The Bonk, Explained

What is The Bonk?

The Bonk, or Hitting The Wall, is a sudden and intense fatigue that ‘hits’ or ‘bonks’ an individual. I will be writing about it from the perspective of the cyclist but it is phenomenon known to runners and other athletes. It is not the same as getting tired or slowing down a bit. When you truly hit The Bonk you know about it; you will feel dizzy and simply coordinating your legs to pedal is an effort, let alone applying serious force.

Why does it happen?

Apologies but it’s science time; I’ll try and keep this short: The Bonk is caused by the depletion of your glycogen stores in your muscles and liver. Glycogen is the body’s store of sugar; it is a large starch-like aggregation of glucose (sugar). Though most of us have significant energy reserves in the form of adipose tissue (read ‘fat’) its release is very slow and gradual in comparison to the liberation of sugar from glycogen. This release is stimulated by insulin which is realised by the liver in response to low blood sugar. That is why your body tends to churn through glycogen when doing intense exercise. More gentle exercise can rely on both glycogen and fat stores and will not cause a sudden ‘bonk’.

For those of you trying to lose or control your weight the good news is if you have bonked you are running on fat. However this is not really the best way to go around losing weight. Weight loss and cycling is big topic that I will tackle in another blog shortly.

For those of you who like detail: An average person has about 1500kcal worth of glycogen stores in their liver and muscle. Intense aerobic exercise can suck up around 750kcal an hour. So in theory your untrained individual will bonk after a couple of hours (there are a lot of variables here so this is an extremely rough calculation.)

So you are hitting the wall: What do you do?

The first thing to do is to reduce exercise intensity but you won’t have a lot of choice in this. Then you need to replace these sugars and, assuming you don’t have a drip connected to your bike, this will be in the form of sugary foods or drinks. Drinks have the advantage of helping hydration too. ‘Sports drinks’ (I hate the phrase) will do this, but so will any sugary drink. Again this is another massive topic that I will enjoy tearing into in another post so I’ll keep it light here: Get some sugar in your system to replace your glycogen stores. If this is done during exercise, this will allow our untrained individual to continue on for longer than 2 hours.

How to avoid The Bonk (prophylaxis)

There are two ways:

  • Ride more.
  • ‘Carb. Loading’ (~groan~) .
  • Years of hard training with a bit of help from ‘carb loading’ means that when Wiggo is rolling out in the Tour he has over 3500kcal worth of glycogen (more than double in our untrained man!), which I reckon would give him 4 hours + of fuel for intense exercise without refuel (please note intense exercise for Wiggo will involve going a lot faster than our untrained individual AND that he has a team of domestiques and cars providing carefully calibrated drinks to keep these reserves up).

    ‘Carb loading’ seems to have entered common parlance in the last few years. As you can tell from my excessive use of inverted commas, I hate this phrase. Personally it is synonymous with guffawing university rugby players chucking down plates of white pasta. This is too big a topic to tackle head on here but the basics involve eating significant, but not ridiculous, amounts ofcomplex carbohydrates up to 48 hours before an event. Complex carbohydrates are released slowly because it takes longer for the digestive system to break them down. Things like refined white pasta are only one step away from eating sugar; they are digested very quickly and result in high blood sugar levels fairly soon after eating and thus thought to be a major cause of type 2 diabetes. You want to be eating things like brown rice, brown pasta, oats potatoes and certain beans. These sorts of foods are effectively huge starchy aggregations of sugar that take a long time to break down and are perfect for slowly releasing energy and building up glycogen stores.

    I cannot overstate the fact that all this information is about improving your athletic performance. Consuming large volumes of carbohydrates in whatever form, whether it be potatoes or certain brands of sugary drink, will not help you lose weight; it will actually prevent you using up fat reserves. However if you’re trying to stick to the back wheel of Jens Voigt on a sunny day in the south of France in July then it is probably a good idea.

    By Tim Hole