Our bodies are hybrid engines which allows us to burn both fat and carbohydrates.
We are always running with a mix of fat and carbohydrates. After we eat a plate of pasta we are fueling ourselves with the carbohydrates. After we sleep we are being fueled with our body fat. The ability to switch between the two fuel sources is what makes it so we can sleep through the night without getting up every few hours to eat.
The holy grail for performance would be to burn fat and carbohydrates at the maximum possible rate and at the same time. For the Low Carb eater who has very little carbohydrate stores understanding how to perform eating a High Fat diet is important.
The human body has very little carbohydrate stores – about a day’s worth of fuel. The human body has nearly unlimited amounts of fat storage – many weeks worth of fuel in even the leanest person.
Running on Both Fuels at the Same Time
It turns out that eating carbohydrates during exercise reduces fat oxidation rates. Surprisingly, there’s a lot of theories on how this fat/carb balance works and the science it not settled on how this all works. Here’s a paper which is a good summary of how this might work (L. L. Spriet and M. J. Watt. Regulatory mechanisms in the interaction between carbohydrate and lipid oxidation during exercise. Acta Physiol Scand 2003, 178, 443–452).
During (low-moderate intensity) exercise in the fasted state adipose tissue lipolysis exceeded skeletal muscle fat oxidation, whereas CHO ingestion resulted in elevated plasma insulin, reduced adipose tissue lipolysis and decreased fat oxidation that equalled the reduction in lipolysis, thus indicating a limitation of FFA availability for fat oxidation.
Fat utilization increases from rest during low to moderate intensity exercise (40–65% VO2max), however, fatty acid oxidation decreases at power outputs above ~75% VO_ 2max (Romijn et al. 1993, Sidossis et al. 1997)
Takeaway to lose body weight exercise at levels in that 40-60% of VO2max. If you exercise above that level it comes from glycogen stores. Either way, win-win.