Which is more important with cycling, calories or carbs?

QUESTION: I was wondering which is more important in cycling – calories or carbs. I’m a type 2 diabetic and have to watch my carbs. – Mike E.

RBR REPLIES: To start with, this is not medical advice and you should always ask your doctor when it comes to controlling your type 2 diabetes with nutrition and exercise.

I am definitely not a doctor, but my personal understanding of blood sugar levels is that when you’re exercising, your body tends to use those carbs immediately to fuel your muscles. So consuming some carbs while you’re actually riding isn’t as big a deal as consuming them when you’re sitting on the couch. This article by Gabe Mirkin, M.D., explains the concept better than I can.

Exercise lowers high blood sugar by making the cells respond more effectively to insulin to drive sugar from the bloodstream into muscles. People who doubled their exercise time, from 150 to 300 minutes per week, reduced their risk of developing diabetes by 36 percent. More than 40 percent of North Americans have high blood sugar levels, to a large degree because fewer than 50 percent exercise at least 30 minutes a day, five days a week. You should try to exercise every day, since the ability of contracting muscles to lower blood sugar without needing insulin lasts less than 24 hours (Am J Clin Nutr. 2008(July);88(1):51-57; J Appl Physiol, 2005;99: 338-343 and 2005;8750-7587).

I have been testing a Levels continuous glucose monitor for the past several weeks. (I am using it for general health and fitness reasons and do not have blood sugar control issues.) What I have found personally is that when I go on a very hard ride (where my heart rate goes above threshold) my blood sugar rises while riding, even if I am riding completely fasted and even if I consume nothing but water. This type of blood sugar rise is not bad for me, according to Levels, the company that makes the continuous glucose monitor. But why not?

What I learned from the Levels support files is that what is happening is that my liver is releasing stored glycogen to help fuel my high intensity exercise. My body sees that I am exercising hard and responds to help my efforts to help, more or less. The liver only holds a limited amount of glycogen though, so if I were to continue on at a high pace without consuming anything, I would eventually bonk. In general, I often ride fasted in the mornings and can ride up to 40 miles before I need to start eating because I have adapted to riding this way.

The Levels blog also has an excellent explanation of how your blood sugar changes during and after exercise.

But what about carbs or just calories? Can you fuel your longer rides with only low carb foods?

The answer is yes, but there is generally an adaptation period where your body learns to help fuel your ride by burning fat instead of sugar. And if you are riding at a high enough intensity, you are still going to burn sugar and potentially bonk. Even the hard core low carb endurance athletes tend to “train low and race high” because it’s difficult to maintain a high intensity without any carbs.

If you are primarily riding at an endurance pace, then you could probably safely skip the carbs and just eat regular calories as needed and would do just fine. If you are riding at a more intense pace, it’s possible that avoiding all carbs might lead to a bonk since the liver can’t replace the glycogen fast enough to keep up and can run out. (It wouldn’t hurt to carry something with carbs in case you do start to feel a bonk coming on, because low blood sugar can be dangerous.) For rides under an hour, most cyclists do not need to consume carbs or calories at all.

What I have found personally by looking at my own continuous glucose monitor results over the last several weeks is that exercise in the morning will tend to help my blood sugar for the entire day. A meal that might ordinarily cause my blood sugar to go up and out of range generally has less of an effect on my blood sugar if I have exercised, even if it was six or eight hours earlier.

One other thing that I learned from Dr. Mirkin’s article and confirmed with my continuous glucose monitor is that going on a walk for 15 minutes after a meal rapidly reduces my own blood sugar, just as he describes. My wife and I already had a habit of going on a walk after dinner, but after seeing how effective it is at bringing my blood sugar down, I’ve become even more motivated not to skip that walk after a big meal.

If you’re interested in learning more low carb bicycling, we’ve covered it in this interview in the past. Interview: A Beginner’s Guide to Ketogenic Diets for Cyclists

Readers, tell us about your experiences and opinions!