Aspects and Flaws of the Carbohydrate Story in Endurance Performance

As I observe it since several years in triathlon “Food” its 4th discipline is flooded by sloppy scientific facts. Study results telling you their truths are available in huge amounts. Critical, controversial and incomplete are the underlying theories, that present the matrix for interpreting the data. Data remains until today fluffy and inconclusive. This applies first and for all for carbohydrates the pillar of nutrition for all athletes.
Since the early 20th century carbohydrates are considered as the fuel for athletes. In the following I don’t want to question carbs as such, I only want to direct your attention to the fact that the scientific foundation on which diets are based is weak and arbitrary. Therefore your personal experience counts more than you may assume. To find the optimal diet and the optimal race nutrition needs you as an experimenter, this is evidence-based research work!

Food – whether a diet works for you or not – is all about experience decorated with some basic scientific facts

The observation that experiences with the same diet regimen among individuals vary on a broad scale underscores this view. What works well for you does not necessarily work another one. During the last months when I was touring through Europe with Chris “MACCA” McCormack from one race to the other I learned about the many gastrointestinal problems such as stomach pain or cramps, nausea, vomiting, diarrhea, cramps etc. that athletes experience during or already shortly before a race. The reasons and explanations given for these problems are in line with the predominant paradigms governing exercise science, it is either salt or/and carbohydrates, or if these explanations don’t work or are unsatisfying it is nervousness about which nobody really knows what it means in the very end.

I was amazed to which extent carbohydrates in form of gels and bars are currently used as race nutrition, be it on the bike or the run. It occurred to me that the gastrointestinal issues may be due to too much of the good, that the quantity of ingested carbs may be too high or the timing may be inadequate. Another thought that popped up was that it might be wrong to narrow all the troubles down to carbs and salt, that the “carbohydrate depletion model” that is the underlying matrix for all the explanatory arguments may be incomplete and insufficient to explain the problems. I think it is justified though to question a model that cannot answer the questions asked anymore.

Beside the whole spectrum from nausea, vomiting, diarrhea and cramping in addition more and more complain about a feeling of distaste for sweet stuff during the race. They are not able to ingest the sweet gels anymore. And many an athlete tells me that it does not occur to them to use gels during training. This feeling may be due to several reasons: the sweet taste as such, the texture of the gel (to my knowledge fluids like coke containing simple sugar don’t induce these reactions), a reflux into the oesophagus induced by the water absorbing carbs, an irritated mucosal lining and disturbances of motility and permeability of stomach and gut, not to forget the additives that they may contain. Especially under stress these may initiate food intolerance. All together this may lead to this feeling of nauseation.
I don’t dare to conclude straightforward that the increasing gastrointestinal (GI) troubles in elite athletes are caused by their race nutrition. But I think it is time to look at these GI health issues that athletes experience in a race in greater detail. Gels are eaten in such huge amounts and their benefits are unquestioned. But the reason why athletes have to eat so much carbs, namely to avoid carbohydrate depletion in muscle and liver in my opinion needs reconsideration.
What I did do now, I went into the literature. If you start digging, you find a huge amount of studies on carbohydrates that are inconclusive not only due to their study design, but also due to the premises they were founded on.

The studies available on carbohydrates are not conclusive, probably because they are extremely difficult to conduct. Most of them have been performed in a laboratory, the athletes tested performed on a steady performance level, and studies have not been controlled and blinded. There are other flaws too such as inter-individual differences, that make data incomparable. The only 2 placebo-controlled double-blind studies for example carbo-loading gave  no indication that carbo-loading is superior to placebo.
The Carbohydrate Depletion Model is so strong per se, that studies that are not double-blinded can easily be flawed from either side, from the investigator’s and athlete’s perspective. If individuals were told that they were ingesting carbohydrates even though they were on placebo, the test results have been as if they had eaten carbohydrates. This is only one more proof on how strong the influence of our mind/belief (one name it) is on performance as such. The results of such studies don’t tell us anything about cause and effect. They tell us more about the investigator’s mindset than giving us answers to our questions. This happens a lot, if you look into studies results.

