Pros and Cons of Low-Glucose Training For High Performance

Lori Russell, MS RD CSSD
by Lori Russell, MS RD CSSD
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Pros and Cons of Low-Glucose Training For High Performance

While we know we need a good balance of macros to perform, many athletes are taking to cycling carbs to optimize their performance. Carb cycling, combined with low-carbohydrate workouts, has gained popularity by those seeking to lose weight and improve performance. “Training low” is a term that relates to fasted or under-fueled training sessions that can include some form of low-carbohydrate availability specific to the training.

While it seems like a simple concept: skip food then workout, there’s much more strategy and specificity involved, especially if high performance is your goal. Training low is not the same as a diet consistently inadequate in carbohydrate intake. Instead, sufficient carbs are consumed, but not before or during training sessions. This is done to begin the workout in a state where muscles and the liver are lacking stores of roughly 500 grams of the macronutrient.


There are two main strategies for accomplishing this.

  • The first has been coined “sleeping low,” which can be achieved by eating a low-carbohydrate evening meal (vegetables and chicken, hold the rice or pasta) and then training in the morning before consuming food.
  • The second option is depleting glycogen between two workouts; the first workout is completed in a fueled state, but carbohydrates are then withheld for 4–6 hours until after a second workout is completed.


Touted benefits of training low are based on the belief the body learns to burn fat for more fat loss. The concept is not having carbohydrates to rely on for immediate energy forces the body to make stored fat more available for use. Burning higher rates of fat during training may promote a lean body composition which is desirable for athletes whose sport is weight sensitive. However, while nice in theory, the body also uses protein as an energy source, which might reduce the body’s lean tissue mass over time.

Fat is also in almost limitless supply, even in lean athletes, while carbohydrates are limited and full stores are likely to be depleted in 2-hour workouts, leading to bonking if external food sources are not consumed. In concept, relying on fat allows an athlete to go further without fatigue. Research comparing carbohydrate-fueled workouts with fasted workouts has shown muscle-oxygen capacity can be improved and blood glucose levels less unstable, potentially improving future performance abilities.

READ MORE: How Endurance Athletes Should Carb Up During Workouts


Those attempting this type of sports nutrition training protocol should understand high-intensity sessions will suffer. While muscles can, in fact, adapt to lower glucose, the researched outcome of low-glucose training on performance is limited. Conversely, a clear performance improvement exists when carbohydrates are amply available. Before key workouts or competitions, the athlete should ingest adequate carbohydrates to stimulate an enhanced utilization of the carbohydrates.

Training with low-carbohydrate availability, especially if the macro isn’t properly replenished, can induce poor performance outcomes, promote chronic fatigue, create mood disturbances, and diminish immune function. It is also possible restricting carbohydrates reduces the ability to metabolize and utilize the nutrient when it is consumed again.

Many athletes report feeling generally crummy in low sessions, as training without carbohydrates is more physically and metabolically demanding on the body. This makes sense as carbohydrates are the body’s most efficient and preferred energy source. Carbs are also the only macronutrient able to be utilized for anaerobic energy needs, meaning if your training has intense efforts, you will suffer without carbohydrates to pull from. Due to this, athletes performing bouts of high intensity and explosive power (jumps, sprints, CrossFit, etc.) might suffer more in training low sessions compared to endurance athletes involved in low-to-moderate efforts where the body pulls from a more equal ratio of fat and glucose as energy sources. Research backs this up as training low was seen to improve 20-minute fitness twice as much as a high-carb training group, however, this same study found the low-carb group did not outperform in sprint efforts.


To reduce the ‘crummy’ feeling, sleeping low appears to be a good strategy as the athlete spends much of the carb avoidance period sleeping, making it easier to adhere to. A study testing sleeping low on triathletes for a 3-week period saw significant improvement using the strategy. However, athletes tend to perform better later in the day as muscle temperature is higher, free fatty acids are mobilized in greater concentrations, and it aligns with natural circadian energy levels.


Ultimately, using a highly specialized nutrition strategy, such as training low, requires a bit of personal experimentation as research is limited and results are mixed. Which style to follow depends on the type of sport, training demands and normal lifestyle habits. Working closely with your coach and sports dietitian is key to ensuring your performances, general health and overall nutrition is on track for success.

Make progress every day while you work on fitness and nutrition goals, like counting macros or going low-carb. Go to “Plans” in the MyFitnessPal app for daily coaching and easy-to-follow tasks to keep you motivated.

About the Author

Lori Russell, MS RD CSSD
Lori Russell, MS RD CSSD

Lori, MS RD CSSD is an accomplished sports dietitian; she holds a Master’s Degree in Human Nutrition and Certification as a Specialist in Sports Nutrition. As a current professional road cyclist and previous elite marathoner and ultra-runner, Lori knows firsthand that food can enhance or diminish performance gains. She understands the importance of balancing a quality whole food based diet with science-backed performance nutrition and strives to share this message with others. Learn more about her @HungryForResults.


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