Carb Burning vs. Fat Metabolism: Are You Burning Carbs or Fats? Thomas DeLauer



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Carb Burning vs. Fat Metabolism: Are You Burning Carbs or Fats? Thomas DeLauer…

Aerobic vs Anaerobic Metabolism: As your body breaks down glucose, a simple sugar molecule, it produces a compound called pyruvate. When this compound enters the mitochondria, aerobic metabolism occurs in which this compound is oxidized and turned into carbon dioxide and water. In the case of anaerobic metabolism, pyruvate enters the fluid, or cytoplasm, of the cell and is turned into lactic acid via a process called glycolysis.

Difference in Efficiency: When one glucose molecule is converted into lactic acid in anaerobic metabolism, three ATP molecules are generated; when a glucose molecule is converted into carbon dioxide and water via aerobic metabolism, it produces 39 molecules of ATP. However, glycolysis requires powerful enzymes that compensate for the lower energy efficiency of anaerobic metabolism - what limits the anaerobic process is the byproduct, or lactic acid.

Aerobic vs Anaerobic Activities:
If you’re performing a low-intensity physical activity over a long period of time, such as running a marathon, you’ll draw the majority of ATP from aerobic metabolism. For these types of activities, fat becomes the chief fuel source. Some activities require the use of both types of metabolism. For example, if you’re cycling in a long distance race, your body will draw on your anaerobic metabolism for a short-term energy boost to climb a hill (1)

When Does Switch Occur:
When your muscles do not have enough oxygen, such as when you're sprinting or otherwise exerting yourself in an extreme manner, your muscles must rely on anaerobic respiration. Your muscles cannot provide the energy they need to sustain your physical efforts given the amount of oxygen available - so when your body is lacking in oxygen, this is when the switch occurs.

Crossover: The anaerobic threshold, also known as the lactate threshold, is the point where lactate (lactic acid) begins to accumulate in the bloodstream. The AT varies from person to person, and, within a given individual, sport to sport. Untrained individuals have a low AT (approximately 55 % of VO2 max), and elite endurance athletes, a high AT (approx. 80 - 90% of VO2 max) (2)

Aerobic & Adult Hippocampal Neurogenesis: Study from researchers at the University of Jyvaskyla in Finland, published in the Journal of Physiology - Looked at how different types of physical exercise effects adult hippocampal neurogenesis. Found that sustained aerobic exercise had positive effects on brain structure and function, including on a process called adult hippocampal neurogenesis (AHN), which basically means nerve growth in the hippocampus (3)

Aerobic & BDNF: Study - "Exercise Induces Hippocampal BDNF Through a PGC-1/FNDC5 Pathway," appeared in the journal Cell Metabolism. Researchers found that a molecule called FNDC5 and its byproduct, irisin, become elevated in the brain through endurance exercise - found that raising levels of irisin in the circulation caused the molecule to cross the blood brain barrier, which then increased the expression of BDNF and activated genes involved in cognition. Mice genetically altered to have low irisin levels in the brain had reduced levels of BDNF. (4)

Additional: Researchers from New York University (NYU) Langone Medical Center published a study explaining why sustained aerobic exercise benefits the mammalian brain on a molecular level
Concluded that, "Exercise Promotes the Expression of Brain Derived Neurotrophic Factor (BDNF) Through the Action of the Ketone Body β-Hydroxybutyrate.” (5)


1) What Is Aerobic & Anaerobic Metabolism? (2017, November 21). Retrieved from
2) Anaerobic Threshold. (n.d.). Retrieved from
3) Physical exercise increases adult hippocampal neurogenesis in male rats provided it is aerobic and sustained. (2016, February 24). Retrieved from
4) Exercise Induces Hippocampal BDNF through a PGC-1α/FNDC5 Pathway - ScienceDirect. (n.d.). Retrieved from
5) Exercise promotes the expression of brain derived neurotrophic factor (BDNF) through the action of the ketone body ?-hydroxybutyrate. (2016, June 2). Retrieved from