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Race Fueling

Want to Avoid Bonking? Learn the Science Behind It

Nutritionist Scott Tindal does a deep dive on bonking, glycogen stores, and the train low diet.

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We’ve all done it—and we all dread it: bonking. That terrible feeling when you know you’ve not consumed enough calories and it’s going to be a slow, painful shuffle home or, worse yet, to the finish line. Yet understanding a little more about how bonking happens can go a long way to helping you prevent it. 

Firstly, let’s start with glycogen. Glycogen is a collection of glucose molecules (or sugar in its simplest form) and the simplest form of carbohydrate. Glycogen particles come in two forms: proglycogen and macroglycogen. These two forms of glycogen particles are important to understand as they are both responsible for differing rates of glycogen particle repletion. The proglycogen acts quickly and is dependent on dietary intake of carbohydrates, thus allowing for rapid replenishment, while the macroglycogen are formed from collections of glucose units that build slowly and allow secondary glycogen particle repletion over a longer period of time to occur. Both are important phases and are described as the “biphasic nature” of glycogen repletion. 

Before worrying about getting depleted and needing to refill your stores, you should know that your body has approximately 600g of glycogen particles in it, dependent on body mass, diet, overall fitness and your most recent bout of exercise. The majority of your glycogen particles are stored in your muscles (~300-700g), and the remainder in your liver (~80-160g) and brain (100 times less than the number of glycogen particles stored in your muscle cells). The liver glycogen particles are constantly used to replenish the small amount (~4g) of glucose in your bloodstream. This amount of glycogen is entirely adequate for a sedentary individual who is not performing bouts of short to long, low to high-intensity exercise most days of the week (that’s not you, endurance athletes!). For triathletes, understanding your glycogen stores and how they affect you—and how you can affect them—becomes a lot more important to understand.

In order to have muscular contractions, ATP (adenosine triphosphate) needs to be produced. This is the energy that powers you during exercise. It is (at the most basic level) the energy that allows cells to function. Without it, you do not perform. It is produced by organelles called mitochondria, which are the energy factories of your muscle cells. ATP is produced by the oxidation of fatty acids from the bloodstream and from intramuscular fat stores, along with glucose supplied by the liver into the bloodstream and the glycogen particles stored in your muscle cells and between the muscle cells. It is widely agreed upon that as exercise intensity increases, reliance on blood and muscular glycogen particles increases. In fact, as intensity approaches 60% VO2max, you will find that the predominant fuel source being oxidized through anaerobic and aerobic processes to produce ATP will be the glucose in your blood and muscle.1 This is in large part due to the type of muscle cells being recruited (i.e. Type II) in order to allow for the intensity and exercise to continue.  

Bonking occurs when your muscle glycogen particle levels drop to an inadequate level that results in reduced muscle contractions, increased fatigue, and suboptimal effort.2 The mitochondria cannot produce ATP rapidly enough to sustain the exercise intensity that you require and as a result you are forced to slow down and ultimately stop. The duration and intensity of the exercise you are performing will have a huge impact on just how quickly you deplete your body of glycogen particles. The other factor that will contribute to bonking is the other workouts you have completed that day or in the days before, as well as your overall dietary intake of carbohydrates. Inadequate consumption of carbohydrates coupled with multiple sessions of moderate to high intensity sessions of prolonged duration will usually result in an athlete complaining of fatigue, lack of enjoyment, and difficulty completing sessions. Note that this can happen with or without that heavy feeling of bonking in a particular session, too.

