Training on Low Glycogen

Interested in training on low glycogen levels? Doing it by running in the morning, without breakfast? If so, you may not be doing what you intend. In this episode, I explain why and provide an actionable and reliable approach to do it right.


Script of Episode

The body stores far more energy as fat than as carbohydrate. If you can change your metabolism so that you rely more on fat to fuel exercise, then you should spare the carbohydrate that's stored in muscle and liver, and improve your endurance capacity. Training when carbohydrate availability is low may force your metabolism toward a greater reliance on fat as a fuel for exercise. At least that’s theory.

Two approaches to creating a state of low carbohydrate availability are often described simultaneously, as if they're simply different but equivalent ways of going about it:  

    a) exercising after an overnight fast - stores of carbohydrates are used while you sleep and not replaced before a workout, or 

    b) exercising after a prior heavy exercise bout - carbohydrate is used up during an exercise bout and not replaced before a second workout.  

Despite being presented as equivalent options, the two approaches are different in their bioenergetics and will lead to different adaptations. This episode is meant to clarify the physiology of the two approaches and to present additional concepts related to fat vs carbohydrate usage for fueling your ultra endeavors. 


In 1853, Claude Bernard discovered that the liver secretes our body’s main carbohydrate, glucose by observing that the blood flowing out of the liver of fasted or meat-fed animals contained glucose while the blood flowing into it contained little (Br Med J, Young 1957, 1(5033):1431–1437). 

When a cell takes in glucose from the blood, it's modified (phosphorylated) in the process. This effectively locks the molecule into the cell because the modified form cannot be secreted back out of the cell. Only the cells of the liver make the enzyme glycogen 6-phosphatase, which modifies glucose back to its original form so that it can be secreted. Therefore, only the liver can secrete glucose. 

The liver is our highest concentration of glucose. But liver doesn’t taste sweet at all. That’s because it's stored as long strands of glucose, called glycogen, and glycogen doesn’t activate sweet receptors in the mouth. Liver contains 100-120 g of glucose, as glycogen. Skeletal muscle contains roughly 1/5th the concentration of glycogen when compared to the liver but we have much more skeletal muscle than liver. In total, our skeletal muscle contains 400-500 g of glycogen overall. The net energy storage, at 4 kilocalories (a nutritional Calorie is a kilocalorie) per gram of glucose, is about 500 kilocalories in the liver and about 1500 kilocalories in skeletal muscle. Trained individuals store a little more, such that the average athlete might store 2,000-2,500 kilocalories in total as usable carbohydrate energy in muscle and liver.

At night, while we sleep, the liver secretes glucose to maintain blood glucose levels and supply the body with much needed carbohydrate. This is especially important for the brain because it relies almost entirely on blood glucose to function, unless a person has starved it of glucose long enough to force it to convert to using fats. By morning, 60-80% of the liver glycogen has been secreted and used by cells throughout the 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 little change in muscle but with low levels in liver.

As you begin your workout, 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 in liver glycogen that blood glucose levels drop. Muscle glycogen levels will also be low at this point because the muscle has been preferentially consuming it.

The Issue

In the running community, the concept of exercising when the body is low in available carbohydrate (i.e., glycogen/glucose) has been promoted as a means to encourage adaptations that favor the use of fats for fuel. In theory, the development of fat utilization pathways will ultimately lead to a sparing of limited glycogen stores and reduce the need to eat as many extra calories while running. 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 bout the next morning. We don’t yet know if doing this leads to long term improvements in endurance. But, this field of research is young and there remains much hopethat periodizing carbohydrate consumption during exercise training can lead to improved endurance performance.  

With that goal in mind, there are two approaches commonly used for training on low glycogen. An athlete may either train a) first thing in the morning without eating breakfast or b) after depleting stores through a prior workout. In the latter case, an athlete would perform a high intensity workout then not eat carbohydrate until after a second workout. The two workouts would be performed approximately 10-12 hours apart (e.g., early morning then late evening or late evening then early morning).  

