Fuel Utilization: How the body burns stuff during exercise

            I suppose the first thought to tackle would be: what is fuel utilization (for not a car)? Fuel utilization is the term used in Exercise Physiology to describe the process of the body using sources of energy, usually from food, to produce movement of some kind. On the larger scale, your muscles take calories, burn them, and do whatever motion it is they are meant to do; this explanation works well as a quick definition, but the purpose of my articles to give people a deeper understanding of exercise and health topics.

            First step, what fuel is used by the body? Fats and carbohydrates (protein if it’s really desperate, we don’t want that). Fuel burning occurs at the cellular level; cells take in oxygen from the fuel, and spit out carbon dioxide. For measurement of this occurrence, researchers and practitioners in exercise science use the Respiratory Exchange Ratio (RER). The RER is “the ratio of the net output of carbon dioxide to the simultaneous net uptake of oxygen at a given site”[1].

            I’ll use my experiences as example here. In the lab at worked at at my university, we would bring in athletes to do a small barrage of tests depending on coach requests or research demands. One of these tests was a combined VO2Max and metabolic test. The athlete puts on a mask and runs a moderately awful jaunt on a treadmill.

It is really just a standard pre-set protocol starting with a walking warm-up followed by increases in speed and incline. The test concludes once the athlete feels they can go no longer

            The mask captures and analyzes the amount of carbon dioxide being breathed out by the athlete, then gives us a number of 0.6-1, this is the RER.

There’s also a whole bunch of other neat information that this test gives, just give me a minute, I’ll get into it.

            So why do we get a number from 0.6-1? Looking back to the definition above for RER, it is the ratio of CO2 out to O2 in. Fat and carbohydrates burn differently, as such, they expel different amounts of CO2, and produce a different ratio. The lower end, 0.6, can be understood as the body burning 100% fat, to do whatever is you are doing. At RER of 1, your body is using 100% carbohydrates.

A quick note: RER is an estimated value, unless you directly go into every single cell and measure the CO2-O2 ratio at the cellular level (known as the Respiratory Quotient (RQ)), you can’t technically get the exact value, but it’s pretty darn close.

Another quick note: You’ll probably know 100% carb use by another name, “Hitting the wall.”

            I chose to write about RER because 1, it’s important, and 2, it’s not really something that every person engaged in physical activity knows about. Granted it is understandable that most people won’t know every number and variable in their metabolic profile, doing the test isn’t readily available to all people. BUT, if you have a hard set goal for your health or performance, I think it is vital to have as much information at your disposal as possible.

            I’ll once again use myself as an example, because I am currently the only person within 10ft of right here. I have been lucky to be part of an exercise science program with a lot of cool “toys” or scientific technology as they would prefer it. Due to this I have gladly volunteered myself at every opportunity to be a test subject, especially when it comes time to do a metabolic and VO2max test. What I learned about myself from these tests was my RER at various increasing intensities, compared with my heart rate. At low intensities I burn fat, but I very quickly switch over to using more carbohydrates, at still fairly low intensities. This isn’t necessarily bad, unless you are say... training for long distance cycling races where prolonging fat burning is quite nice...Needless to say, this was good to learn; it has allowed me to adapt my training to better suit my individual needs.

            Takeaway points; learn as much about yourself as possible, match your goals to your capabilities, match your training to your goals. If you can find a place to do a metabolic test, try it out, but definitely make sure you’re prepared for it, about a 30 minute run to maximum effort.

R. Andrew Paxton,  B.S., M.S. Kinesiology

Head Coach & Trainer, Coeur Ninja

e: robertpaxton11@gmail.com

p: (615) 424-3239

REFERENCES

Respiratory exchange ratio. (n.d.) Medical Dictionary for the Health Professions and Nursing. (2012). Retrieved April 16 2019 from https://medical-dictionary.thefreedictionary.com/respiratory+exchange+ratio