We all know that cardiovascular exercise is beneficial both to our health and fitness as well as a good way to burn stored fat. Many different options exist in how to best achieve our goals. In an effort to burn as much fat as possible, many athletes are turning to something called fasted cardio. Fasted cardio means doing cardio, usually first thing in the morning, after fasting for at least 12 hours. Fasting, in this case, means not ingesting any food or alcohol or other drink or calorie containing substance for at least 12 hours prior to exercising. The theory is that by doing cardiovascular exercise in a fasted state you are burning mostly fat, since the body has burned glucose (stored in blood stream) and glycogen (stored in liver and muscle) throughout the night and the body must burn stored fat for energy while exercising. Many studies have been performed to determine if this theory indeed holds true. One such study was performed on 20 healthy female volunteers who were divided into two groups. One group did fasted cardio for one hour and the other group did the same slow steady state cardio in a fed state. Both groups received the same diet with a calorie deficit and were monitored for compliance.
Both groups showed a significant loss of weight and fat mass from baseline, but no significant between-group differences were noted in any outcome measure. These findings indicate that body composition changes associated with aerobic exercise in conjunction with a low caloric diet are similar regardless whether or not an individual is fasted prior to training.
How can this be? We theorize that a fasted state will force the body to burn more fat. However, this ignores the dynamic nature of the body, which is constantly adjusting its use of substrate for fuel. Evidence now also points to the fact that any increase in fat burning from fasted exercise might be mitigated by the T.E.F. (thermic effect of ingesting food) prior to exercise, which results in an increase in calorie burn and metabolism. A study of 10 male subject supports this finding. They were randomized to perform exercise in a fed state and in a fasted state for four weeks. The study concluded that exercising in a fed state increased E.P.O.C. (excess post oxygen consumption,) which increases caloric burn for up to 24 hours post exercise. The E.P.O.C. effect was greater in the fed group than in the fasted group, thereby allowing the fed group to burn more calories for a longer period of time post exercise. This was true whether the exercise performed was high intensity or low intensity.
In conclusion, these findings indicate that body composition changes associated with exercise are very similar regardless of the subject being fasted or fed prior to exercise. In fact, as mentioned, being fed increased calorie burn over time, which may potentially be used to an athlete’s advantage. So the choice is really yours. However you feel best, fasted or fed, is the way that you should do your exercise. Like the Nike quote says; “Just do it!”
Unni Greene. C.M.T., C.N.S. is the owner of SoMi Fitness, where she helps clients gain control of their fitness and health. For more information, visit www.somifitness.com or feel free to email Unni at firstname.lastname@example.org