The primary goal of this episode is to explain why it’s important to appreciate that these two methods are very different with respect to their bioenergetics. Let’s evaluate each in turn: 

a)    Method ‘a’ is running 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 over night. Skeletal muscle will rely on its own stores of glycogen during the morning run, which it prefers anyway. While you run, your muscles may run out of their stored glycogen a little 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. The stress in this scenario is placed on the liver, which can’t respond to the stimulus to maintain blood glucose once it runs out of what it has left from your overnight fast. We can estimate roughly 5-10 miles depending on bedtime levels 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.

b)   Method ‘b’ is completing a high intensity workout or a long run to deplete muscle of glycogen followed by a near-zero carbohydrate diet for 10-12 hours. During the depleting workout you force both muscle and liver to use up most of their glycogen stores. During the 10-12 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 10-12 hours later, muscle and liver will be low in glycogen reserves. You’ll feel the same level of ‘bonk’ but, in this case, it’s not just your brain bonking, it’s your muscle, too. Muscle will be forced to rely much more on fat for fueling your run, and it may adapt so that you use a little more fat in the future if you do this regularly. In the research showing the benefits of this method, subjects depleted glycogen stores with a high intensity interval session in the evening followed by no carbohydrates until after a long run the following morning; they repeated this protocol for the first three days of each week for three weeks.

Words Of Caution

From this evaluation, it may seem that depleting muscle glycogen through prior exercise (method ‘b’) is the way to go if you want to really force muscle into 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 a workout, your immune system infiltrates skeletal muscle for repair and adaptation. By depleting carbohydrate and not replacing it, these repair and adaption systems are unable to function optimally. The second bout of exercise, where the goal is to stress fat utilization, comes with the added consideration that the muscle is functionally compromised. So, the second bout should be of low or moderate intensity. Carbohydrate feeding should begin immediately upon completion of the second bout.

How might all of this translate into performance in ultra-marathons?

Longer ultras are performed at average intensities near the first group of thresholds, near the division between the moderate and heavy intensity domains, which I spoke about last time in episode 68. This occurs at around 60-70% HRmax, where fat utilization is maximal in most athletes and nearly 60% total energy expenditure, progressing to 65+% by 2 hrs. (AJP Romijn et al. 1993, 265:E380-E391). After 12 hrs of running at 60-70% max during a 100-km race, glycogen had decreased by 64% in vastus lateralis (a thigh muscle) and was still high in gastrocnemius (Ann NYAcad Sci, Essen 1977, 301:30-44). These observations beg the question of practicality in doing anything extreme in an attempt to influence fat utilization in athletes competing in ultra-marathons. Any shifts in fat utilization will likely pale in performance impact when compared with the many other factors in an ultra-marathon including temperature regulation, foot care, hydration, and especially mindset.

Concluding Thoughts

Overall, exercising while carbohydrate stores are low may be something you choose to dabble with but it’s unlikely that doing so will have a large impact on your performance. And, you may compromise the efficacy of your training while you figure out what might work for you because exercising while carbohydrate depleted is a substantial stress and the quality of a workout (intensity, form, total work) usually suffers. If your goal is to get on the podium and you're routinely coming in 4th place, then this may be something you want to explore. 

If you choose to train in a glycogen depleted state, the biggest take home recommendation of this episode is to make sure that you're actually depleting the tissue you're trying to stress. If you want to stress skeletal muscle’s fat burning, an overnight fast won’t do it. For that, you have to deplete muscle of its stored glycogen with a bout of high intensity or long duration exercise and then not replace the carbs until after the next workout. 

This is an area of research where we’re likely to see large inter-athlete variability in outcomes as more studies are completed. There’s a good chance we’ll also see high- and low-responders even if the periodization of diet and exercise is optimized for each athlete.

I hope you learned something useful in this episode. If you’d like to support the podcast and keep it advertisement-free, go to If you want to take your training to the next level, go to  Until next time, pay attention to all the facets of your training…how you eat, sleep, think, and move all play a role in helping you become your ultra-best.