From (Med Sci Sports Exerc. 2017 Jan;49(1):133-140. Maximal Fat Oxidation Rates in an Athletic Population. Randell RK, Rollo I, Roberts TJ, Dalrymple KJ, Jeukendrup AE, Carter JM.):
1121 athletes (933 males and 188 females), from a variety of sports and competitive level, undertook a graded exercise test on a treadmill in a fasted state (≥5 h fasted). Rates of fat oxidation were determined using indirect calorimetry.
The average Maximal Fat Oxidation (MFO) was 0.59 ± 0.18 g·min, ranging from 0.17 to 1.27 g·min. Maximal rates occurred at an average exercise intensity of 49.3% ± 14.8% V˙O2max, ranging from 22.6% to 88.8% V˙O2max.
[I think I had a fundamental error in the following. I have corrected it in later posts. The error is that the minimum point in the RER curve isn’t the sweet spot for fat oxidation. The sweet spot is where the RER crosses the 0.7 line. That’s where the energy starts to be drawing from carbohydrates. For me that occurred at 63% of VO2max]
This is what I observed in my own VO2 max testing. My RER was minimal (0.59) at VO2 of 14.3. My max was 34.2. This is 41.8% – reasonably close to the 49.3% from this study especially noting the wide variation in this number (± 14.8%). Exercising at rates higher than that turned to burning carbohydrates. This also explains some of my own exercise tolerance and why I run out of gas at higher intensities. This is the plot of RER vs time for my VO2max test:
Here is my heart rate for the timeframe. Note that the heart rate which corresponded to the minimum RER value was 100 bpm. The range of 80-120 was where I was burning purely fat. This goes against common opinions that you burn the most fat at high intensities.
I really wish I could correlate this to my watch.
Here’s my VO2max over the same time.
Comparing both genders:
In absolute terms, male athletes had significantly higher MFO compared with females (0.61 and 0.50 g·min, respectively, P < 0.001). Expressed relative to fat-free mass (FFM), MFO were higher in the females compared with males (MFO/FFM: 11.0 and 10.0 mg·kg·FFM·min, respectively, P < 0.001).
Interesting, the MFO is in terms of Fat Free Mass (FFM) not Fat Mass (FM). I wonder why that is the case? It seems to me that FM would be more relevant.
Soccer players had the highest MFO/FFM (10.8 mg·kg·FFM·min), ranging from 4.1 to 20.5 mg·kg·FFM·min, whereas American Football players displayed the lowest rates of MFO/FFM (9.2 mg·kg·FFM·min). In all athletes, and when separated by sport, large individual variations in MFO rates were observed.
Here’s the part that relates to my supposition:
Significant positive correlations were found between MFO (g·min) and the following variables: FFM, V˙O2max, FATMAX (the exercise intensity at which the MFO was observed), percent body fat, and duration of fasting. When taken together these variables account for 47% of the variation in MFO.
So there is a positive correlation between percent body fat and MFO. That makes a lot of sense. If you have less fat then there’s less substrate to oxidize.
What does this mean for fat burning?
30 minutes at this average rate of fat oxidation is .6*30 = 18g of fat burned. That’s less than 1 ounce of fat. At 1 gram per minute, 30 minutes would be 30 g or a bit over 1 ounce of fat. This is also at a fairly low level of activity.
Credit to Ben Greenfield for pointing to this.
I consistently burnt far above 1g/min of fat during the entire run, and often went above 1.5g/min, which is unheard of (notice the -0.16 g/min carbs in the photo below…I was burning so much fat the carb count registered as negative)
My RER during the 3 hour treadmill slog showed a fat burning level higher than most modern carb-consuming people burn at complete rest.
During the majority of hard, sustained exercise, I was burning close to zero carbohydrates. -I continued to burn nearly 100% fat and 0% carbohydrate for hours after the run.
Note that Ben’s VO2max value during that activity was around 36. His VO2max peak was measured at 61.1. So this exercise was at 59% of his VO2max which fits within the max fat burning range numbers from this study. It is possible that at lower level of exertion his fat oxidation numbers might have been even a bit more.