It has been known in the scientific community since the late 1960s that the ability to perform endurance exercise is strongly influenced by the amount of pre-exercise glycogen (carbohydrate) stored in skeletal muscles, with muscle glycogen depletion becoming the decisive factor limiting prolonged exercise at moderate intensities (65 to 75 percent of maximum aerobic power, VO2max). Any marathoner who has hit the wall knows this intimately as well. It is also well known that more glycogen in muscles before exercise results in a greater use of glycogen during exercise, and therefore increases the ability to sustain a high intensity (e.g., a faster pace). Research has even shown that fatigue can be delayed with carbohydrate supplementation during exercise.
The well-documented decrease in muscle glycogen content that accompanies endurance exercise results in an empty/refill-more cycle. When muscle glycogen is depleted by prolonged exercise, muscles respond to the empty tank by synthesizing and storing more than what was previously present, a process largely controlled by the hormone insulin. Empty a full tank, and you get a refilled larger tank in its place. (Imagine if your car, after driving for long enough that it ran out of gas, created a larger gas tank when sitting on the driveway. That’s what your muscles do.)
Studies Show: Doubles Deplete and Expand the Tank
When it comes to refilling a larger tank (and inducing greater mitochondrial enzyme activity, which enhances aerobic metabolism), training twice every second day is superior to training once daily. That was the conclusion of researchers at RMIT University in Victoria, Australia, after 18 endurance-trained, male cyclists and triathletes trained for three weeks. Half of the study participants trained six days per week, alternating days of 100 minutes of cycling at 70 percent of their VO2max with days of high-intensity interval training. The other half did both of those workouts on the same day (separated by one to two hours of rest) every other day. The twice-per-day training strategy resulted in a marked decrease in muscle glycogen after the first workout, such that the participants started the second workout with significantly lower muscle glycogen than before the day’s first workout. After three weeks of training, muscle glycogen significantly increased in the twice-per-day training group but not in the once-per-day group.
Aerobic enzyme activity and the amount of fat used during submaximal exercise also increased more by training twice every second day compared with training once daily. However, despite the physiological adaptations that favored twice-per-day training, endurance performance (measured as the average power maintained during a 60-minute cycling time trial 15 minutes after completing a 60-minute ride at 70 percent VO2max) didn’t differ between the two types of training. (It’s plausible that three weeks of training, although having an effect on muscle glycogen storage, enzyme activity, and fat burning, was not long enough to elicit differences in cycling performance, as measured in this study.)
A clever experiment at the Copenhagen Muscle Research Center in Copenhagen, Denmark also found that training one leg twice every second day for ten weeks caused greater muscle glycogen storage and greater endurance (measured as time to exhaustion during knee extension exercise at 90 percent of peak power output) compared to training the other leg of the same person once daily.
Starting Out On Low
Starting workouts with low muscle glycogen increases the transcription of specific genes and proteins involved in training adaptation, making it a promising strategy to enhance glycogen storage, which is a crucial factor for long races (marathon, ultramarathon).
Training consists of a series of threats to different aspects of your body’s survival. Because carbohydrate is your muscles’ preferred fuel during exercise, a low carbohydrate (glycogen) fuel tank is threatening to the muscles’ survival. When that threat exists, your DNA gets busy transcribing genes that ultimately lead to making a bigger glycogen fuel tank to assuage the threat.
To prepare your body to store more glycogen for long races, low-glycogen training can be accomplished several ways:
(1) Training twice per day without consuming carbohydrate between workouts
(2) Running long (at least 90 minutes) on consecutive days (and consuming a low-carbohydrate diet between runs)
(3) Consuming a low-carbohydrate diet during periods of long-endurance, low-intensity training
(4) Not consuming carbohydrate during long runs.
Running on Empty
Regarding that last method of avoiding carbs during the run, which runs counter to what most marathon runners do, one of the main purposes of long runs is to deplete (or at least severely lower) muscles’ store of glycogen. Glycogen-depleted muscles force your muscles to more effectively rely on fat for energy, stimulate your liver to make new glucose from non-carbohydrate sources (a process called gluconeogenesis), and stimulate a greater synthesis and storage of glycogen during recovery, all of which are important adaptations to prepare for long races, most notably marathons and ultramarathons.
If you consume carbohydrate during your long runs, you won’t deplete your glycogen fuel tank and will blunt these adaptations from occurring to their potential. To create the largest muscle glycogen storage possible, you need to deplete muscle glycogen on a regular basis. In the marathon race itself, it’s important to consume carbohydrate to prevent a severe drop in blood glucose. Since you should never do anything different in the marathon that you have not done in training, you must balance the physiological adaptations with the practical concerns. To facilitate this balance, I suggest alternating long runs during which you consume and don’t consume carbohydrate. When you do consume carbohydrate, use the same gels and sports drink that you’ll use on the marathon course.
If you run twice on the same day for low-glycogen training, the timing of the second run is important—do your second run before enough glycogen is synthesized and stored in your muscles (within a few hours). Don’t do low-glycogen training all the time, since carbohydrate is necessary to fuel high-intensity training. Train with normal or high muscle glycogen during high-intensity training periods, and with low muscle glycogen during low-intensity training periods.
Adapted from the forthcoming book, Running Periodization: Training Theories to Run Faster by Jason R. Karp, PhD
A competitive runner since sixth grade, Dr. Jason Karp quickly learned how running molds us into better, more deeply conscious people, just as the miles and interval workouts mold us into faster, more enduring runners. This passion that Jason found as a kid placed him on a yellow brick road that he still follows as a coach, exercise physiologist, author of 11 books and more than 400 articles, speaker, and educator. He is the 2011 IDEA Personal Trainer of the Year and two-time recipient of the President’s Council on Sports, Fitness & Nutrition Community Leadership award. His REVO₂LUTION RUNNING™ certification has been obtained by coaches and fitness professionals in 25 countries. Follow him @drjasonkarp on Instagram, Facebook, and YouTube.