It is a current theory in bodybuilding and the fitness culture in general that the performance of aerobic exercise in a fasted state is optimal for body fat loss when compared to fed cardio. While proper research on the topic is minimal, and the research that has been completed is lacking in many factors – no study to this point has directly related fat loss to fasted cardio. This seems counterintuitive, I know, so allow me to summarize the current literature:
Schoenfeld, JB; Aragon, AA; et al. Body Composition Changes Associated with Fasted versus Non-Fasted Aerobic Exercise. JISSN. 2014.
25 healthy, young females performed 1 hour of steady-state aerobic exercise 3 days a week for 4 weeks
All subjects followed a hypocaloric diet (eating at a deficit)
Half completed exercise after an overnight fast, the other half consumed a meal replacement shake immediately prior to exercise
Both groups showed significant weight loss and body fat loss, however there was no statistical difference between the two groups’ losses.
Zoladz, JA; Konturek, SJ; et al. Effect of Moderate Incremental Exercise, Performed in Fed and Fasted state on Cardio-Respiratory Variables and Leptin and Ghrelin Concentrations in Young Healthy Men. Journal of Physiology and Pharmacology. 2005.
8 healthy men performed moderate incremental exercise testing fed vs. fasted with one week’s time between testing
10 bt . min-1 lower HR at each power output when fasted than the same exercise performed in fed state
Effect on HR is independent of plasma leptin and ghrelin concentrations, which were not affected by the overnight fasting and the applied exercise
Ghrelin – The Hunger Hormone
Leptin – The Satiety Hormone
The idea behind fasted cardio anchors on the possibility of increased lipid oxidation. Meaning fasted cardio is hypothesized to utilize fat for energy at a higher rate than fed cardio per bout of exercise. However, this theory seems to look at the human body and its energy systems as all or nothing – wherein the true nature of the human body is much more dynamic. De Venne and colleagues in their 1991 study “Influence of the feeding frequency on nutrient utilization in man: consequences for energy metabolism” posed the idea that a higher fat utilization at one given time throughout the day will be adjusted and compensated for by a higher carbohydrate reliance at a later period throughout the day. Therefore, we should instead consider fat burning as an average throughout a larger timespan, and not on an hour to hour basis.
These findings together place less emphasis on the feeding state during aerobic exercise, and indicate a need simply for the aerobic exercise for body fat loss – when all other factors are controlled for.
Put in the work, don’t obsess over the minutia, the results will follow
Fat loss and body changes are a slow process to be monitored on a weekly, not hour by hour basis
No definitive differences in fat loss have been found between fasted and fed cardio, however, neither are detrimental. Whichever suits your preferences is best for you.