Warning - This will be a long technical post, but if you want to figure out how exercise intensity and fuel utilization (ration of Carbs to Fat burned) are related, I suggest taking the time to hear me out.
When reading John Berardi’s column about glycogen depletion, he talks about the ratio of fuels (C and F) being burnt as a function of exercise intensity. I want to present another point of view that one of my professors was working on until he unfortunately passed away from a heart attack a year ago. When exercising you consume oxygen (VO2 = oxygen consumption rate) and produce carbon dioxide (VCO2 = rate of carbon dioxide production). There is a ratio called the respiratory exchange ration (RER) which is used to determine the ration of C and F being used. The equation is as follows: RER = VCO2/VO2. There is also another ratio called the respiratory quotient (RQ) and it has the same equation: RQ = VCO2/VO2. You need to know that the RQ is what is going on at the cellular level and we cannot measure this. Thus, we have to use the RER as an estimate of the RQ, but this is measured at the mouth, not the cell. Why is this a problem? There are sources of CO2 that are not linked to aerobic metabolism (the primary other source is anaerobic CO2 production). In general an RER of 0.7 is considered to be 100% F consumption and 0% C consumption. A ratio of 1.0 is considered to be 100% C, 0% F. The generally accepted theory is that as the RER increases you burn more C and less F. Remember that I am only talking about aerobic metabolism now. When you do a VO2 max test though, you will often see RER values of 1.2 +. When the generally accepted theory is asked to explain this phenomenon, it says that you are just burning 100% C and no F if you are at 1.0 or over. That does not seem to sit well with me. Why? Because it ignores the fact that on top of aerobic metabolism, there is a SIGNIFICANT amount of anaerobic metabolism at high intensity exercise.
OK, what is my point. At rest, we tend to generally burn the ratio of foodstuffs that we consume. Eat a high fat diet, you will burn more fat. Eat a high C diet, you will burn more carbs. My professor made the assumption that this ratio of foodstuffs will be burned throughout aerobic exercise. We can safely assume that RER = RQ at rest because there is no significant anaerobic metabolism. When we start to do cardio, we increase our energy expenditure and must increase metabolism to keep up. If you look at a graded exercise test where you work your way from low intensity exercise to max exercise, the RER does not change significantly at low workloads. Why, there is very little anaerobic metabolism. But as the intensity increases more and more the RER increase close to and then past 1.0 at max. Does this mean that we are only burning carbohydrate at high intensity. I don’t think so and I’ll try to explain why.
Lets just say for simplicity that your resting VO2 is 1.0 L/min (I know that is high, but I’m shooting for easy calculations) and your max VO2 is 4.0 L/min. I will also say that resting VCO2 is 0.7 L/min and max VCO2 is 5.0 L/min. IF you do the calculations: resting RER = VCO2 (0.85 L/min)/VO2 (1.0 L/min) = 0.85 and max RER = 5.0/4.0 = 1.25. Pretty big change in the RER and remember that generally accepted theory says that you are burning 50%F, 50%C at rest and 100%C at max. I don’t think that we really shift this much. The reason is that there are 2 primary sources of CO2 – aerobic metabolism and the CO2 produced by buffering Lactic Acid with bicarbonate. Lactic acid is a product of anaerobic metabolism and the bicarbonate is secreted by the kidney to help control pH. The generally accepted theory ignores the anaerobic CO2 production and I think that this is a mistake.
So we need to assume 2 things: 1) the resting RER is constant for AEROBIC metabolism and 2) the RER is changing because we are adding CO2 from anaerobic pathways. At a resting VO2 of 1.0 L/min we burn about 5 calories (1 L O2 consumed = 5 calories burned) and at a 50/50 ratio, we burn 2.5 cals of F and 2.5 cals of C per minute at rest. At max, we burn 20 calories (4 L O2 * 5), but we also have significant anaerobic metabolism going on that is burning an extra 10 calories (for this to work, you have to assume that I have my subject in a direct calorimeter and I can measure all heat produced while also using indirect calorimetry to calculate aerobic metabolism). So with the 20 calories of aerobic metabolism, we are still 50/50 and burning 10 cals F and 10 cals C. The other 10 calories coming from anaerobic metabolism are all C because we do not use fat anaerobically. So at rest, we were burning 2.5 cals F and 2.5 cals P (50% F, 50%P). At max, we burned 10 cals F and 20 cals C (33%F, 67%C). So yes, we do burn more C when exercise intensity increases, but we do not stop buring F. Aerobic metabolism is never shut down, rather you add anaerobic metabolism on top of it. So at max, those 5L of CO2 produced per minute were not all from aerobic metabolism, but rather 3.4 L were coming from aerobic CO2 production and the other 1.6 L of CO2 production were coming from anaerobic CO2 production that comes from buffering lactic acid with bicarbonate.
Note of caution: This really only applies to shorter durations of cardiovascular activity (generally less than 1 hour), but what bodybuilder does more than 1 hr of continuous cardio. Also, I have really simplified an incredibly complex subject. Also, the calculations were just an example. The message is that you need to read studies that talk about the RER/RQ with a note of caution. I don’t think that fat metabolism is shut off at any exercise intensity and I hope that I have at least provided some evidence to support this. Finally, I must thank Dr. Paul Mole for opening my eyes to this theory and helping me to learn that everything you read in a journal is not necessarily the truth.