Want to Avoid Bonking? Learn the Science Behind It

deep dive on bonking, glycogen stores, and the train low diet.

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. 

WHAT YOU HAVE

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.  

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