Glycogen stores, hypoglycemia and fatigue

From the Carbohydrate Depletion Model infers that full glycogen stocks are essential for peak endurance performance. This assumption entails the various approaches to carbohydrate ingestions, be it high-carb diets, carbo-loading or race nutrition. I think there is currently little doubt about the fact that you should enter a race with restored glycogen stores in the muscles and the liver. About how this is achieved a debate may be allowed, moreover about the viewpoints for whatever biological reasons carbohydrates or glucose respectively are needed during a race. Studies have been performed that suggest that the muscles’ glycogen stores even after strenuous workouts are never completely emptied, that general fatigue and finally exhaustion and dropping out of the race occurs before that. If this is the case, then the use of carbs in endurance performances longer than about 90 minutes has to be revisited. By the way, an energy-depleted muscle would develop rigor, this is when muscles become completely stiff.

May be we only need the carbs to avoid hypoglycemia, a notion that is not completely in line with the Carbohydrate Depletion Model. To prevent hypoglycemia may be crucial for sustaining performance over 2 hours. It is the liver’s glycogen stores that are responsible for feeding glucose into the blood stream. Glucose (simple carbohydrate, sugar) is not only fuel for the muscles to move, but for many other body parts/organs that have to function properly. The brain needs glucose badly to perform. Symptoms that occur, if the brain is undernourished are dizziness, fatigue, loss of orientation, movements disorder (coordination), impaired cognition. As soon as blood glucose levels are back to normal the symptoms disappear. You may have experienced this as an athlete! Hypoglycemia during a race may happen due to the fact that glucose intake was inadequate, and the liver supplies do not suffice anymore.

General fatigue is another symptom that violates the depletion model. A couple of studies show that even at complete exertion the muscles are not empty of glycogen. Pacing, for me the fantastic ability of a human mind to anticipate a race, cannot be explained by carbs or glycogen stores either. The decision to quit a race due to fatigue or exhaustion happens in the brain, where the commands for muscle movements come from. Neuromuscular recruitment becomes impaired, temperature may increase due to autonomic nervous system dysregulation in cases of general fatigue. This happens not due to a lack of fuel within the muscle tissue, the brain stops the exercise before this can happen. To replenish glycogen stocks is not the solution, and won’t work. Under stress, and racing is definitely extreme stress for the body, the body does not have time nor power to save stuff for storing, even if you feed it with big amounts of carbs. But what happens if you do so, you start suffering from symptoms of gastro-intestinal distress. There are probably only a few athletes out there who don’t know what I am talking about.

Here some figure to ease your mind: The current consensus is that the fastest initial rates of muscle glycogen resynthesis is achieved when 1.2 g carbohydrate per kg body weight per hour are eaten every 30 minutes over 5 hours (= about 400 g). It is improbable that such a high rate of carbohydrate ingestion could be sustained for much longer than 5 hours without producing diarrhea and gastrointestinal distress. It is usually presumed that optimum carbohydrate loading can be achieved if about 600 g carbohydrate daily is ingested for 2 or 3 days. Data indicate that it may not be of importance whether you eat simple (glucose) or complex (starch) carbohydrate. This applies for a resting condition only, under stress the situation changes completely.

Running on fatty acids not on carbs?

