Carb Loading <sigh> - A late night meander

I was asked recently about carbohydrate loading and my first reaction was "don't" but I reined that in and asked the sensible questions: what sport, what level of activity/duration, what standard of event/competition?

The response was amateur rugby, so I reverted to my first answer! However, this answer seems a little trite and unhelpful. So I got to thinking (not unusual) and what started life as a short answer rapidly escalated into a lengthier one. I'll explain not only my reasoning but go some way to tightening up the definition of the term itself. That last act in itself may go some way to leading to your own conclusion as it the term is frequently abused which I feel leads to much of the misapplication.

I have skipped some of the physiology in the interests of not extending the answer any more than I already have. I have tried to be faithful to the science but by skimping have probably done it a massive disservice. If more information/clarity would be useful feel free to ask.

First some physiology (short version)

When a cell in your body needs energy it commonly uses ATP which is made available to the body through the process of cellular respiration. Carbohydrate is stored in the muscles and liver as glycogen (approximately 350g in the muscles of an untrained individual and a further 100g in the liver - enough to sustain you for a day or two hours of exercise). When you exercise the body needs to generate ATP as fuel via one of its three energy systems: ATP-PC; anaerobic glycolytic, and aerobic (made of glycolytic and lipolytic systems) systems. ATP-PC generates maximal bursts of speed and strength for up to six seconds of high intensity exercise, at which point the glycolytic system takes the lead. After 30 seconds the system contributes up to 60% of your energy down to 35% after 2 minutes1 The aerobic system increases its contribution during this time, until it takes over. The various times do not suggest checkpoints at which one system takes over, rather they all work concurrently from the beginning with the emphasis shifting over time.

Carb Loading

For endurance athletes, the limits of the glycogen supplies in the muscles were felt to limit the performance of the athlete in longer events and the 1960s witnessed a period of investigation and research into overcoming this limitation.

The method proposed by Karlsson and Saltin in 1971 based was thought to stimulate the hormone glycogen synthase and via a period of glycogen depletion and subsequent refuelling, would promote a hypercompensation such that the stores would be greater than previously possible. The initial depletion was achieved by exhaustive training on day one, coupled with a low carbohydrate diet for days 1 to 3. This was followed by three days of loading, accomplished via reduced training and a high carbohydrate diet.

The downside of this approach was the negative of the depletion phase. Anybody who has tried a low carb diet or cutting weight for boxing will tell you how rubbish you feel in the short term. Athletes who tried carb loading reported feeling weak, irritable and unable to train during the depletion phase. This, coupled with athletes failing to load sufficiently during the next three days led to further thought.

Sherman et al (1981) suggested omitting the three-day depletion phase. Their method was to undertake endurance training for approximately 1 hour to reduce the hepatic (liver) glycogen. Days 2 through 4, taper training and consume a moderate carbohydrate diet (5-7g Carbohydrate per kilogram of bodyweight). Days 5 through 7, continue to taper the training or rest completely while increasing the carbohydrate intake to 8-10g per kilogram of bodyweight.2

The final piece of advice that accompanies carb loading (after testing it in training to see what effect it has on performance and any gastric distress) is that it should not be attempted more than 3 times per year.

So we see that, even with the truncated model, there are a few constraints that do not suit the application of the concept to rugby (or indeed any team sport). And with the explanation ringing in our ears, we suspect that this is probably not what was intended! If you have not already questioned the wisdom of the exercise on the basis of normal stores (and more of that follows below), there is a useful comment in the NSCA's primary textbook, almost hidden in a section on recovery (my emphasis)

"Most research on recovery has focused on glycogen repletion, and since glycogen is usually not depleted during intermittent activities such as many team and skill sports..."

But it is not just glycogen

During the glycolytic phase, your cells produce lactic acid as a byproduct. It has been held for years that lactic acid, which is responsible for the burning sensation in the muscles, is a toxic byproduct which impairs performance. Increasingly, research has demonstrated that lactic acid is used as a fuel in its own right and goes some way to creating the bridge between the glycolytic and aerobic energy systems.4 The difficulty is that at high intensities, the lactic acid is produced at a greater rate than the body can utilise it.

However, as Verkoshansky and Siff remind us, once sufficient oxygen again becomes available either at rest or a decrease in exercise intensity, the lactate is converted to pyruvate and via gluconeogenesis is used to manufacture glucose in the Cori cycle 5 (which explains why 15 minutes after exercise the muscles are clear of lactic acid). Far from being remorseless sadism (although that does have its part to play), the high-intensity sessions that a good coach will be applying should be slated with this in mind, to prepare the body for processing the lactate efficiently, not to mention the mental aspects of getting used to working while in discomfort.

