Physiological Testing- Fuel Utilization

Last May I did some testing at the Monfort Human Performance Lab at Colorado Mesa University. I performed their standard Ventilatory Threshold Test that gave me an array of useful information that I delved into here. A little while ago I reached back out again to do a similar test, but this time slightly different. My goal this time around was to get a more precise measurement of the amount of fuel I burn at different intensities. Why? Well, because at it’s core, racing an IRONMAN to your fastest potential becomes a math problem. How much fuel do you start with, how much are you burning and at what rate during the race, how much you can eat and digest to replace lost fuel, and when will you run out? And trust me, it’s really painful when you get the math wrong on race day.

A day I got the math wrong.

So while I once again did a cycling ventilatory threshold test, this time we changed the protocol a bit to make things a bit more specific to my needs. Rather than starting lower at 210 watts, we went ahead and began at 240 watts knowing that I burn hardly any carbohydrates below that point from the prior testing. And I also know that I likely won’t be racing at those intensities at any point. We stuck with 3 minute increments as before to allow the body to settle at each step before moving on. This time we took the steps at only 20 watt increments instead of 30 watt increments to really tease out smaller differences. We also knew that since we were mainly focused on sub-threshold intensities, that I the smaller steps were unlikely to fatigue me and effect results in that way.

I attempted to show up to the testing session in a very similar state to how I would approach a race. I had just finished a decently large aerobic block of training with a couple rest days leading into it while still maintaining a hefty diet leading in. I even completed a 4km swim session slightly before the testing with similar to race day fuel. Before long, I was hooked up to “the mask” and ready to go!

The actual process of the test wasn’t extraordinarily interesting, and we finished the test just after reaching threshold. Meaning that it had pretty much just started getting uncomfortable in the final step or two. Besides the constant discomfort of the mask, an excessive amount of saliva and a heart rate monitor that kept wanting to cut out. It took a couple days to get the results processed and now I have more good information to work with for training and racing.

While there are a lot of things I could go into with the results, there are a couple key take-aways that I’ll get into. First of all is the increase in threshold from 330 watts in my prior test to 340 watts now. This was likely a combination of gains from the end of last season, as well as the increased focus of being able to focus primarily on training in the prior month. The steps were a bit different during the testing which may make things murkier there, but I think it is apparent that my cycling has taken a step up since then and this reflects that.

FEIGH_ADAM_AT Test 2_19_2020F


As expected, my aerobic fitness is still definitely up there. It is apparent that I could likely cruise around at 240 watts and keep up with fueling just about indefinitely. Assuming there are no shifts in the data due to dehydration, muscular fatigue, or other variables. Increases above that simply come down to how much I would be able to digest. 260 watts and I could potentially keep up if I really push the calories hard. 280 watts and I am likely to start running a deficit with increases above that simply pulling more stores out. I fully expect to be running a deficit during the bike portion of an IRONMAN, but the key is how much do I think would be safe to still be fueled well enough to run a marathon afterwards?



There are still a couple variables that could change these numbers though, and mostly in my favor. First of all is training. Obviously this is all able to be manipulated through both volume and intensity of training to make the body more efficient. Otherwise I could just sit around and not train at all. With this data I will know how better to adapt my body in training to my expected race demands. The second in my favor is altitude. While Grand Junction isn’t super high in elevation (4600-ish feet), it is certainly enough that I notice it in my training data and in how I feel. While the power required to produce those watts still have to come from somewhere even at sea-level, ideally my body will be more efficient and rely a bit less on carbohydrate use at any given intensity. Somewhere around 5% is used around here as a general rule of thumb for when returning to sea-level in regards to power numbers, and I would certainly take every bit of that!

There are potentially negative variables that could effect these numbers too. As mentioned before, dehydration and muscular fatigue are definite realities in the latter end of IRONMAN racing. Though ideally training and a smart hydration and pacing strategy can help counter those. An often overlooked variable could also be stress. Stress and the release of cortisol can potentially cause the body to shift to using carbohydrates more as a fuel source. And race day can certainly be stressful. All the more reason to try to keep a relaxed and focused mindset when entering competition if possible when expecting that running out of fuel may be a possibility.

So congratulations to anyone who made it this far into my post! You are either a data geek like myself, or a potential competitor looking to find out and exploit my weaknesses by delving into my data. Either way, I hope you enjoyed the read!

Charleigh getting potty trained!


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s