One on the most received messages as an athlete is to consume carbohydrates, pre, during and post exercise to aid with adequate fuelling, support athletic performance and optimise rates of recovery.
Carbohydrate provision and its role in enhancing exercise performance are well documented, where carbohydrate availability through glycogen stores and diet are a key indicator for sustaining prolonged exercise performance. In this instance, carbohydrates are still considered to be ‘king’ and the most important nutrient for optimising athletic performance.
This is primarily through maintaining muscle and liver glycogen stores which become compromised during the latter phases of exercise – however is dependent on the type, intensity and duration of the exercise session or event. For example, continuous endurance exercise sessions under 60-90 minutes or intermittent higher intensity exercise (team sports) less than 45-60 minutes would not require additional carbohydrates through drinks or gels during as muscle and liver glycogen stores will most likely be adequate. In comparison, untrained individuals who follow a balanced diet would have glycogen stores ~ 80-90mmol kg−1 w.w., where exercising endurance athletes have higher stores ~150mmol kg−1 w.w., and even higher following a 3 day carbohydrate load in excess of 200mmol kg−1 w.w.. Since the availability of glycogen stores drives exercise performance, higher glycogen stores prior to an event or training sessions would be more favourable in reducing fatigue and improving exercise performance. In some instances, a carbohydrate load can improve exercise performance by 2-3% over set distances and reduce fatigue by 10-20 percent in long duration endurance events before reaching critically low levels of muscle glycogen ~ 25mmol kg−1 w.w.. Therefore during longer duration events, the higher the starting point, the better.
Exercise sessions over ~60-90 minutes may require carbohydrates during to ‘spare’ glycogen stores and offset a decline in work rate, fatigue and exercise capacity. Therefore, longer exercise sessions or events become more reliant of carbohydrate provision during. As carbohydrate intake is paramount for increasing longer duration exercise performance, it should be stressed that competition strategies utilise carbohydrate rich foods in order to supercompensate muscle and liver glycogen stores prior to an event whilst consuming carbohydrate drinks and gels during an event to sustain performance.
The aim of competition and exercise sessions are different, therefore nutrition and fuelling practices will adjust accordingly to maximise the outcome. In recent years, emerging evidence of the concept ‘train low’ has demonstrated significant improvements with regards to enhancing the endurance training adaptation. Train low involves training on reduced carbohydrate availability, where both dietary and stored carbohydrates are reduced to bring about favourable training adaptations; enhanced signalling pathways, oxidative enzymes, whole body and intramuscular lipid oxidation in return improving exercise capacity. The aim of any training session is to adapt in order to have improved performance for subsequent training sessions. Research appears to consistently show that manipulating carbohydrate intake around endurance type training sessions (3-10 weeks) increases mitochondrial biogenesis, which results in increased mitochondrial mass. Increasing mitochondrial mass allows for the athlete to train at higher absolute intensities for a longer period of time by maintaining metabolic homeostasis, such as; smaller decreases in ATP, phosphocreatine, muscle glycogen, lactic acid and smaller increases in ADP and AMP.
To keep the mechanistic side brief, low levels of muscle glycogen increase signalling enzymes known as AMPK and p38MAPK, in return activates the downstream transcription coactivator; PGC-1alpha which stimulates mitochondrial biogenesis. Therefore, in order to further increase the end goal of mitochondrial biogenesis, carbohydrate restriction appears to be favourable. It’s worth noting that the endurance training adaptation with a traditional carbohydrate fuelling strategy also yields a training adaptation through increases in VO2max, however not to the same magnitude of carbohydrate restriction as many of the muscular adaptations become blunted when carbohydrates are consumed.
As with any intervention, a trade-off or compromise will become apparent – this is no different to the ‘train low’ paradigm. The favourable components of ‘train low’ have been mentioned above, however with any positive approach, comes some negatives. Below outlines the main concerns with training for long periods of time without the provision of carbohydrates.
A misunderstanding occurs with ‘train low’ as it is often believed to be a low or zero carbohydrate diet, which simply isn’t the case. Training low involves carbohydrate manipulation on a daily basis, where total daily carbohydrate intakes are still met through withholding carbohydrates in some meals; before, during and after training sessions and compensating in other meals. For example, Marquet et al (2016) compared a three week carbohydrate equated diet (6 grams per kilogram of bodyweight ~ 420g from a 70kg athlete), however one group manipulated carbohydrates timing over the course of the day for low and high intensity training sessions, where the control group consumed an equal amount of carbohydrate spread throughout the day. At the end of the three week period, the group that manipulated carbohydrate intake around their sessions improved submaximal cycling economy, as well as supramaximal cycling capacity and 10-km running time. Therefore, manipulating the timing of a high carbohydrate diet (6g per kilogram of bodyweight) can improve exercise performance and highlights that the ‘train low’ paradigm is not a low or zero carbohydrate diet.
Training with low carbohydrate availability is still in its infant stages where many unanswered questions remain with regards to an optimal approach and best practice. With this in mind, here are strategies to implement ‘train low’ into a training programme and offset the likeliness of any of the negative factors mentioned prior.
In closing, carbohydrate intake should not be considered as linear or universal approach where all training sessions require the same nutrient provision. Therefore, an individualised and periodised nutrition approach may be required to maximise the outcome of each and every training sessions by following the motto;
‘Fuel the work required’
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