Big Fat Theory

Lately, I have been thinking a lot about the ratio of fat and carbohydrate expenditure while running at varying paces.

Drs. Jeff Volek and Steve Phinney are currently working on a study through the University of Connecticut Department of Kinesiology (dubbed the FASTER Study), which is comparing the rate of fat and carbohydrate metabolization at various paces in ultramarathon runners. One of my coaching clients, Kevin Grabowski, just returned from the study. Kevin is incredibly fat-adapted. The man consumes 3+ quarts of heavy whipping cream a week! I won't spill the beans on his results, but it's safe to say he burns pure fat at his "easy pace," which is often the same as race pace on longer distance ultras. This knowledge has a lot of implications for racing something like a 24-hour event.

I am up to participate in the FASTER Study in March, so until then I can only speculate. However, this doesn't mean I can't experiment.

My theory is that I metabolize fat exclusively beginning around 7:10-7:40/mi (on flat terrain, that is). If this is true, it means I could deplete my carbs and still expect to maintain 7:20/mi pace without trouble. I set out on a run Saturday morning to test this out. My goal was to try to deplete my glycogen stores with an aggressive, fasted workout so that I could then, during the cool-down, identify what pace I could still maintain comfortably without the help of glycogen. Here's the protocol I followed:

  • Overnight fast
  • Pre-workout nutrition (coffee, Vespa Ultra Concentrate)
  • 11 miles at 6:40-6:50/mi
  • 5 mile progression (5:51, 5:46, 5:42, 5:29, 5:35)
  • 4 miles at what felt comfortable (average about 7:30/mi)
Anecdotally I am pretty certain that the workout depleted my carbohydrate stores: When I finished the progression run, I was drained. I went into the 4 mile cool-down without a break. I quickly noticed that I could push my pace down close to 7:00/mi with a bit of effort, but if I just ran to comfort—my "run forever" speed—it was closer to 7:30/mi. Assuming I was successful at depleting my carbohydrate stores, I believe this was a good indicator of what pace I can currently maintain, while burning fat as my sole fuel source. (Again, terrain and other conditions play a role.)

My performance at my past few ultra races seems to support this. In my recent 50 milers (Tussey Mountainback and the Chicago Lakefront 50), my pace was well under 7:00/mi. According to my little experiment, I would have required some carbohydrate intake to maintain those speeds; my fueling of roughly 1,000 calories during each of those events makes sense.

One might argue, however, that I should still have had enough glycogen stored up to adequately meet the demand placed on me even at the quicker pace. With that thought in mind, I would argue that I did not do a "carb load" in any traditional sense leading into each of those efforts. Maybe this is something to play around with in the future. Ultimately, I think it will come down to whether I would rather take in a few more carbs during the race, or would it be easier to stock up a bit the 1-2 days prior to racing.