Carbohydrate and Cycling: Could Training with 'Low Glycogen' Offer Any Benefits?

Introduction

Most of us that are into sports nutrition are aware of the importance of carbohydrate on performance. Each gram of carbohydrate provides 4 kcal worth of fuel and at higher intensities, carbohydrate is the preferred substrate during exercise. Moreover, carbohydrate is also an important fuel that is used by the brain, the liver and the central nervous system. We have known for years now that carbohydrate availability is a critical factor for events lasting longer than 90 minutes as total body carbohydrate stores are limited, and are often substantially less than the fuel requirements of the various training and competition sessions undertaken by cyclists. The purpose of this article is to examine whether adaptations to endurance training can be enhanced by manipulating carbohydrate intake. More specifically, could training with low carbohydrate stores rather high, a practice discouraged by many nutritionists, enhance the body's ability to respond to the training stimulus? This article will answer this question and provide some practical recommendations of carbohydrate intake for training and competition.    

Why Carbohydrate is Important?

In the body, carbohydrate is stored as glycogen. Around 80-100 g is stored in the liver and 300-400 g is stored in the muscles however, factors such as training status and dietary intake of carbohydrates will affect the exact amount stored. These stores should provide most individuals with approximately 90 minutes worth of fuel if exercising at a moderate intensity. The American College of Sports Medicine published a position statement¹ on nutrition guidelines for athletes in 2009 and carbohydrate recommendations suggested range between 6-10 g/kg of body weight. However, more recent studies have examined whether lower amounts of carbohydrate during certain training sessions could be beneficial in enhancing endurance training adaptations which we will discuss later. We have known for over 40 years now that carbohydrate loading, an ergogenic practice where you increase the amount of carbohydrate eaten in the days preceding an event, the body will increase its glycogen stores which will prolong endurance performance. Various different carbohydrate loading protocols exist. The early ones in the 1960's suggested that in the week leading up to an event, carbohydrate intake should be reduced and training continues as normal, which depletes the glycogen stores, followed by a very high carbohydrate intake 3 days before the event, which would supercompensate glycogen stores. As we started to learn more about these mechanisms, this practice was refined and we now know that there is no longer a need to deplete glycogen stores before supercompensating them. Simply by increasing carbohydrate to 10-12 g/kg of body weight during the taper phase a few days before an endurance event lasting longer than a few hours has the same ergogenic effect of prolonging endurance exercise performance. Carbohydrate loading is usually reserved for races however, many cyclists also train on a fairly high carbohydrate diet of around 6-8 g/kg of body weight. More recently however, nutrition research has looked at whether cyclists would get more benefit from their training by following a low carbohydrate diet. Scientifically, this concept makes a lot of sense. When there is reduced carbohydrate availability, the muscle will oxidize more fats. If you deliberately ‘encourage’ the muscle to oxidize fat it becomes more energy efficient therefore, the rationale for training with low carbohydrate could offer beneficial adaptations in the long term.

Training 'Low'

One of the first studies to investigate training with low carbohydrate was done in 2005 by Hansen and colleagues². This was a very clever study where one leg was trained in a low glycogen state and the other leg was trained in a high glycogen state. This was achieved by eating 8 g/kg of carbohydrate per day whilst the 'low’ leg trained twice a day and the 'high' leg trained every other day, very clever stuff!    Hansen and co-workers found that 10 weeks of endurance training resulted in a 90% increase in time to exhaustion in the ‘low’ leg during a knee extensor exercise. It was also found that the activity of citrate synthase and 3-hyroxyacyl-CoA dehydrogenase, two key enzymes involved in fat metabolism, was increased. These findings suggest that training in a glycogen depleted state could increase the muscle's ability to use more fat as a fuel during exercise.  Three years after this study, Yeo and colleagues³ carried out a similar study but the participants where 14 trained endurance cyclists. The cyclists were required to carry out six cycling sessions per week where three sessions were 100 min at 70% intensity and the other 3 were high intensity intervals (8 x 5 min with 1 min recovery). 7 Cyclists in the 'low' group had to train every other day but twice a day (moderate intensity session followed the high intensity session) which meant that the 100 min was done with low glycogen, and cyclists in the 'high' group trained once a day but every day alternating between moderate intensity and high intensity which meant they were training with more glycogen. Interestingly, despite the cyclists in the low group experienced difficulty in doing the interval training, fat oxidation and activities of citrate synthase and 3-hyroxyacyl-CoA dehydrogenase had increased in the low group. These results are similar to the study done 3 years ago by Hansen and co-workers but this time in endurance trained cyclists suggesting that training with low glycogen can enhance fat oxidation and adaptations to training in comparison to high glycogen stores. Two years later, an almost identical study done by Hulston and colleagues (2010)⁴, found similar results in cyclists where fat oxidation in the 'low' group increased during submaximal cycling. In 2009 a research group from Liverpool led by Morton and colleagues⁵ found that when runners trained with 'low' carbohydrate stores over a six week period, mitochondrial enzymes responsible for fat metabolism were improved compared to the high carbohydrate group.