This is were fat comes into the game. I know that it got a bad reputation, for wrong I think. There is a large body of evidence showing that athletes can adapt to high-fat diet without sacrificing endurance performance. Some have even suggested that exercise performance, especially during prolonged exercise, might actually be enhanced following adaptation to high-fat diet. In fact, blood concentration of the protective HDL-cholesterol fraction increased on this diet, whereas unfavorable changes in total cholesterol and triglyceride concentration were measured in the group ingesting a high-carb (about 70%) diet.
Nurture your body from fatty acids spares liver glycogen, and in the end glucose supply. High-fat diet is still very controversially discussed. But to me it is seems to make sense to train the fat metabolism, and test how it works for you, if you rely more on your fat metabolism and fatty acids as fuel. You don’t gain weight and you don’t provoke insulin secretion that may lead to bouts of irresistible hunger episodes that make you gain weight, you may even run into more severe health hazards later.

To cut a very very long story short:
The Energy Depletion Model cannot explain how elite competitors in the Ironman Hawaii triathlon race are able to swim, cycle, and run at more than 65% VO2max for about 8 hours and more without slowing down. Without any doubt you develop already a profound muscle glycogen depletion in your active leg muscles after cycling, and you are still able to run a 2:45 marathon. If muscle depleted of glycogen is unable to sustain a high work rate of about 65% VO2 max, then these performances are physiologically impossible. We all know that this is not the case!

Maybe you run on fatty acids, keep blood glucose levels stable with carbs and save the glycogen stores in the muscles to prevent a premature onset of fatigue or exhaustion. Could this be a recipe!

The whole 4th discipline of triathlon is so incredibly complex, and we know so very little about it, that experiments and studies are still needed. But what is needed too is an open mind that questions the established models, especially when its weaknesses start to become obvious. As an athlete you have to dare to leave the beaten path, if a concept does not work for you. In the end it seems to me that much more has to be taken into the equation than only focusing on carbohydrates to fully understand peak endurance performance on the one hand and fatigue and exhaustion on the other.



Susann is the biest prototype and head of the team. She is Austrian, has studied medicine, meaning she is a medical doctor and the Biesters' alpha wolf. Susann continuously produces new ideas, is strong in making concepts and is practically always ON FIRE. Without her BIESTMILCH wouldn't be where and what it is today, and anyway - not possible.

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  1. Hello I live in Sweden and I would love to be able to order your products but I don´t know how? Hope this problem is solvable, best regards,


  2. Hello I live in Sweden and I would love to be able to order your products but I don´t know how (I get no through the webshop)? Hope this problem is solvable, best regards,


  3. So the conclusion of this article is that we should be burning fat, and we should train our body to burn fat, right? Now what is the recipe for this training? How do we train our body from being carbohydrate burners to fat burners? Does that mean we change our diet? Or change the timing of our diet? During training? Racing?

  4. Great article

    the answers to your questions may lie in IMTG’s… I think they play a huge role… let me know what you think

  5. Hey Fredrik,

    please, order in our web store, we ship the goods all over Europe. What kind of problems did you have with our store?
    We need some more details to work on this issue. Orders from other European countries get in.

    Sorry for delayed answer, best regards, Susann

  6. Hello David,

    to answer your question is not easy at all. In my view, you have to take the time to experiment in training what works best for you. An overload of carbs in training, I think is not good. Provocation of insulin secretion, and an acidic metabolic state in tissues is a fact that influences well-being and recovery. On the other hand it is necessary to test race nutrition in training. I know from many athletes that it does not occur to them using gels or bars during training sessions. You test it and see how sensitive your gut reacts to these carbs/sugars during high-intensity sessions.

    Generally spoken, fatty acids are the fuel for endurance performance. But of course, it depends on your performance level too, on how you should compose your nutrition.

    I don’t know whether this brief answer is of any help, because at the very end everybody is left to his/her own devices.

    All the best, Susann

  7. I’ve got problems with seeing your blog correctly through the most recent version of Opera. It is fine in IE7 and Firefox though.Hope you have a nice day.

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  9. So the conclusion of this article is that we should be burning fat, and we should train our body to burn fat, right? Now what is the recipe for this training? How do we train our body from being carbohydrate burners to fat burners? Does that mean we change our diet? Or change the timing of our diet? During training? Racing?

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