So is it the worst thing in the world? Will you cause yourself harm? Of course not, it just sucks while you are in that moment. If it is a one-off occurrence, you will not lose muscle, you will not cause an injury, and you will certainly bounce back. But if you are continuing to see these signs and symptoms of fatigue, lack of enjoyment and poor performance in your training week after week then there are some long-term negative ramifications that can occur. These include increased muscle protein breakdown resulting in the potential of muscle mass loss and, of course, your training suffering and needing to cut back volume and intensity. In short, you will not be getting the same out of your training as you possibly could or should be. In addition, there is an increased risk of illness, namely in the form of upper respiratory chest infections due to the fact that carbohydrates have immuno-suppressive functions.3 And perhaps the biggest performance risk will be the reduced ability to utilize carbohydrates at higher intensities when required most.4 This refers to the negative physiological changes that occur when you remain on a low carbohydrate diet for too long. Your cells and actual function of the mitochondria become less efficient at utilizing carbohydrates at higher intensities when needed most and thus you struggle to compete at the highest intensities.5,6,7

There is also a school of thought that some periods of training with low glycogen stores can be beneficial in terms of physiological adaptation and improvements in fat oxidation and subsequent glycogen sparing during exercise. When an athlete trains in a low muscle glycogen state, it can enhance the intracellular signaling that assists the oxidative capacity of muscle cells (mitochondria) and possibly improve endurance performance. Several forms of “training low” have been studied. The original concept of training with low carbohydrate stores for a second training session on the same day was found to result in a reduced capacity to train and sessions being reported as psychologically harder. As a result, there were no clear performance gains or benefits to improve glycogen storage.4 Researchers then applied a different model to the “train low” paradigm and had athletes fuel up prior to exercise, perform a glycogen particle depleting exercise session, followed by little or no carbohydrates prior to sleep, and then exercise in the morning in a fasted state at low intensity (50% VO2max) for two hours. This resulted in improved fat oxidation (fat burning) with evidence of physiological changes in the muscle, yet overall performance gains were equivocal.5 When reading on this topic, you will often see a lot about improved mitochondrial function, improved fat burning, and better utilization of fat—yet the overall take-home message is that while all these wonderful physiological changes are occurring, the performance gains are not being seen, in research papers at least. 

This leads us to the obvious question: Why train in a low glycogen state? For me, I believe the reason is the potential for improved body composition by eating fewer calories overall. As a result of this, there is less body weight and/or improved power-to-weight ratio for the athlete. It is a way of manipulating your total energy intake and allows for an energy deficit which ultimately pushes your fat to be reduced. This is how I tend to use it in a periodized approach when training low with athletes who have body composition complaints or needs. Inadvertently, some of the physiological gains being measured in research are likely to be occurring as well, especially because often the training hours are well above anything researched to date with continual “train low” programs. But should it be used in high-performing athletes who already have very low body compositions? The answer to that is possibly, yes, if it’s to assist with any gut issues that are being experienced as a result of prolonged processed carbohydrate consumption due to high training intensities and race schedules. In terms of the physiological gains for improved performance from training low, the evidence is equivocal (63% of 11 studies completed showed no improvements in exercise performance). In short, the potential physiological gains are at the back of my mind and certainly discussed with an athlete. When comparing random “train-low” sessions or weeks to actually periodizing the carbohydrates relative to the overall training load, I believe the latter to be a far better approach.8

If you’re considering altering your body’s glycogen stores, there are a few things to be mindful of. There should always be structure and consideration to any periods of lower carbohydrate fueling and this should be in line with your session intensity and duration, as well as the time of year/point in the race season. A review of your schedule with your coach and, if possible, your nutritionist, will identify the parts of the week, month, and year that could allow for successful implementation. It is important to factor in some training sessions that incorporate higher carbohydrate consumption too so as to ensure your ability to utilize glycogen is not blunted.6 A good pattern is five days of lower carbohydrate fueling (i.e. <3g/kg/body weight) followed by two days of moderate-high carbohydrate fueling (>5g/kg/body weight).  This approach can fit well in two ways:

  1. Accommodating for lower intensity sessions mid-week to avoid fatigue at work while having harder sessions on the weekend when rest can be more plentiful. 
  2. Hitting a few days of short, sharp, intense sessions mid-week to free up time for your work and family and then the back end of the week have longer, lower intensity sessions as the weekend can allow for extra training time.

It is also important to ensure adequate protein intake during times of lower carbohydrate availability.  This means consuming at least 0.4-0.5g protein/kg of bodyweight pre- and post-training to assist with glycogen particle synthesis and reducing the potential for muscle protein breakdown.9 It also has the effect of reducing any potential heat stress injury to your gut when carbohydrate availability is low. 

