Many elite athletes schedule training sessions without eating beforehand to optimise fat metabolism. In the past, this was referred to as fasted training, but nowadays, other strategies are also used to train with low carbohydrate reserves. The idea behind this approach is to force the muscles to burn fat for longer and at higher intensities, thereby preserving more carbohydrate reserves for the final stages of a ride or race. Elite athletes do this under professional supervision, but can all cyclists benefit from this strategy?
During exercise, the body burns fats and carbohydrates
Your body derives its energy during exercise from carbohydrates or fats. Even with a low body fat percentage, your body has a vast fat reserve available for energy, whereas carbohydrate stores are limited. These limited carbohydrate stores are crucial for every cyclist. As intensity increases, carbohydrates become the primary energy source for muscles because they are easier to access and break down faster than fats.
If the intensity remains low enough, below 65% VO2 max, the body primarily burns fats during exercise in an aerobic process. As intensity rises, fats can no longer provide energy quickly enough, leading the body to rely more on carbohydrates while fat utilisation decreases. Training and nutrition can help improve these processes.
Train-low or train-high strategies
Just as training is structured in phases, nutrition is also periodised1. This means that food intake is adjusted based on the type of training to be performed. For a high-intensity or long, demanding session, a substantial fuel supply is needed, primarily sourced from carbohydrates due to the intensity. Therefore, ensuring optimal carbohydrate stores for such a session is crucial, known as the train-high approach. Train-low refers to training sessions where carbohydrate availability is deliberately reduced as a fuel source for the muscles.
Fasted training is perhaps the most well-known method of training with low carbohydrate reserves, but other approaches with different effects exist2. One alternative is training twice a day: after a proper breakfast, a strenuous session depletes the carbohydrate stores. Following this, a low-carb meal is consumed before a second training session is performed with reduced carbohydrate availability.
The benefits of train-low
It is possible to artificially limit the muscles' ability to store and burn carbohydrates by reducing carbohydrate availability. A scientific study comparing a train-high group with a train-low group found that fat oxidation during steady-state exercise improved by 31% in the train-low group. However, performance during a High-Intensity Interval Training (HIIT) session was significantly lower in the train-low group compared to the train-high group. This outcome is expected, as the HIIT was performed with low carbohydrate reserves, which are essential for high-intensity efforts3.
Muscles contain their own energy-producing structures called mitochondria. Research indicates that train-low strategies can increase the number of mitochondria in muscles. Three hours after an intense running session, the gene expression responsible for mitochondrial production was higher in the train-low group compared to the train-high group4. Although train-low strategies enhance ‘cellular markers’ needed for performance improvement and can enhance fat oxidation during submaximal exercise, it remains unclear how these effects translate into actual performance gains5.
The downsides of train-low
Fasted training is one of the train-low strategies. Many cyclists choose to train before breakfast, with some performing structured sessions in the morning, while others commute to work or school and have breakfast upon arrival. One major drawback is that this can compromise safety. The brain also relies on carbohydrates, and when carbohydrate stores are too low, reaction time and balance can suffer.
Each train-low strategy seeks to reduce carbohydrate stores before a session in a different way. While fasted training is the most common approach, overnight fasting primarily depletes liver glycogen. Muscle glycogen, provided it was replenished after the previous training session, remains relatively unchanged from the night before. Therefore, muscles still have carbohydrate reserves available for a morning session. Other train-low strategies also reduce muscle glycogen stores before training.
Training quality is the most crucial factor for improving performance. When applying train-low strategies, a balance must be struck between training adaptation and recovery. Prioritising adaptation can come at the expense of recovery, reducing the quality of subsequent training sessions. This can lead to minimal progress that is offset by a decline in training quality. Poor recovery also increases the risk of falling into a negative spiral.
Many cyclists believe fasted training helps with weight loss. However, improved fat metabolism does not equate to fat loss. Weight loss occurs when energy intake is lower than energy expenditure. If you train before breakfast, it is essential to replenish all energy afterwards; otherwise, you may end up snacking throughout the day, potentially leading to weight gain. A personalised nutrition plan tailored to your training sessions can help you reach your target weight.
With proper guidance, train-low strategies can enhance performance
There is increasing discussion around ‘marginal gains’. In elite sports, minor improvements can mean the difference between winning and losing. Professional athletes have support teams that focus on the smallest details. Although train-low strategies enhance ‘cellular markers’ and fat oxidation during submaximal efforts, scientific studies have yet to show clear performance improvements. The question remains whether the benefits of fasted training or other train-low strategies outweigh the disadvantages. Without proper guidance, train-low strategies can be challenging to implement effectively.
1. Jeukendrup AE. Periodized Nutrition for Athletes. Sports Med. 2017;47(Suppl 1):51-63. doi:10.1007/s40279-017-0694-2
2. Hawley JA, Burke LM. Carbohydrate availability and training adaptation: effects on cell metabolism. Exerc Sport Sci Rev. 2010;38(4):152-160. doi:10.1097/JES.0b013e3181f44dd9
3. Hulston CJ, Venables MC, Mann CH, et al. Training with low muscle glycogen enhances fat metabolism in well-trained cyclists. Med Sci Sports Exerc. 2010;42(11):2046-2055. doi:10.1249/MSS.0b013e3181dd5070
4. Bartlett JD, Louhelainen J, Iqbal Z, et al. Reduced carbohydrate availability enhances exercise-induced p53 signaling in human skeletal muscle: implications for mitochondrial biogenesis. Am J Physiol Regul Integr Comp Physiol. 2013;304(6):R450-R458. doi:10.1152/ajpregu.00498.2012
5. Philp A, Burke LM, Baar K. Altering endogenous carbohydrate availability to support training adaptations. Nestle Nutr Inst Workshop Ser. 2011;69:19-37. doi:10.1159/000329279
