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“Fasted cardio helps you burn fat, you’ve no food in your system so it has to use the fat to fuel it….duh” –  We’ve all heard this at some stage, it’s still being echoed by many coaches and gym goers seeking to get #shredded.

Let’s break it down in simple terms:

  • We eat for energy; our body needs energy to perform all activities & functions.
    • We categorise what we eat into carbs, fats and proteins (macro-nutrients).
  • ATP (Adenosine Triphosphate) is the body’s cellular currency for energy.
  • Transforming food into energy is a chemical process known as metabolism.

A (simplified) analogy is that within our body there is a factory, we supply it with raw materials (food) to process (metabolise) and provide us with an end product (ATP /Energy).

During exercise the primary sources of energy are carbs and fats. Protein contributes only a small amount of total energy used (Brooks, 2012), so let’s omit that raw material for now. In our analogy we have two production lines one for fat and one for carbs. The production of turning carbs into energy can be a much faster process than the process of converting fat into energy.

As with all production, supply and demand plays a role. The demand is based on what we ask our bodies to do. When we exercise we are asking our body to satisfy the energy demand of the exercise intensity.

The thinking behind fasted cardio is that fat is used as a fuel, as we’ve no carbs in “stock”, therefore we must lose more fat?

Schoenfeld et al in 2014 examined twenty volunteers, young females, 10 per group of Fasted and Fed. They did an hour of treadmill walking x3/week and dependent on group, fasted or fed cardio. All nutrition was monitored, managed and supported to ensure a deficit for each volunteer in the group, no significant difference was found after 4 weeks. Indicating that fat loss changes associated with aerobic exercise (cardiovascular training aka “cardio”) and a calorie deficit are the same irrespective of the cardio performed fasted or fed.

A meta-analysis and systematic review by Hackett et al 2017 showed that across 5 studies matching their desired criteria, a total sample size of 96 participants showed no significant difference in weight loss or fat loss when using fasted cardio in comparison to fed cardio in an effort to lose weight / reduce body fat.

It is well known that the underlying mechanism of weight loss is a calorie deficit and not fuel substrate (raw material) utilisation.

Bottom-line: Performing exercise in a fasted state did not influence weight loss or changes in fat mass. If you’re looking to lose fat then focus on creating a sustainable Calorie deficit over a period of time.

Doing fasted cardio comes down to preference, maybe you dislike eating before early morning workouts as you’ve experienced some gastrointestinal discomfort or eating your food earlier or working out later isn’t an option, then fire away.

Doing fasted cardio in the belief it enhances fat loss is a non-runner (pun).

References

  • Brooks, George. (2012). Bioenergetics of Exercising Humans. Comprehensive Physiology. 2. 537-562. 10.1002/cphy.c110007.
  • Hackett, D.; Hagstrom, A.D. Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis. J. Funct. Morphol. Kinesiol. 20172, 43.
  • Schoenfeld, Brad & Aragon, Alan & Wilborn, Colin & Krieger, James & Tiryaki-Sonmez, Raziye. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition. 11. 54.
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