What's the fuel source? HR zone vs. RPE

I guess it is never black or white.

I've not an expert at all but I imagine the body always use a proportion of glycogen and fat as fuel.

The higher the intensity, the higher the glycogen % is and the lower the intenity is, the higher the fat % is as a fuel.

So your gray zone should be in the middle

Correct me if i'm wrong

How could you test for this? Stick someone on a treadmill and draw their blood throughout the workout? Not sure even that would do it. What impact would the answer have? Would that drive some decision?

Certainly training to target different fuel sources can be beneficial. Just one example would be someone on a ketogenic diet. Having little to no carbs to fuel from, during high intensities, the body will create glucose from gluconeogenesis (conversion from protein to glucose). While staying in certain zones may produce more ketones to get into a deeper ketogenic state. This is just one example of why someone would want to be conscious of their fuel source during exercise. Other reasons could be performance, training plan, tapering, weight loss, etc...

Actually pretty simple - it's called RER or Respiratory Exchange Rate. It's the ratio of O2 inhaled vs CO2 exhaled. It's measured during a VO2 max test or any test using a metabolic cart. From here, we are able to see a percentage breakdown of energy from glycogen stores vs fat stores.

Problem is, HR% Max and RER values do not correlate for individuals. I have a high VO2 max, but I also have a high RER ratio for a given exertion, meaning I go through carbs at a higher than average rate. The only way to know is to do a test in a lab with a metabolic cart.

It's never either or. For most running, it will always be a mix of the two.

https://www.physionic.org/respiratory-exchange-ratio

That's informative. Thank you.

Was your RER ratio tested with your VO2 max? I had an official VO2 max test done before, but cannot remember if this was also measured.

With RER ratio in mind, I gather:

60% or below max HR = 100% fats (or at least predominantly fats)

75% max HR = mixture of both fats & glucose

85% or higher = 100% glucose (or at least predominantly glucose)

And given the example I mentioned of rate of perceived exertion being low but heart rate being elevated, in the examples I used of sprinting, then returning to a slow jog... I imagine the RER ratio would be on the lower end, given the ability to hold a conversation. Therefore, you would be using more fat than glucose, despite the elevated heart rate? My understanding is the fuel source comes down RER ratio over heart rate.

FastFeet11 wrote:

That's informative. Thank you.

Was your RER ratio tested with your VO2 max? I had an official VO2 max test done before, but cannot remember if this was also measured.

With RER ratio in mind, I gather:

60% or below max HR = 100% fats (or at least predominantly fats)

75% max HR = mixture of both fats & glucose

85% or higher = 100% glucose (or at least predominantly glucose)

And given the example I mentioned of rate of perceived exertion being low but heart rate being elevated, in the examples I used of sprinting, then returning to a slow jog... I imagine the RER ratio would be on the lower end, given the ability to hold a conversation. Therefore, you would be using more fat than glucose, despite the elevated heart rate? My understanding is the fuel source comes down RER ratio over heart rate.

Yes, My RER was charted along with my HR during the test. The test was over when I ether quit, or my RER went over 1. The problem is it will be different for everyone, again my RER is slightly higher than average which means I burn more CHO than average.

Those zones sound pretty good, at threshold pace and above, it will be predominately glucose. Anything below will be a mix. You are correct that fuel use comes down to RER over heart rate, but in your example of basically using a fartlek, When sprinting or going fast, you'll use anaerobic energy sources, therefore spiking O2 demand and going into EPOC or elevated post oxygen consumption. This means exhaled CO2 values will be elevated for a period of time until balance is restored (This is where afterburn comes from for anaerobic workouts and elevated calorie burn for 24 hours etc etc).