04/11/2024
Once we reach that level where we are out of breath and we cannot chat with the hottie on the treadmill next to us, we are pushing into anaerobic exercise intensities. Anaerobic means “without oxygen.” At this point, adequate oxygen isn’t present to use fat as fuel, so carbs take over as the primary fuel source.
Most of us have more than enough stored body fat to sustain hours of physical activity at moderate intensity levels. Yay fat storage. Fats, as opposed to glycogen (carbs) are available in somewhat unlimited supply. This makes fats preferable during longer-duration, lower-intensity types of training. The chart below demonstrates that fats are the primary fuel source in the lower-intensity training exercises. You can see where the orange line rides above the blue line up to 120 watts of intensity. However, once we reach higher intensity levels, carbs begin to overtake fats as our primary source of energy. This concept is referred to as “The Crossover Effect.”[1]
[1] G. A. Brooks and J. Mercier, “Balance of Carbohydrate and Lipid Utilization during Exercise: The “crossover” Concept,” Journal of Applied Physiology 76, no. 6 (June 1994): doi:10.1152/jappl.1994.76.6.2253.
