As a member of a wellness team at my workplace, I gave a recent seminar on basic nutrition. In follow-up chats with people who attended, a couple of people asked about fasted cardio (doing cardio on an empty stomach).
As with many exercise and diet topics, there seems to be strong feelings one way or the other. As with most of those topics, proponents can point to conflicting studies to support their argument…so, conclusively proving one is better than the other is not always clear cut.
The theory behind fasted cardio is that doing at least 20 minutes of cardio upon rising has greater fat loss effects because the body’s glycogen (stored form of glucose in the muscles) stores are low, so the body will shift energy utilization to stored fast. Countering that is, that even if so, the fat has to be converted to fatty acids and that has to be converted to fuel for the body to use. The body does that process slower than the fatty acids becomes available, so some of the fatty acids are not converted for energy end up being re-stored as fat after exercise. Given the number of studies and data available, however, it is not really possible to conclusively argue that either fed or fasted cardio is superior over the other form. (Schoenfeld, Brad. “Does Cardio After an Overnight Fast Maximize Fat Loss?” Strength and Conditioning Journal, Feb 2011, Vol 33 Issue 1.). Other critics of fasted cardio are concerned that some amount of muscle may be burned off in addition to fat.
I am fortunate to work out at and train people at Destination Dallas, a serious training facility in suburban Dallas. We have a several dozen pro and top-level amateur competitive bodybuilders who desire an edge to get to extremely low body fat levels for competitions, where the difference between 4% and 5% body fat is huge. Many work with well-known contest prep coaches. The result — some do fasted cardio and some do not. So, there is not a consensus in the real life either. Typically, most coaches use the cardio options (fasted or not, steady state or HIIT) as prep tools to choose from depending on a person’s schedule, preference, etc.