The Low Carb diet requires use of fat as the primary fuel source. Fat as fuel is only available at a particular maximum rate. The rate fat is used as fuel is the fat oxidation rate.
The other ordinary source of energy is from carbohydrates. Carbohydrate stored in the human body are in the form of glycogen and are much more limited than fat as a fuel. On the Low Carbohydrate diet, Glycogen stores are reduced from other diets so there’s less substrate available for fuel. In the body, glycogen is stored in the liver and muscles. Glycogen in the liver is used for Gluconeogenesis which produces glucose for the parts of the body which require glucose. Glucose in the muscles is used for energy.
This paper is a good source of material on this subject (Sports Med. 2014; 44(Suppl 1): 87–96. New Insights into the Interaction of Carbohydrate and Fat Metabolism During Exercise. Lawrence L. Spriet).
Fuel Source Depends on Diet
We are always a hybrid mix of carb and fat fueled. Someone on a High Carb (HC) diet is primarily carb fueled. On a Low Carb (LC) diet that person is primarily fat fueled.
With Low Carb, at t=0:
Carb oxidation rate is about 0.4g/min (1.6 kCal/min) and fat oxidation rate is 1.2g/min (10.8 kCal/min). That’s a combined 12.4 kCal/min or 744 calories per hour.
With High Carb, at t=0:
Carb oxidation rate is about 1.75g/min (7 kCal/min) and fat oxidation rate is 0.55g/min (4.95 kCal/min). That’s a combined 11.95 kCal/min or 744 calories per hour.
The fat oxidation efficiency is noted in other studies at about 10% lower. These rates are in trained athletes. These are also at longer endurance paces where the fat fueled athlete is on equal or better footing.
Fat Oxidation Rates
There is a study of the maximum amount of fat that can be oxidized at various levels of exertion (Nutrition. 2004 Jul-Aug;20(7-8):716-727. Optimizing fat oxidation through exercise and diet. Achten J, Jeukendrup AE.).
With increasing exercise intensities, the fat oxidation rate increased to a maximum of 0.60 ± 0.07 g/min at 64 ± 4% VO2max (range 42-84), which corresponds to 74 ± 3% HRmax (range 54-92).
The intensities of the Fatmax zone were located -8.9 ± 1.3% and +8.2 ± 1.2% from Fatmax. The Fatmax zone was found to be between 55 ± 3 and 72 ± 4%VO2max, which is equivalent to 68 ± 3 and 79 ± 3%HRmax, respectively.
At exercise intensities above the high border of the Fatmax zone, fat oxidation rates decreased markedly. The contribution of fat oxidation to energy expenditure became negligible above 89 ± 3%V02max (range 71-99) and 92 +I %HRmax (range 84-98).
My Own VO2max Numbers
On my own case, my fat max oxidation was at a heart rate of between 90 and 120 BPM (nominal peak value of 100 BPM). My RER reached 0.85 (carb/fat fuel mixed evenly) at a heart rate of about 138 (85% of my max heart rate).
A rate of 0.6 g/min (36g/hr) is 5.4 kCalories per minute or 325 kCalories per hour (maximum rate). The highest fat oxidation rate recorded was Ben Greenfield who burned at 1.5g/min or 810 calories per hour.
My Three Hour Bike Ride
I took a three hour bike ride where my watch said I burned 900 kCalories. That’s 300 kCalories per hr which approaches matches the maximum fat oxidation rate.
My heart rate was around 138 which is 85% of my HRmax. That is close to my 50-50 crossover (half carbs and half fat burning) from my VO2max test.