Sugar vs. Fat: The Crossover Concept of Carbohydrate and Fat Utilization During Exercise

If you want to go long, burn fat. If you want to go fast, burn carbohydrate. Athletic performance is not that simple, of course, because you burn both – and a little protein – at all times, during all activities, and at any intensity level. However, the basic premise still stands: you rely more heavily on fat during longer, lower intensity exercise and call on higher amounts of carbohydrate for high-intensity, race winning efforts. So, since you’re loaded up with mountains of sugary Halloween candy this week, there’s a great workout below that will put that carbohydrate energy to good use.

First, though, let’s talk about why high carbohydrate availability is good for high intensity training.

Respiratory Exchange Rate

The Crossover Concept is part of the way we know whether a workout is primarily fueled by the oxidation of fat or carbohydrate. One way we determine it in the lab is to use a face mask and metabolic cart to measure the composition of the gases you’re inhaling and exhaling. We then calculate Respiratory Exchange Rate, which is the volume of CO2 produced divided by the volume of O2 consumed.

Respiratory Exchange Rate = VCO2/VO2

You are always burning both fat and carbohydrate for energy, but the relative contributions from each changes based on energy demand. The reason RER provides insight on the balance of fuel usage is because oxidation of glucose produces more CO2 than the oxidation of fat. At rest, there is a lot of individual variability in RER, but values close to 0.7 are most common. Oxidizing fat generates less CO2 per molecule of oxygen consumed, so burning more fat decreases VCO2 and decreases RER.

The Crossover Concept

As energy demand increases (as determined by percent of VO2 or power output), the oxidation of carbohydrate increases and the oxidation of fat decreases. This is known as the “Crossover Concept”, which is illustrated below. As the contribution of carbohydrate increases, RER rises and a value of 1.0 means you are deriving nearly all your energy from carbohydrate. For an in-depth explanation of this concept, read this article by CTS Coach Corrine Malcolm.

Purdom, T., Kravitz, L., Dokladny, K., & Mermier, C. (2018). Understanding the factors that effect maximal fat oxidation. Journal of the International Society of Sports Nutrition, 15(1). doi: 10.1186/s12970-018-0207-1

The intersection point can be affected by training, as you can see in Fig 2. The top graph shows that professional athletes produce higher maximum power (when RER would be about 1.0) than moderately trained athletes. That’s no surprise. But also notice that elite athletes reach a higher percentage of their maximum power while still relying primarily on fat for energy. This is one of the primary goals of endurance training, and the reason so many people focus training and nutrition strategies on maximizing fat oxidation.

San-Millán I, Brooks GA. Assessment of Metabolic Flexibility by Means of Measuring Blood Lactate, Fat, and Carbohydrate Oxidation Responses to Exercise in Professional Endurance Athletes and Less-Fit Individuals. Sports Med. 2018 Feb;48(2):467-479. doi: 10.1007/s40279-017-0751-x. PMID: 28623613.

It is also important to focus training on increasing the power you can achieve when burning almost entirely carbohydrate, and on increasing the amount of time you can sustain efforts at that intensity. That’s what the workout below is all about. You’re going to use high-intensity intervals to keep your RER elevated and burn through a lot of glucose (sugar).

Sugar-Buster Cycling Workout: Descending Intervals

In truth, this is all a bit of tongue-in-cheek fun. In our coaching and training plans, we don’t really prescribe workouts specifically to burn carbohydrate calories. Yes, high-intensity workouts burn a lot of carbohydrate, but in the grand scheme of things we use workouts like this to increase performance at lactate threshold and VO2 max. But as a fun training session, we’ll focus on burning sugar.

The workout is called Descending Intervals. No, not downhill intervals, the interval times and recovery times descend across the set. Following a good warmup, the main sets in the workout below start with a 2-minute max effort, 2 minutes easy spinning recovery, and then the max efforts get shorter, along with the recovery periods.

For those who want to download the workout to a device or use it on virtual platform, you can download the workout files. If you’re doing these on a smart trainer, use the level mode instead of erg mode so you can use your internal motivation to put out lots of power, rather than the trainer setting the power.

Click to enlarge.

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Workout details:

Warm up

5 min @ 50-60 % of FTP

Ramp up in 3 steps
1:30 @ 65-75 % of FTP
1 min @ 75-85 % of FTP
30 sec @ 85-95 % of FTP

5 min @ 50-60 % of FTP

Ramp up in 3 steps
1:30 @ 65-75 % of FTP
1 min @ 75-85 % of FTP
30 sec @ 85-95 % of FTP

4 min @ 50-60 % of FTP

Descending Intervals Set #1

2 min @ 125-200 % of FTP
2 min @ 50-60 % of FTP
1:30 @ 125-200 % of FTP
1:30 @ 50-60 % of FTP
1:15 @ 125-200 % of FTP
1:15 @ 50-60 % of FTP
1 min @ 125-200 % of FTP
1 min @ 50-60 % of FTP
45 sec @ 125-200 % of FTP
45 sec @ 50-60 % of FTP
30 sec @ 125-200 % of FTP

Recovery
8 min @ 50-60 % of FTP

Descending Intervals Set #2

2 min @ 125-200 % of FTP
2 min @ 50-60 % of FTP
1:30 @ 125-200 % of FTP
1:30 @ 50-60 % of FTP
1:15 @ 125-200 % of FTP
1:15 @ 50-60 % of FTP
1 min @ 125-200 % of FTP
1 min @ 50-60 % of FTP
45 sec @ 125-200 % of FTP
45 sec @ 50-60 % of FTP
30 sec @ 125-200 % of FTP

Cool Down

10 min @ 40-50 % of FTP

By Jim Rutberg,
CTS Pro Coach, co-author of “Ride Inside“, “The Time-Crunched Cyclist”, and “Training Essentials for Ultrarunning”