New Blog post by Brad Schoenfeld at his site Lookgreatnaked on the subject of fasted cardio and his latest research study.
In the late 1990’s, Bill Phillips authored “Body for Life,” which went on to become one of the biggest selling fitness books of all time. In the book, Phillips claimed that performing 20 minutes of high-intensity aerobic exercise (HIIT) after an overnight fast has a greater effect on fat loss than an hour of cardio performed following consumption of a meal. The rationale for the hypothesis was based on research showing that low glycogen levels cause your body to shift substrate utilization away from carbohydrates, thereby allowing greater mobilization of stored fat for energy.
While the theory that fasted cardio is superior for fat loss is certainly intriguing, it is based on an extrapolation of findings that might not translate into practice. Several years ago I authored a review of literature that discussed the contradictions of the research on the topic. While my review highlighted a number of inconsistencies that suggested fasted cardio might not work as claimed, one little issue continued to nag at me: The entire debate was based on acute data; no study had actually investigated the effects of fasted cardio on body fat when subjects were in an energy-deficit sufficient to produce weight loss.