Whilst these findings are exciting and promising there have been other studies conducted by De Bock and co-workers (2009)⁶, Cox and colleagues (2010)⁷ and Akerstrom and colleagues (2009)⁸ using the training 'low' protocol that found no benefit on either fuel use or performance. One thing to consider is that although fat oxidation and enzyme activity for fuel use were enhanced in some of these studies performance remained the same in both the high and the low carbohydrate groups. However, these studies were conducted during a relatively short period (4-10 weeks), and the benefits offered by training low in the longer term, may be more beneficial in increasing fat utilisation and preserving glycogen which can prolong and possibly enhance cycling performance. 

What are the Mechanisms?

Although more research is being conducted to further investigate the exact mechanisms, it is believed that when we train in a glycogen depleted state, our bodies have an improved ability to use fat as a fuel to provide energy for training. It has been suggested that a transcription factor called PPARfb is activated when training in a glycogen depleted state which in turn increases the enzymes in the body such as citrate synthase and 3-hyroxyacyl-CoA dehydrogenase which are responsible for increasing the use of fats to fuel exercise. These enzymes are less active if training is done when plenty of glycogen and carbohydrate is available.

  

Some Problems with Training 'Low'

Although there is some exciting evidence that training with low glycogen stores can enhance endurance training adaptations and increase enzyme activity that regulates fat use during exercise, there are a few disadvantages. Firstly, training with lower glycogen stores will result in a reduced training intensities and volumes as the muscle requires more carbohydrate as exercise intensity and volume increases. Secondly, training with low carbohydrate intake for prolonged periods increases the likelihood of getting ill through upper respiratory tract infections (common cold). And finally, low carbohydrate intakes before and during longer rides will result in 'bonking' or 'hitting the wall' which result in fatigue and loss of power.

Practical Implications

It is not recommended that all training sessions are done 'low' but instead a more strategic approach may offer greater benefits. For example, easy and moderate rides lasting anywhere between 1-2.5 hours can be done first thing in the morning in a fasted state and no food should be consumed during the ride. This will obviously limit how hard you can ride which is why it should be kept to an easy/moderate pace. It is also important that you don't eat a high carbohydrate diet the day before to ensure that glycogen levels remain low the following day. Longer sessions and interval training sessions can be done later in the day after breakfast and lunch where more carbohydrate has been consumed. It is not recommended that cyclists race 'low' as the high intensities and durations during competition are reliant on carbohydrate therefore increasing carbohydrate to 8-10 g/kg of body weight a few days before a race lasting longer than 90 minutes remains a sensible ergogenic race nutrition strategy.  

Conclusion

In the real world, many endurance athletes already practice training 'low' without being conscious of it as described in the practical implications section. As there is still a discrepancy regarding the science on training 'low' there is still ongoing research being done in this area as it is still not that well understood. However, this is an exciting area of research and there are clearly studies that have shown there could be some benefits in strategic 'low' training to enhance the endurance adaptations to training. 

References

1.    American College of Sports Medicine position stand. Nutrition and athletic performance. Med Sci Sports Exerc. 2009 Mar;41(3):709-31

2.    Hansen et al. Skeletal muscle adaptation: training twice every second day vs. training once daily. J Appl Physiol. 2005 Jan;98(1):93-9

3.    Yeo et al. Fat adaptation followed by carbohydrate restoration increases AMPK activity in skeletal muscle from trained humans. J Appl Physiol. 2008 Nov;105(5):1519-26.

4.    Hulston et al. Training with low muscle glycogen enhances fat metabolism in well-trained cyclists. Med Sci Sports Exerc. 2010 Nov;42(11):2046-55.

5.    Morton et al. Reduced carbohydrate availability does not modulate training-induced heat shock protein adaptations but does upregulate oxidative enzyme activity in human skeletal muscle.J Appl Physiol. 2009 May;106(5):1513-21

6.    De Bock et al. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake. J Appl Physiol. 2008 Apr;104(4):1045-55.

7.    Cox et al. Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. J Appl Physiol. 2010 Jul;109(1):126-34.

8.    Akerstrom et al. Glucose ingestion during endurance training does not alter adaptation. J Appl Physiol. 2009 Jun;106(6):1771-9