New "Fasted Cardio"-Study Falsifies the Myth of Superior Long-Term (4 Week) Fat Loss on a Moderate Energy Deficit

If we go by the convincing results of the study at hand, the fasted cardio myth is obviously busted.
Sometimes the day you've been waiting for comes faster than you'd thought... no, I am not talking about a teen's eighteens birthday or Christmas (reminds me, I still have to buy a ton of presents), but rather of the recently hinted at "fasted cardio study" by Brad Jon Schoenfeld, Alan Albert Aragon, Colin D Wilborn, James W Krieger and Gul T Sonmez.

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.
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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)
not for one or two testing days, but for 4 weeks! The training itself consisted of 1 hour of steady-state aerobic exercise on a regular treadmill (0% incline) and was performed for 3 days per week for the previously mentioned total study duration of 4 weeks.
"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)
In addition to their regular diet, the adherence to which was monitored on a regular basis, the subjects received a meal replacement shake either
  • immediately prior to exercise for the FED group or
  • immediately following exercise for the FASTED group,
with this nutritional provision carried out under the supervision of a research assistant. The "Pursuit Recovery" (Dymatize Nutrition, TX) shake you could also buy at your local GNC contains 250 calories, total, and 40 g carbohydrate (from maltodextrin and organic cane sugar), 20 g protein (from whey protein isolate + added leucine), and 0.5 g fat (residues).

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)
As Schoenfeld et al. rightly point out, their findings clearly "indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training" (Schoenfeld. 2014) - in other words, in this pretty realistic scenario (I hope nobody starves himself after a 1h morning cardio session for another 4-8h) the myth that morning cardio on an empty stomach would accelerate fat loss is thus busted.
Bottom line: The assumption that the consumption of an insulinogenic pre-workout meal as it was used in the study at hand and a subsequent reduction of fatty acid oxidation during the workout would induce a shift from fat to carbohydrate oxidation (not measured in the study at hand, but previous studies show that this is the case) and have significant effects on an individual's long-term fat loss on an energy reduced diet is thus falsified.

The study at hand shows that the 50% increase in fatty acid oxidation w/ fasted cardio does not translate into increased fat loss | more
You could still argue that it may be beneficial if there is no energy deficit involved, for example by improving glucose levels as it was reported by Van Proeyen et al. (2013) in a study with a hyper-caloric energy intake (~bulk), but that's a whole different story.

Or you could argue that there is an albeit non-significant trend for an increased loss of fat mass in the FASTED group (inter-group difference = 33%, but the latter was (a) paid dearly for by an almost 2x higher increase in lean mass loss (inter-group difference = 60%) and stands (b) in contrast to the non-significant greater reduction in abdominal fat in the FED group as it is signified by changes in waist circumference.

For the time being, the long-standing "myth" that fasted cardio would lead to a significant acceleration has thus to be considered "questionable", if you put 100% faith the statistical accuracy of the study at hand (with only 10 participants in both groups, I am inclined not to do that) even "busted". For so long, at least, until another study, maybe one with more participants (which would allow to really figure out how "significant" the difference actually was), but a similar strict standardization, will show that it works. In that case, we would have to find out could have been that made the difference - could be the sex or training status of the subjects, the extend of the caloric deficit, the total protein intake (which was comparatively low), the type of the pre-workout meal or the form of cardio training that was used... Comment on Facebook!
  • 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)
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