When exercising, your body will draw on two types of fuel, glycogen (from carbohydrate) and/or fat. Ketones can supply some substrate for work, but that is a topic for another day. In general, easier efforts draw primarily on fat for fuel. These are known as “aerobic” efforts. More intense “anaerobic” training burns glycogen. There is a point at which a person switches from primarily burning fat to primarily burning glycogen. This is known as a “crossover point”, and it occurs at different efforts for different athletes. We all have fat stores which can provide many, many hours of fuel for aerobic efforts, but glycogen stores are limited. The more judiciously those glycogen stores are used, the more you’ll have left for efforts later in training. When you deplete those stores, you will “bonk”. But sometimes an athlete “bonks”, only to find that glycogen stores are not empty. There are many factors that influence this. It’s hard to say you outstripped your glycogen stores if your blood sugar is 140, and only with something like a CGM can you know this data.
Let’s look at two different training sessions during which I wore the CGM. The first is a 3+ hour bike ride with some intense efforts and a “race” up a 4 mile climb. The other is a long run (for me…12 miles) where I started the run having fasted for more than 14 hours, then maintained an easy pace without fueling for over 90 minutes.
I knew the ride would be a “spirited” group ride, so I fueled ahead of time for an effort which I presumed would be highly glycogen-dependent. (Remember, more intense efforts use more glycogen and therefore require more carbohydrate intake.) I started the day with a carbohydrate-rich breakfast (oatmeal, fruit, maple syrup) so that my stores were topped off. This morning saw one of my highest average blood sugars on my CGM, but that was expected given my nutrition. You can see that during my ride, my blood sugar remained in the 120 - 150 range, which is appropriate for the effort. This was fueled by both intake of simple carbohydrates and release of stored glycogen while riding. Given the intensity and duration of this ride, I would say that this was appropriate fueling. I was able to keep those glucose levels at a reasonably high level through refueling on the bike, though we’re not talking about a gel every 20 minutes. During the ride, I had 2 bottles of Skratch Labs hydration mix, a Clif Bar Banana Mango Coconut pouch, and a homemade oat and nut bar.
Interestingly, you will notice that my blood sugar is quite low for the rest of the day. This is likely due to increased insulin sensitivity (consumed carbohydrate is quickly taken from the bloodstream and stored after exercise) and plenty of room in the tank to store that glycogen. What you can’t tell from this graph is that I ate fairly indiscriminately after this ride. We were at the USA Cycling National Championships and there was plenty of barbecue, cornbread, beer, and desserts. My blood sugar stayed low for the next 12 hours, despite stuffing my face! While this was an abnormally intense day on the bike for me, it shows how beneficial such exercise can be, especially when I don’t splurge like this very often.
The fasted run was a very different effort. For this workout, it had been 14+ hours since I’d last eaten. I woke up, had a big glass of water, did some mobility exercises, and headed out for a long, easy run. The majority of this run was in Zones 1 & 2, so the goal was to draw primarily on fat stores for fuel. Even after an overnight fast, I knew I’d have 1000-1500 calories of glycogen stores I could use when needed. Having the CGM attached let me see that, indeed, my blood sugar levels remained stable through the run. To be clear though, this was not the first time I’d done this type of fasted effort. I do similar training weekly, but this was different due to the CGM and the data gathered.
The trick here is that every athlete is different, and every athlete’s physiology is trainable. Your diet and training may be causing beneficial adaptations, or it may be detrimental for your health and performance. You may think you are doing the right thing in your training and racing, but it could be that you are constantly digging a hole for yourself. Unless you check, you can’t be certain.
While I was compiling the information for this post, the Training Peaks Blog published a great 2-part series on the same topic. For more information on CGM, I recommend reading it. HERE's a link to the first article in that series.
At Podium Sports Medicine, we’ve put together a package to help you assess your individual glucose metabolism and make personalized recommendations on improving your health and performance. This is a great tool for both diabetic and non-diabetic patients, regardless of your athletic or wellness goals.