Today, we know that an athlete’s diet can influence, positively or negatively, the physiological adaptations targeted through training. In endurance sports, carbohydrates play a central role because they determine the endurance and power capacity of the muscles.
The current evolution of training methods involves modulating energy intake, especially carbohydrate intake before, during, or after training. One of the first ways to adjust carbohydrate intake was fasted training, which consists of training in the morning without having eaten breakfast beforehand.
Physiological reminder
Blood glucose refers to the level of glucose in the blood. Under normal conditions, glucose is the only energy substrate used by the brain. The brain consumes around 110 g of glucose each day. The second biggest consumer of glucose is muscle, especially when it is subjected to repeated contractions.
Blood glucose fluctuates throughout the day within a range of about 90 to 100 mg/dl. At times, it can drop as low as 50 mg/dl — this is the well-known hypoglycaemia. Conversely, it can rise to 180 mg/dl or more after excessive carbohydrate intake.
The liver is the regulator of glucose and is also referred to as the “glucostat.” It can store glucose in the form of glycogen and release glucose again by breaking down its glycogen stores.
During exercise, energy needs — especially glucose needs — depend mainly on duration, intensity, and environmental conditions such as heat, cold, or altitude. The longer and more intense the effort, and the more extreme the environment, the greater the need for glucose.
The benefits of fasted training
In the morning upon waking, after a night of sleep and without any food intake beforehand, the following can be observed:
- a lower-than-normal blood sugar level
- a higher blood cortisol level, the stress hormone
- reduced liver glycogen stores
- an empty stomach
This context is particularly favorable for influencing energy metabolism. Over time, and through regular fasted sessions, the body gradually adapts to drawing energy from where it is available in almost unlimited quantities — namely fat mass — while preserving glycogen stores. Session after session, the body therefore learns to become more efficient and to make better use of circulating glucose.
Studies conducted by Professor Peter Hespel’s teams at the University of Leuven in Belgium showed that after 6 weeks of fasted training, consisting of 3 sessions of 1 to 2 hours per week at 75% of VO2max:
- muscle glycogen is better preserved
- the oxidative capacity of the muscle is improved, especially through enzymatic adaptations
- fat oxidation is positively modified, notably through improved breakdown of intramuscular triglycerides
The main benefits of this method are therefore:
- improving endurance by teaching the body to draw on fat reserves
- teaching the body to stabilize blood glucose in situations of carbohydrate deficit and physical exercise
- training the body to use gluconeogenesis to produce energy
Protocol 1: A classic fasted training approach
It generally consists of:
- drinking a large glass of still water immediately after waking up
- limiting the effort to 20 minutes at the start
- increasing intensity very gradually without exceeding 80% of maximum heart rate
- keeping a gel on hand in case of sudden hypoglycaemia
- drinking a large glass of water after the session
- having a restorative breakfast after the session
Progression in fasted training mainly comes from increasing session duration, without exceeding 2 hours, and from performing technical exercises such as one-leg pedalling or cadence variations, while staying at a low to moderate intensity (< 80% of max HR).
After several months of regular fasted training, it becomes possible to introduce short phases of intensive work. Many elite athletes train in this way, especially African runners. The goal is to teach the body to use lipolysis and gluconeogenesis to produce energy.
Protocol 2: Lower glycogen stores, then train fasted
Training at the end of the day and then eating a low-carbohydrate dinner makes it possible to begin the next morning’s fasted session with very low liver and muscle glycogen stores. The fasted workout that follows therefore places the body in a more pronounced state of liver glycogen depletion than simple fasted training alone. The aim is to teach the body to use lipolysis and gluconeogenesis to produce energy.
Protocol 3: Start fasted, then fuel during the second half of the session
For fasted sessions lasting more than one hour, it can be beneficial to finish the workout by consuming carbohydrates, for example through an energy drink. This helps limit the negative impact of fasted training while maintaining a meaningful training intensity. This method can also help train the digestive system to absorb carbohydrates in a specific context.
Protocol 4: Train twice a day
When training twice a day, the goal of the first session is to reduce glycogen stores and then eat without consuming carbohydrates. The second session is then carried out with low reserves at both the muscular and liver level. Several studies have shown that this type of training can improve fat metabolism at the mitochondrial level.
Protocol 5: Do not refuel immediately after training
Under normal circumstances, it is essential to take a recovery snack containing carbohydrates and proteins after a long and/or intense session. The goal is to optimize glycogen restoration and recover more quickly. In the case of the “recover low” approach, carbohydrates should be avoided for 1 to 2 hours after the session. This process slows recovery, but it may help stimulate certain physiological adaptations at the level of gene expression.
Protocol 6: Overfeed
Consuming very high amounts of carbohydrates during exercise can help prepare the digestive system to absorb large quantities of carbohydrates. To do this, 80 to 120 g of carbohydrates per hour of exercise should be consumed.
Conclusion
Although the links between training and nutrition have been studied for 50 years, there is still a long way to go in fully integrating training and nutrition in order to optimize an athlete’s performance and health. Personalizing carbohydrate intake according to the training day and training load is undoubtedly one of the keys to making progress.
Reference
Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations — Hearris MA et al. Nutrients 2018, 10(3): 298.