“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 is still being echoed by many coaches and gym-goers seeking to get lean.
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 too but only a small amount of total energy is 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 process of turning carbs into energy can be a much faster process than converting fat into energy.
As with all production, supply and demand play 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?
A study (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. In our analogy, a calorie deficit is producing more than we’re supplying and weight loss is effectively taking from stock we have in our factory/system – losing weight.
Bottom-line: if fat loss is your concern, prioritise achieving and sustaining a calorie deficit.
It’s really down to preference, maybe you dislike eating before early morning workouts as you’ve experienced some gastrointestinal discomfort and getting in your food earlier or working out later isn’t an option then fire away.
- 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. 2017, 2, 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.