in liver.
As you begin your workout in the morning skeletal muscle prefers its own fat and glycogen stores but gets some of its energy from fats and glucose in the blood. The liver breaks down glycogen to ensure that blood glucose levels are maintained to support all the cells of your body. During exercise a person hits the wall or bonks when they run so low on liver glycogen that the blood glucose levels drop. Muscle glycogen levels will also be low at this point because the muscle has been preferentially consuming it.
In endurance community the concept of exercising when the body is low in available carbohydrate (“training low“, meaning low in glycogen and glucose), has been adapted by athletes to encourage adaptations that favor the use of fats for fuel. In theory, improving fat utilization will lead to a sparing of limited glycogen stores and reduce the need to consume as many calories while exercising. Some recent research has shown that fat utilization can be enhanced by depleting muscle and liver of glycogen through high intensity interval training in the evening and then stressing the muscle in a prolonged moderate exercise about the next morning without eating any carbohydrate in between. With that goal in mind there are two approaches commonly used by athletes for training on low glycogen:
1. Exercise first thing in the morning without eating breakfast.
2. Exercise after depleting stores through a prior workout. In this case athlete would perform a high intensity workout then not eat carbohydrate until after a second workout. The two workouts would be performed approximately 10 to 12 hours apart. For example: early morning and then late evening or late evening and then the following morning.
These two methods are very different with respect to the bio energetics.
THE FIRST METHOD
Exercise first thing in the morning after an overnight fast and without breakfast. This method stresses your liver glycogen stores because only the liver uses its glycogen stores overnight. Skeletal muscle will rely on its own stores of glycogen during the morning training which it prefers anyway. The glycogen that is stored in muscles is for “locals only”. In other words, once it’s stored in muscle, it’s not capable of being transported to other areas of the body to provide fuel. Instead, it must be used at the site. When your body can’t push for one more rep, this likely means that the glycogen has been depleted in those muscles. While you exercise, your muscles may run out of their stored glycogen sooner than if the liver was supplementing them with additional glucose but muscle won’t be very compromised energetically and any stress for adaptations in fat metabolism will be small at best. Exercising in this manner you primarily stressing the liver because it can’t maintain blood glucose once it runs out of what it has left from your overnight fast. A rough estimate would be 5 to 10 miles run, depending on bed time and duration of sleep. After that you may feel the bonk because your brain isn’t getting the glucose it needs but your active muscles aren’t very stressed.
THE SECOND METHOD
Complete a high intensity workout or long run to deplete muscle of glycogen followed by a near zero carbohydrate diet for ten to twelve hours. Depleting workout forces both muscle and liver to use up their glycogen stores. During the 10 to 12 hours without carbohydrate the liver will metabolize any remaining glycogen to maintain blood glucose levels as best it can. When you start a second workout 10 to 12 hours later muscle and liver will be low in glycogen reserves. You’ll feel the same level of bunk but in this case it’s not just your brain bonking, it’s your muscle to muscle too. The muscles will be forced to rely much more on fat for fueling your exercise and they may adapt so that you rely a little more on fat in the future if you do this regularly.
It may seem that depleting muscle glycogen through prior exercise (second method) is the way to go if you want to really force muscle into a greater fat utilization, but be careful and consider the fact that the first workout places a substantial stress on muscles, it’s a depleting workout after all. Following such workout your immune system works hard to repair your muscles and it needs carbohydrate to do so. By depleting carbohydrate and not replacing it, these repair and adaptation systems are unable to function optimally. The second exercise, where the goal is to stress fat utilization, comes with consideration that the muscle is functionally compromised. Therefore the second bout should be of low or moderate intensity. Carbohydrate feeding should begin immediately upon completion of the second workout.
Endurance events are typically performed at average intensities at roughly 60 to 70 percent of maximum heart rate. At such intensities fat utilization is highest in most all athletes and nearly 60 percent of total energy expenditure progressing to 65 plus percent after exercising longer than 2 hours.
A study was conducted, where after 12 hours of running at 60 to 70 percent max during a 100 km race, glycogen levels were measured to decrease by 64 percent in the thigh muscle and it was still very high in the calf muscle. These observations led to question practicality of doing anything extreme in an attempt to influence fat utilization in athletes competing in endurance events. Any shifts in fat utilization will have very little impact on performance, when compared with the many other factors, such as temperature regulation, hydration, fueling and especially mindset.
Overall exercising while carbohydrate stores are low may be something you choose to experiment with but it’s unlikely that it will lead to a large impact on your performance. Because exercising in glycogen depleted state makes it very hard to keep the intensity high, and places additional stress on your body and nervous system, you may compromise the quality of your trainings while you try to figure out what works for you. If you choose to train in a glycogen depleted state the biggest take home recommendation is to make sure that you’re actually depleting the tissue you’re trying to stress. If you want to stress skeletal muscles fat burning, exercising in the morning without breakfast won’t do it. To do so, you need to deplete your muscles with a bout of high intensity or long duration exercise and then not replace the carbs until after the next workout.