Numerous studies over the last decades have highlighted the importance of nutrition for improving athlete performance and recovery, with the role of carbohydrates (CHO) being of special interest. During physical exercise – especially if it is carried out at high or moderate intensity – CHOs are one of the main sources of energy, sharing prominence with fats in long-duration sports.
CHOs are stored in the body as glycogen mainly in the muscle and liver, although it is now known that another important place of accumulation is the brain. When performing physical exercise, these deposits are depleted and the concentration of glucose in the blood decreases, which leads to the development of fatigue (known as the familiar "wall" or "bonking") especially if we do not have an optimal metabolism of fats.
Carbohydrates during exercise
To avoid fatigue caused by glycogen depletion, it is necessary, apart from starting exercise with glycogen stores as full as possible, an adequate exogenous intake of CHO.
Providing CHO during exercise will improve our performance. However, it is important to take into account some basic considerations since not all CHOs provide energy or are digested and absorbed as quickly. The maximum absorption rate of CHO is 60 g/h, which corresponds to CHO such as glucose, sucrose, or maltodextrin. On the other hand, other CHOs such as fructose or galactose are absorbed more slowly, which can cause gastrointestinal discomfort.
Although the maximum absorption ratio for a type of CHO is 60 g/h, in long-term exercise it may be advisable to provide a higher amount of CHO (up to 90 g/h). To do this, and taking into account that the intestinal transporters for a single type of CHO become saturated above 60 g/h, it is advisable to provide CHOs that use different transporters to enter the cell. Thus, it is possible to provide more than 60 g/h of CHO (up to 90 g/h) if CHO is used with different transporters (e.g. glucose and fructose), which has been shown to reduce gastrointestinal discomfort and increase performance to a greater extent than with the consumption of a single type of CHO (Currell and Jeukendrup, 2008).
In addition, CHOs could increase performance not only through metabolic mechanisms (greater glucose intake) but also central mechanisms (at the brain level). In fact, it has been seen that rinsing with CHO without ingesting them is enough to produce changes at the brain level and improve performance in tests of less than 1 h duration (Carter et al., 200; Chambers, 2009). Therefore, rinsing with a CHO solution could be a suitable strategy for short efforts such as a 10 km run.
Figure. Carbohydrate intake recommendations based on exercise duration. Retrieved from Jeukendrup (2014).
Carbohydrates after exercise
After an effort in which glycogen stores have been expended, the enzyme that is responsible for glycogen synthesis (glycogen synthase) is stimulated, increasing insulin sensitivity and the permeability of the cell membrane to glucose and thus favoring glycogen re-synthesis after exercise.
A classic study that popularized the term "metabolic window" found that glycogen synthesis was higher if CHO intake was done in the two hours post-exercise than if it was done later. Therefore, if you have little time between two exercise sessions (e.g. two sessions in the same day) it is important to consume CHO as soon as possible. More recently, however, it has been observed that, as a general rule, it is more important to consume an adequate amount of CHO (>1 g/kg/h) in the hours after exercise than at the exact time intake occurs (Burke, van Loon and Hawley, 2017). In addition, glycogen synthesis seems to be facilitated by the consumption of high-glycemic index or rapidly absorbed CHO (e.g. glucose) as well as by the combination of CHO together with protein (by increased insulin response) or creatine (by increased cellular osmolarity) (Burke, van Loon and Hawley, 2017).
High-carb or low-carb diet?
Although traditionally maintaining a diet with at least 60% CHO has been encouraged, vilifying fats regardless of their origin, more and more evidence puts CHOs in the spotlight, especially in the face of the growing prevalence of pathologies such as obesity or diabetes. In fact, there is ample evidence showing that low-carb diets appear to be more effective than low-fat diets for weight loss (Mansoor et al., 2016).
Diets with low CHO intake are also a physiological stimulus of relevance for endurance athletes. As we have mentioned, performance in these sports is highly conditioned by the availability of glycogen. It has been observed that restricting CHO intake (e.g., ketogenic diet) increases the ability to consume fat during exercise (McSwiney et al., 2017), which will result in glycogen savings and therefore delay fatigue. Thus, in long-term endurance sports (e.g. long-distance triathlon) where lipids are the main energy substrate and the ability to oxidise fats is directly related to performance, the adaptations produced to low-carb diets could be beneficial.
However, the evidence regarding the benefits of low-carb diets for performance is not unanimous. While it could provide certain benefits in long-term exercises, low-CHO diets have been shown to worsen energy efficiency (higher oxygen consumption for the same intensity) and decrease performance in short events such as 10 km (Burke et al., 2017). Therefore, although low-carbohydrate diets can provide interesting physiological benefits such as a greater capacity for fat oxidation - something essential especially in very long sports - they also seem to compromise the ability to perform high-intensity efforts. Faced with this situation, strategies such as the periodization of CHO intake have been shown to offer an effective balance, with high-intensity sessions with a high availability of CHO and lower intensity sessions restricting the intake of this macronutrient (Marquet et al., 2016).
Conclusions
During moderate or high-intensity exercise, CHOs become the main energy substrate, with glycogen depletion being a limiting factor in performance. Therefore, it is advisable to provide an optimal amount of CHO during exercise, as well as to replenish glycogen levels after exercise. Varying the intake of CHO in our diet will also involve physiological adaptations of relevance to our performance. Thus, low-CHO diets will increase our ability to oxidize fat during exercise, but will also limit performance in high-intensity efforts. Periodization of CHO intake (alternating periods of low and high CHO diet) along with periodization of training (alternating low and high intensity sessions) may be a suitable strategy to optimize performance.
References
- Burke, L. M. et al. (2017) ‘Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers’, Journal of Physiology, 595(9), pp. 2785–2807. doi: 10.1113/JP273230.
- Burke, L. M., van Loon, L. J. C. and Hawley, J. A. (2017) ‘Postexercise muscle glycogen resynthesis in humans’, Journal of Applied Physiology, 122(5), pp. 1055–1067. doi: 10.1152/japplphysiol.00860.2016.
- Currell, K. and Jeukendrup, A. E. (2008) ‘Superior endurance performance with ingestion of multiple transportable carbohydrates’, Medicine and Science in Sports and Exercise, 40(2), pp. 275–281. doi: 10.1249/mss.0b013e31815adf19.
- Jeukendrup, A. (2014) ‘A step towards personalized sports nutrition: Carbohydrate intake during exercise’, Sports Medicine, 44(Suppl 1), pp. 25–33. doi: 10.1007/s40279-014-0148-z.
- Mansoor, N. et al. (2016) ‘Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials.’, British Journal of Nutrition, 115(3), pp. 466–479.
- Marquet, L. A. et al. (2016) ‘Enhanced Endurance Performance by Periodization of CHO Intake: “Sleep Low” Strategy’, Medicine and Science in Sports and Exercise, (JANUARY). doi: 10.1249/MSS.0000000000000823.
- McSwiney, F. T. et al. (2017) ‘Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes’, Metabolism. Elsevier Inc., 81, pp. 25–34. doi: 10.1016/j.metabol.2017.11.016.
AUTHOR
Pedro Valenzuela
Researcher in the Physiology Unit of the University of Alcalá and in the Performance Control Unit at the Sports Medicine Center (AEPSAD, CAR of Madrid).
Web: www.fissac.com
Degree in Physical Activity and Sports Sciences, Official Master's Degree in Integrative Physiology, University Specialist in Personal Training, NSCA – CPT.
