High Intensity Cardio Versus Moderate Intensity and How Our Bodies Adapt

Maximizing fat burn can be tricky in that during exercise the body performs an elaborate dance between using carbs and fats for energy.  The degree to which the body favors one over the other depends on various factors, many of which are still being debated by scientists to this day.


Fat is typically stored in what is known as adipose tissue.  However, it can also be stored within muscle tissue (a.k.a. intramuscular fats or intramuscular triacylglycerols) and be present in blood.  The advantage of fat storage, even in muscle, is the ability to sustain longer duration of exercise.  This is particularly important for endurance athletes.Friends Running


Muscle tends to rely on intramuscular fats and carbs, from glycogen, during moderate and high intensity whole body aerobic exercise.  Some studies have shown that by suppressing either the amount of intramuscular fats or glycogen available the alternate source of energy will be more heavily used.  Therefore, it is possible to manipulate the use of fats for energy under the right dietary conditions.

When exercising fasted (i.e. after a night of sleep), the use of adipose tissue fat for fuel exceeds that of intramuscular fat due to the increased needs of muscle.  This is particularly the case at moderate intensity (25-65% of maximal oxygen consumption) exercise.

Note that after consuming carbs, there is an inhibitory effect on adipose tissue breakdown and muscle ability to use fat for fuel.  When you increase to high intensity exercise, the shift to usage of muscle glycogen has a similar effect. Research has shown when exercise is high-intensity (>70% of maximal oxygen consumption), total fat burned drops to below that of moderate-intensity exercise.

The reason for this decrease isn’t exactly known.  Some speculation includes: decrease in blood flow to fat tissues, decreased release of fatty acids into the blood and decreased delivery to muscle, or decrease in transport of the fatty acids into the muscle cells and their mitochondria.  Others suspect that it has something to do with a decreased capacity of muscle to use fatty acids for energy due to a shift in the pathways in muscle cell metabolism, such as an increased reliance on muscle glycogen.  Once the glycogen is consumed, in order to still draw from fat either the exercise must stop or the intensity needs to be reduced to prevent the breakdown of muscle protein.  Thus, according to science, maintaining moderate exercise intensity maximizes the body’s ability to burn fat.


There is a greater ability to burn fat in trained muscles than in untrained muscles.

Trained Verus Untrained Muscle
The estimated contribution of various sources to energy metabolism during exercise when the limb is trained or untrained. Similar results were found in many studies.

Some advantageous adaptations of trained muscle that help maximize fat burning ability include:

  1. Increase in the amount of enzymes needed for metabolic cycles to produce energy, especially for burning fat
  2. Greater amount of carnitine and other proteins that are important for making fat more accessible – to certain components of cells – so that it can actually be used for energy
  3. Creation and spread of more capillaries per muscle fiber, thereby increasing the effectiveness of blood and nutrient flow to the muscle (this means if there is more fat released into the blood from our fat tissue, it will be more easily transported to the muscle for use for energy and essentially more fat can be burned under the right conditions)
  4. There may be more mitochondria (components that provide our cells energy by breaking down fats and carbs) in trained muscle


Research has shown that such advantages of trained muscle over untrained muscle were felt even when under the same concentration of hormone stimulation (think: epinephrine).  So next time you start planning your routine, embrace the dumbbells in addition to hitting the stair climber!



Saltin, B., & Astrand, P. O. (1993). Free fatty acids and exercise. Am J Clin Nutr, 57(5 Suppl), 752S–757S; discussion 757S–758S. Retrieved from

Horowitz, J. F., & Klein, S. (2000). Lipid metabolism during endurance exercise. The American Journal of Clinical Nutrition, 72(2 Suppl), 558S–63S.

Jeppesen, J., & Kiens, B. (2012). Regulation and limitations to fatty acid oxidation during exercise. The Journal of Physiology, 590(Pt 5), 1059–68.

Morton, J. P., Croft, L., Bartlett, J. D., Maclaren, D. P. M., Reilly, T., Evans, L., … Drust, B. (2009). Reduced carbohydrate availability does not modulate training-induced heat shock protein adaptations but does upregulate oxidative enzyme activity in human skeletal muscle. Journal of Applied Physiology (Bethesda, Md. : 1985), 106(5), 1513–1521.

Robinson, S. L., Lambeth-Mansell, A., Gillibrand, G., Smith-Ryan, A., & Bannock, L. (2015). A nutrition and conditioning intervention for natural bodybuilding contest preparation: case study. Journal of the International Society of Sports Nutrition, 12(1), 20.

Spriet, L. L. (2014). New Insights into the Interaction of Carbohydrate and Fat Metabolism During Exercise. Sports Medicine, 44(S1), 87–96.

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