|If we go by the convincing results of the study at hand, the fasted cardio myth is obviously busted.|
The study of which I wrote only 2 days ago in my article about the 50% increase in fatty acid oxidation in fasted vs. fed morning cardio (learn more). And it is in fact the study which may finally solve the "Is fasted cardio good for your weight loss?"-question.
In contrast to the previously discussed paper, Schoenfeld et al. who started with the common hypothesis that "performing aerobic exercise after an overnight fast accelerates the loss of body fat" (Schoenfeld. 2014), did not content themselves with measures of acute fatty acid oxidation. What they did was a study to investigate the actual changes in fat mass and fat-free mass following four weeks of volume-equated fasted versus fed aerobic exercise in young women adhering to a hypocaloric diet.
Needless to say that this study has the potentially to give us reliable insights with respect to the previously formulated question, because their subjects, twenty healthy young female volunteers were randomly assigned to 1 of 2 experimental groups,
- a fasted training (FASTED) group that performed exercise after an overnight fast (n =10) or
- a post-prandial training (FED) group that consumed a meal prior to exercise (n =10)
"Subjects performed a warm-up for the first 5 minutes at an intensity equating to 50% of maximal heart rate (MHR), determined by the formula 220 - age, then increased intensity to 70% MHR for the next 50 minutes, and finished with a 5 minute cool down at 50% MHR. Heart rate monitors (model F7U, Polar Electro Inc, Lake Success, NY) were used to ensure that exercise remained at the appropriate intensity." (Schoenfeld. 2014)To ensure that (a) the subjects actually trained and they would (b) only do the prescribed standardized volume of exercise, all training sessions were supervised by research assistants who were upper level undergraduate students in exercise science and the subjects were instructed to refrain from performing any additional structured exercise for the duration of the study.
One thing to consider: I would not fully discard fasted cardio, yet. Even if the resulrs of the study are convincing. It's one study that simulates a specific scenario. In a real world scenario you will often have people, who do shorter fasted cardio sessions, extend the fast and thus reduce their overall energy intake. This is similar to breakfast skipping, which works magic if you don't compensate for the lack of energy intake in the AM (learn more). In the study at hand this "side effect" of morning cardio didn't exist, because of the standardization of the dietary intakes of the female participants. This is perfectly correct from a science perspective, but may still be a reason the real world results you or your clients see may differ from the null-result in the study at hand.Subjects were provided with customized dietary plans designed to induce a caloric deficit. In that, their total caloric intake was calculated on the basis of the Mifflin-St. Jeor Equation, which yields adequate, but obviously not 100% precise measurements of the resting metabolic rate (max. 10% off in non-obese adults according to Frankenfield. 2005). Since the same method was used for both groups, any possible inaccuracies, due to which the real caloric deficit among the women may not be identical to the calculated one, should carry no real weight, though. And we can simply assume that all women were in the same ~500kcal/day energy deficit the researchers thought to create.
|Figure 1: Nutrient composition and total energy intake of the subjects in both groups (Schoenfeld. 2014)|
- immediately prior to exercise for the FED group or
- immediately following exercise for the FASTED group,
Let's take a look at the results now
As you can see in Figure 2, both groups showed a significant loss of weight (P =0.0005) and fat mass (P =0.02) from baseline, but no significant between-group differences were noted in any outcome measure (which means, that all the differences you see are "random").
|Figure 2: Pre- vs. Post-study body composition measures (Schoenfeld. 2014)|
- Frankenfield, David, Lori Roth-Yousey, and Charlene Compher. "Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review." Journal of the American Dietetic Association 105.5 (2005): 775-789.
- Van Proeyen, Karen, et al. "Training in the fasted state improves glucose tolerance during fat-rich diet." The Journal of physiology 588.21 (2010): 4289-4302.
- Schoenfeld, Brad, et al. "Body composition changes associated with fasted versus non-fasted aerobic exercise." Journal of the International Society of Sports Nutrition 11.54 (2014)