Training in a low carbohydrate (CHO) state has been used in many years by endurance athletes to manipulate the body's fuel utilization during training. The desired outcome has been to improve fat oxidation in order to rely more on fat stores, and spare valuable glycogen for the decisive moments in a race.
Some scientific studies has shown good results with CHO manipulation ie. training in a carbohydrate depleted state, using methods like:
‘Sleep-low’: Doing a hard CHO-depleting workout late in the day, then sleeping in a low-CHO state, and completing a moderate intensity workout the following morning.
‘Fasted-training’: Completing a longer endurance session in a fasted state after an overnight fast, and with no or very restricted CHO intake during the session.
‘Twice-a-day sessions’: Performing two sessions in the same day, where the second session is performed with lower CHO levels.
The reasoning for training in a state with low CHO availability, is that it promotes favorable muscle cellular adaptations. Thus optimizing endurance performance. Other studies have shown no performance enhancing effects with CHO manipulation. So the questions is, whether
or not it is worth trying CHO-manipulation in your own training?
Recently a study used a method called ‘fuel for the work required’, where CHO-intake is adjusted to the goal, intensity and duration of a given training session. This could be beneficial due to the fact that feeding the appropriate amount of CHO required to complete a certain session, could possibly promote a higher quality of the outcome of the training session. This is indeed interesting in itself. But it becomes even more interesting, if you compare the effect to the effect of the same training session being completed in a high-CHO state (ie. having high CHO availability in a session). In other words: is a ‘fuel for the work strategy’ more beneficial compared to a constant high CHO availability?
The study was conducted with U23 elite cyclists and lasted for 5 weeks. The cyclists were divided into two groups. One group using CHO-periodization, where CHO intake was adjusted to the intensity and duration of the sessions. The other group trained on high CHO availability in all sessions. Both groups completed the same training program for all the five weeks.
The CHO-periodization group completed 13 sessions on low CHO, described as an endurance session below the first lactate threshold (AeT/VT1, training zone 1-2 on a 7 zone scale), which is below a 2 mmol lactate concentration. The CHO-high group completed theese sessions with normal CHO availability. Both groups performed 13 moderate to high intensity sessions (zone 3 - zone 6) with high CHO availability, and also completed 9 strength training sessions during the 5 weeks. The overall energy intake was matched between the two groups, and therefore the CHO-periodization group did consume more fats and proteins to keep up with the CHO-high group.
To assess how the two groups did respond to the 5 weeks of training, a maximal lactate steady state (MLSS) and time to exhaustion test (TTE) was performed before and after the training intervention. Body composition and anthropometric data were also measured before and after.
After the 5 weeks of training and CHO-manipulation there were no significant differences between groups in the following parameters. Both groups improved to the same extent.
MLSS: Both groups improved to a similar extent in watts and watts/kg.
TTE: No differences in improvement TTE performance between the groups.
CHO and FAT oxidation. No differences in CHO and FAT oxidation between groups.
Muscle mass and % body fat: No differences found between groups.
To sum up! Despite the differences in CHO-availability there were no differences in the effect of the 5 week training program. Thus CHO-periodization with adjustment of CHO availability from day to day, was not superior to a high steady CHO availability.
So what to choose if both interventions seem to work for improving performance? Looking at the latest trends in cycling, where CHO intake during training and competition is getting higher and higher, it is most likely best to stick with high CHO availability. Why? It improves performance and recovery - quite simply.
‘Old’ recommendations on CHO intake are outlined below.
Light exercise <60 min: Carbohydrate supplementation is not needed.
Sustained high intensity training between 45-75 min: Up to 30 grams of carbohydrate.
Endurance exercise of 1-2½ hour in duration including steady state efforts or interval training: 30-60 grams of carbohydrate per hour. The longer the session, the more it is recommended to go to the higher end of the range (50-60 grams of carbohydrate per hour).
Prolonged endurance exercise >2½ hours including steady state efforts or interval training: Up to 90 grams of carbohydrate per hour. This is the highest theoretical rate of carbohydrate uptake, and many athletes find it hard to consume that much during training and competition. So it needs to be adjusted to individual preferences, and carbohydrate uptake needs to be practiced during training to adapt to high consumption rates.
As you can read 90 g CHO/hour is set as the highest theoretical amount the body can take in during exercise. And that is only recommended for session lasting 2½ hours or longer. More and more practical evidence from leading experts and teams in pro cycling suggest that taking in between 80-100 grams of carbohydrate per hour (g CHO/hour) as a general rule has greater advantages on performance and recovery. Further it is more and more common that pro riders consume up to 120 g CHO/h. Thus, it seems that practical knowledge from the real world of cycling is ahead of the scientific recommendations.
The possible problem with going above 90 g CHO/h is that it can cause stomach related issues, as the body can have trouble tolerate and absorb such a high amount of CHO. But as the quality of energy products has improved a lot in recent years, new formulas for CHO drinks and gels have made it easier for the body to tolerate and absorb these high amounts of CHO. So staying in the range of 80-100 g CHO/h should be doable, tolerable and beneficial. Hence, recommended for demanding long duration training sessions or shorter high intensity sessions.
So if you are not already trying to top up your CHO intake during training and competition, it might be beneficial to start doing so. Especially if you are a competitive cyclist trying to improve your performance!
Be aware that you might need to train your ability to take up and tolerate a high amount of CHO. So instead of starting straight out on 80-100 g CHO/h, you might start at 60-70 g CHO/h, and then gradually increase the amount of CHO.
References:
Prieto-Bellver et al. Nutrients (2024) 16:338. A Five-Week Periodized Carbohydrate Diet Does Not Improve Maximal Lactate Steady-State Exercise Capacity and Substrate Oxidation in Well-Trained Cyclists compared to a High-Carbohydrate Diet.
Kerksick et al. Journal of the International Society of Sports Nutrition (2017) 14:33: International society of sports nutrition position stand: nutrient timing.
Kerksick et al. Journal of the International Society of Sports Nutrition (2018) 15:38: ISSN exercise & sports nutrition review update: research & recommendations.
Vitale et al. Nutrients (2019) 11:6: Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations.
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