Incidentally, it is also increasingly believed that it is the increased presence of Hydrogen ions in the muscles, as opposed to the lactic acid, which cause the post-exercise muscle soreness.

Supply and Demand

Not only is carb loading difficult to get right, but as mentioned above, under normal conditions there are sufficient supplies of glycogen in the liver and muscles to fuel approximately two hours of high intensity exercise.

Tim Noakes demonstrates the point by saying that for a 70kg athlete, with average running economy, running at world record marathon pace (19.8 kmh at the time of writing his book) there would be sufficient glycogen stored to enable him to run for 125 mins.6 For kicks I ran the same equations but factored in one of the lads I train with. Weighing in at 115kg and assuming a low-average running economy his stores would last for 112 minutes. Even if we downgrade the estimation of his glycogen storage to that of the untrained individual, he would still have sufficient stores to last 82 minutes (bear in mind that this is ignoring other fuel supplies and the creation of new glucose within the body).

Timing is Everything

Rugby Union matches are slated for 80 minutes of game time, however, the ball is in play for substantially less than that. The advent of professionalism increased the amount of time that the ball was in play to a colossal 32.1%7 further increasing to 43% in the 2003 World Cup8 and ranging from 39%-46% during the 2009 Tri Nations Tournament9 In the most recent Rugby World Cup, over the course of the quarter and semi-finals the average time that the ball was in play was just over 48%.10 So, we can reasonably safely say that for rugby union players, especially amateur ones, glycogen depletion is unlikely to be a limiting factor in performance.

Alright smart-arse, what would you do?

There will be a need to keep an eye on training loads in the days immediately preceding matches. Physical and mental fatigue, coupled with a need to adequately refuel may present issues which will compete with optimal performance on the day. However, with the prevalence of carbohydrates in standard shopping and the modern diet, not to mention the cuisine of the average office worker, satisfying the needs is not likely to be an issue. Further, aside from the additional weight associated with the 3g of water that is stored for every gram of carbohydrate, once the glycogen stores are full, the liver converts the excess carbohydrate to fat, releasing into the blood stream for transport to adipose tissue (body fat).

Personally, I would be happier to see my players (amateur or professional) spending some time on the foam roller, making sure that they are properly rested and hydrated, and, instead of pounding their bodies with additional carbohydrates (often in the form of unfeasibly large bowls of rice and pasta), managing their inflammation with some oily fish/fish oil supplementation and some light mobilisation exercises. This approach has less possibility of impacting negatively on their performance and a lot of scope for improvements on the field, if only as a result of mentally clearing the decks.

Returning to the carbohydrate question, with the game requirements, the alternative fuelling methods (I have not even touched on fat adaptations in this), lack of individual awareness of quantities and types of carbohydrate, I find myself leaving the last word to Tim Noakes

"This raises the question of whether such diets [high-carbohydrate] are really necessary during training (Sherman and Winter, 1991; Noakes, 1997) [...] It is possible that low muscle glycogen stores may not affect muscle performance as adversely as is generally believed (Grisdale et al 1990). Any diet that prevents the development of hypoglycemia during exercise may be all that is required."11


1 Bean, A,Sports Nutrition 5th Edition, (London, A & C Black, 2006), P7

2 Ackland, J, Complete Guide to Endurance Training (3rd Edition), (A&C Black, London, 2007)

3 Baechle, T & Earle, R, Essentials of Strength Training and Conditioning (3rd Edition), (Human Kinetics, Champaign, Illinois, 2008), P 222

4 Hashimoto, T, Hussien, R & Brooks G, "Colocolization of MCT1, CD147, and LDH in mitochondrial inner membrane of L6 muscle cells: evidence of a mitochondrial lactate oxidation complex",American Journal of Physiology Endocrinology and Metabolism, 290:1237-1244 (2006)

5 Verkoshansky, Y and Siff, M,Supertraining (Sixth edition), (Verkoshansky SSTM, Rome, 2009)

6 Noakes, T, The Lore of Running (4th Edition), (Human Kinetics, Leeds, 2002), P101

7 Eaves S, Hughes M, "Patterns of play of international rugby union teams before and after the introduction of professional status", International Journal of Performance Analysis in Sport, 3 (1) 1 December 2003, PP103-111

8 Morgan, R, "Law Changes and injury in Rugby Union", Journal of Sports Therapy, 2 (1) Spring 2009, Pp3-6

9 International Rugby Board, "Statistical Review and Match Analysis Tri Nations 2009", P17


11 Noakes, P153

Location:Bristol,United Kingdom


Popular Posts

Competition without competing

Burpees Anybody?

100 day burpee ladder...the conclusion

Review and Apology

The 100 day burpee ladder...the beginning