As mentioned, improved fat oxidation as a result of training in a low glycogen particle state can occur, yet this improved ability will only be as good as your total diet and ensuring you are in a calorie deficit across the day, week, and month if one of your goals is fat loss. This is a very important practical step to consider and one that is often lost on a lot of athletes attempting to use the “train low” and/or fasted training in their schedules. If you do not maintain a deficit in calories/energy, you will not lose fat and you will not lose weight. The initial weight you may lose as a result of a low-carb diet or fasting will likely be in the form of water weight. For every gram of carbohydrates, it will attract 3-4g of water. When planning your “train low” strategy, please consider your overall calorie intake and how you are going to structure protein and fat alongside your carbohydrates. A suggested starting point would be at least 2.2g/kg of bodyweight for your protein intake and 1g/kg/bodyweight for fat. The remainder of your energy intake (calories) would be carbohydrates. The caveat to this is a menstruating female who should consider adjusting her “train low” strategy around phases three and four of her cycle. I will also mention that a lot more research needs to include female athletes and the effect that differing fuelling strategies have on them because a lot of the research to date has been performed on men.  

In conclusion, it is always worth remembering that the level of your glycogen stores is unknown unless you go for an invasive muscle biopsy (which is very painful and non-advisable). Therefore, all of this information is theoretical to an extent and you need to consider it alongside thinking about how you train, eat, and feel while training and competing. I would advise that you, your coach, and, if possible, a nutritionist, review your training loads and your macronutrient dietary intake to see if it marries up to the level of work and exertion being applied. If you have data from sources such as heart rate monitors and power meters coupled with laboratory assessments (e.g. DEXA, VO2 and FATmax testing) then even better and, of course, your own subjective ratings of perceived exertion (RPE) will help build the complete picture. Always ask yourself the question: “Are you fueling adequately for the work required?”


  1. Jensen TE, Richter EA. Regulation of glucose and glycogen metabolism during and after exercise. J Physiol. 2012 Mar 1; 590(5):1069-76.
  2. Shulman RG, Rothman DL. The “glycogen shunt” in exercising muscle: A role for glycogen in muscle energetics and fatigue. Proc Natl Acad Sci U S A. 2001 Jan 16; 98(2):457-61.
  3. Costa RJ, Jones GE, Lamb KL, Coleman R, Williams JH. The effects of a high carbohydrate diet on cortisol and salivary immunoglobulin A (s-IgA) during a period of increase exercise workload amongst Olympic and Ironman triathletes. Int J Sports Med. 2005;26:880–6.
  4. Bartlett JD, Hawley JA, Morton JP.. Carbohydrate availability and exercise training adaptation: too much of a good thing?. Eur J Sports Sci. 2015;15:3–12
  5. Hawley J. Manipulating carbohydrate availability to promote training adaptation. Sports Sci Exchange. 2014;27:1–7.
  6. Murray, Bob, and Christine Rosenbloom. “Fundamentals of glycogen metabolism for coaches and athletes.” Nutrition reviews vol. 76,4 (2018): 243-259. 
  7. Burke LM, Ross ML, Garvican-Lewis LA, Welvaert M, Heikura IA, Forbes SG, Mirtschin JG, Cato LE, Strobel N, Sharma AP, Hawley JA. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2017;595:2785–807.
  8. Impey, S.G., Hearris, M.A., Hammond, K.M. et al. Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports Med 48, 1031–1048 (2018).
  9. Betts JA, Williams C.. Short-term recovery from prolonged exercise: exploring the potential for protein ingestion to accentuate the benefits of carbohydrate supplements. Sports Med. 2010;40:941–959

Scott Tindal is a performance nutrition coach with 20 years of experience working with pro and amateur athletes. He has a Masters degree in sports medicine and a post-graduate diploma in performance nutrition. He is the co-founder of FuelIn, an app-based personalized nutrition coaching program.

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