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Add fuel to the fire … or take it away?

Competitive endurance athletes manipulate their carb intake in various ways, and those aren’t always based on evidence. A new carb-cycling strategy may help to shave off precious seconds.

Study under review: Enhanced Endurance Performance by Periodization of CHO Intake: “Sleep Low” Strategy


Many athletes aim to become more efficient at burning fat during exercise. This is due to reasons such as preserving glycogen stores, requiring a smaller caloric intake during prolonged activity, and improving body composition.

Another advantage to using fat as a fuel source is that it's essentially unlimited relative to carbohydrate sources. Generally speaking, lower intensity exercise relies more heavily on fat oxidation for fuel, while higher intensity relies more on carbohydrate. This is due to the rate at which energy in the form of ATP can be created. Fat burning requires the presence of oxygen, while carbohydrate can produce energy with or without oxygen (and produce it more quickly than fats can).

A number of recent[1] studies[2] have shown that performing workouts with low muscle glycogen levels (the body’s carbohydrate “gas tank”) and/or low exogenous carbohydrate availability (reduced carbohydrate intake from things like sports drinks) can result in an increased ability to burn fat during exercise as well as enhancing training adaptations in skeletal muscle. As appealing as that sounds, practical improvements in sports performance are rarely found. Part of this is due to the fact that the low-carb dietary strategies that improve fat-burning also tend to reduce[1] the athlete’s ability to train and perform at high intensities.

If you are familiar with the world of endurance sports, you may have heard the phrase “train low, race high” in reference to training with low carbohydrate availability but racing on a high-carb diet. The latest (and perhaps most promising) strategy for training manipulation is a “train high, sleep low” strategy for carbohydrate intake. Train high, sleep low refers to performing a high-intensity training session after consuming substantial carbohydrate intake throughout the day, but sleeping with low carbohydrate stores by eating a low-carb meal after the high-intensity, glycogen-depleting workout. It has become well[3]-established[4] that the amount of energy available to the muscle before, during, and after exercise can have an effect on fuel metabolism and utilization during exercise as well as on gene expression and cell signaling, and can play an important role in training adaptations. Recent research[5] has begun investigating this unique concept of train-high sleep-low, and has found an upregulation of several markers of fat metabolism.

An important aspect to consider when manipulating carbohydrate intake during training is that it does not need to be an all-or-nothing approach. Rather, one can use an approach to carbohydrate intake that varies each day, in order to ensure that the goals of each training session are met with an appropriate intake. What this means is that high-intensity training sessions that rely quite heavily on carbohydrate for fuel could be performed after consuming adequate dietary carbohydrate, while low-intensity training sessions that are performed to improve fat oxidation can be performed in a carb-depleted state.

With this novel concept of train high, sleep low in mind, investigators set out to see if it could improve endurance performance. This study incorporated three strategies. The first one involved high-intensity training with high-carbohydrate availability aimed at maximizing high-intensity adaptation and performance. The second used overnight carbohydrate restriction to enhance the signaling responses that mediate adaptations after exercise. The third utilized a submaximal training session undertaken in the fasted state in order to promote lipid metabolism.

A novel training and dietary concept called “train high, sleep low” is one nutritional scheme that could support endurance training. It involves coupling high-intensity training with high-carbohydrate availability (training high) followed by overnight carbohydrate restriction (sleeping low). It also includes low-intensity training sessions done after an overnight fast. The goal of this study was to determine if real-world measures of endurance performance in triathletes could be improved using this strategy.

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