Metabolic Fuel Test Results

Fats vs Carbs at Increasing Paces and Heart Rate

Last week I went to the Kinesiology department's Exercise Physiology Lab at Illinois State University to do a metabolic fuel test. The test lasted about 40 minutes and cost $100. They strapped a mask over my face, hooked it up to some machine that would measure O2 and CO2, put a heart rate monitor on me, and had me walk, then jog, then run on the treadmill. Started at 11:20 pace (5.3 mph) and every 3 minutes they increased the pace by 0.3 mph. I ended at 6:58 minute/mile pace (8.6 mph). With the data on pace, heart rate, and mixture of O2 vs CO2 they could calculate the portion of carbs vs fats I was burning at each pace. Essentially they are tracking your respiratory quotient (RQ). Joe Friel has a nice blog post on the subject of fat vs sugar burners and RQ.

Additionally, they calculated training zones for me based on the test data. Lactate threshold was not specifically calculated, but they said it's around the point that you shift to full carb burning (for me, around 8.3 mph or 7:13 pace). That pace also corresponded to a sharp increase in heart rate as seen in the graph above.

According to them, my most efficient "race pace" would be 8:30 minute/mile at about HR=140. Not exactly sure what that means. Maybe it is the high end of the aerobic training zone with highest amount of fat burning? I certainly feel comfortable at that pace. Running at an OK speed, but not hard. The good news is that I was a reasonable fat burner and didn't shift to full carbs until 7:13 pace (8.3 mph). I'd like to repeat this test in about 6 months when I've moved from my current focus on speed and stamina training to pure endurance as I shoot for marathons and ultras this summer and fall. I assume I'll be a more efficient fat burner. Hopefully faster paces at same heart rates too!