I am not an expert on body chemistry, but I have 10 years of personal experimentation in this area with my own long runs as well as a lot of input from family members and friends who are hikers, runners or cyclists. I have read dozens of articles and tried many different things as I have become a distance runner.
There are general principles that apply, such as
- You have glucose in your blood stream. This is what your body uses first when you begin to exercise.
- Your body stores glycogen in your muscles, which is a source of energy when glucose gets low. As glycogen is consumed, blood sugar levels go back up.
- When glycogen reserves are low, the body can burn fat to derive more energy. Sometimes, the body can break down muscle tissue for energy as well.
- Carbohydrate consumption during long periods of exercise can help maintain energy levels and lead to better athletic performance and in extreme cases, prevent injury and bodily harm.
This article from NIH explains the basics of glycogen. It's a long read, but has good information and really helped me.
I have also come to the conclusion that one size does not fit all. Depending on the individual, the body's blood sugar response during endurance events can be very different. This also means that the nutritional needs of each individual can vary significantly during exercise.
- A neighbor (ultra-marathoner) - the more she runs, the better she feels. Her body seems to prefer a high protein diet during exercise. She can eat cheeseburgers during a long run and go forever. If I do this, I will throw up. Sugar tends to make her feel sluggish or even nauseous during exercise.
- My Sister - Type I diabetic. Usually, weightlifting causes blood sugar to drop in Type I diabetics and cardio can often cause blood sugar to go up. For her, the opposite is true. If she runs much more than 5 miles, her blood sugar can drop to dangerous levels. She has to stay close to home and consume a lot of sugar during long cardio. High sugar like this causes me to crash.
- My Wife - Pre-diabetic/Gestational diabetic - cardio causes a short term increase in blood sugar, but long events eventually bring her blood sugar down slowly. She has never experienced a blood sugar low during exercise even if she doesn't eat anything, though she doesn't participate in high intensity activities. She can do a 15 mile hike without any food and her blood sugar may be 50-100 points lower at the end, but usually still above 100. I get really weak and light-headed long before.
- Coworker (triathlete) - Has tried most of the commercial nutritional products with less than stellar results. Only thing that works for him to keep steady blood sugar level and be able to stay in an event is a corn starch supplement. Works great for him, not so much for me.
- Coworker (cyclist) - Needs a mix of protein, complex carbs and simple carbs to keeps his energy levels right for long rides. He typically uses packaged nutritional products designed for cyclists, but will often also drink a mountain dew to keep his blood sugar up.
- Me (running/cycling) - Cardio activity over 1 hour usually requires some type of nutrition. Plain sugar alone, i.e. candy, causes a spike and drop during exercise. I have felt faint and even passed out once on a run when using just candy. Complex carbs combined with more natural sources of sugar like fruit work much better for me. My best half marathons and marathons were done using waffles (commercial supplement) and dried fruit.
I'm pretty sure if all of us were analyzed before, during and after exercise, the blood sugar charts would show similarities, but look quite different. I don't understand the chemistry behind it, but I know the general rules have exceptions, so it's important to learn how to listen to your own body and find a system that works well for you. As you train, your body will also adapt to the training, so over time, your nutritional needs before and during exercise may change.