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Determining how your body prefers to burn fuel is valuable information for endurance athletes. Why? When you’re competing in events longer than 3-4 hours, getting your carbohydrate (CHO) intake right is key. If you don’t take in enough, you risk running low or depleting your CHO stores—which means you might also be risking a DNF (did not finish). But if you take in too much, your gut will shut down; leaving you feeling bloated and nauseated. Neither scenario plays out well in the end.

Sugar burners burn a larger percentage of CHO from total calories burned and need to be hyper vigilant about continuous fueling in longer events. On the other hand, fat burners use fat as their main fuel source, sparing sugars stored in the body. Fat burners have a metabolic advantage in endurance events, which can be attributed to good genetics and/or effective training.

How do you know if you are a sugar or fat burner, and furthermore, how much CHO do you need to avoid running low on sugar stores? The most effective way to find out is via a Metabolic Efficiency Test (MET) from a reputable sports/fitness testing center. The MET determines your efficiency for using stored fat and CHO at various exercise intensities. The MET can be performed on either a treadmill or bike (with power measuring capabilities). It starts at a very easy pace (walk or light spin), and speed increases 0.2–0.4 mph in 3–5-minute stages until you are well above your “crossover point.” More on this below.

The body stores between 1400 and 1800 calories (depending on size, gender, and fitness level) as glycogen (long-chain CHO) in the liver and muscles and blood glucose. With moderate intensity, these valuable stores can be depleted within 2 hours. On the flip side, the body has up to 80,000 calories stored as fat—even thin bodies! As exercise intensity increases, you burn more CHO as fuel.

The “crossover point” determined in the MET test is where your body switches from using mainly fat to mainly CHO as a fuel source. An MET measures your heart rate, O2 (oxygen), and CO2 (carbon dioxide) from exhaled air, and this data helps determine exactly how many calories from fat and CHO you burn at each specific exercise intensity.

A fat burner will start the test at low intensity with a RER (Respiratory Equivalency Ratio) of 0.80 or less. This equates to 33% CHO/67% fat being used as fuel. A similarly fit sugar burner may start the test at same low intensity with a RER of 0.90. At this RER, 67% CHO/33% fat are being used as fuel. When both athletes reach their anaerobic threshold, they will be at 1.00 RER—or 100% CHO/0% fat.

This valuable data can be applied in training to improve performance, take out the fueling guesswork, and improve metabolic efficiency. All you need to do is find your goal race intensity (heart rate, power, or pace) from your test’s results, and this will tell you exactly how many calories you burn at that point and what percentage of your calories come from CHO. These are the calories that need to be replaced in endurance events.

Over the years I have had annual METs to monitor my aerobic fitness, resting metabolic rate, crossover point, and % fuel burn at increasing heart rate intensities. A good friend at the UNC Wellness Center is excellent at administering this test, and I was fortunate to get on her schedule recently. I wanted see how my 2015 metabolic efficiency stacked up to my 2014 results.

Fat is my body’s preferred source of fuel—and this works out well for me since I am not gifted in short, fast distance events and have always gravitated to endurance sports. My total test time was 25 minutes and consisted of 6 five-minute stages. My RER (Respiratory Equivalency Ratio) at a walk was 0.78 (25% CHO/75% fat). In heart rate zone 4, my RER was 0.88 (60% CHO/40% fat). I stopped the test shortly after that. I was extremely thirsty (breathing into a tube) and never feel super comfortable burning that much sugar. My body innately knows it needs to slow down.

We calculated my resting metabolic rate (RMR) at 1872 calories/day, which I think is 200-300 calories high. I had accidentally (oops) biked for 2 hours right before this test—which would have increased my heart rate largely from  dehydration and given a false high metabolic rate. So, we are retesting soon to compare data. Otherwise, my RER data is correct and almost identical to my 2014 MET test.

If you are an endurance athlete and have fueling issues, or want to determine at what heart rates you should train to increase fat utilization, I highly recommend you get this test. Expect it to take no more than 90 minutes and cost around $150.

For more information on where I took my test, visit:

http://uncwellness.com/services/performance-center/