Body composition changes associated with fasted versus non-fasted aerobic exercise

Abstract (provisional): It has been hypothesized that performing aerobic exercise after an overnight fast accelerates the loss of body fat. The purpose of this study was to investigate 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. 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). Training consisted of 1?hour of steady-state aerobic exercise performed 3?days per week. Subjects were provided with customized dietary plans designed to induce a caloric deficit. Nutritional counseling was provided throughout the study period to help ensure dietary adherence and self-reported food intake was monitored on a regular basis. A meal replacement shake was provided 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. 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. These findings 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.

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Alex’s Notes: I’m sure many of you are well aware (and for those who aren’t, surprise!) of the recently published study by Brad Schoenfeld, Alan Aragon, Colin Wilborn, and James Kriegar in the Journal of the International Society of Sports Nutrition. As the title alludes, Brad and colleagues sought to examine the changes in fat- and fat-free mass following a caloric deficit coupled with a fasted or fed moderate intensity aerobic training program.

Like many of Schoenfeld’s studies, he aims to fill gaps in the literature. You see, the idea of fasted cardio is entirely based on hypothetical assumptions that are in turn based on our understanding of biology. For instance, when training in the morning following an overnight fast, the low(er) levels of glycogen and insulin cause the body to rely more heavily on fat for energy rather than carbohydrates, and the fact that you haven’t eaten anything makes the fat have to come from our body stores. This idea is supported by acute studies looking at 24 hour metabolic differences between fasted and fed training. One such study in nine endurance athlete males found that fasted cardio increased 24 hour fat oxidation by 47% relative to consuming breakfast before the cardio session. It stands to reason that if done chronically, then perhaps there would be greater fat loss when dieting.

But as we should know by now, our body is annoyingly complex and what sounds good on paper doesn’t tend to pan out in the real world. Hence Schoenfeld’s recruitment of 20 healthy and young university females. They were all between the ages of 18-35 years (average 22.4) and all reported performing aerobic exercise several days per week on a regular basis; some were even off-season collegiate track and field athletes. None, however, were involved in a resistance training program.

The ladies were divided into two groups: fasted and fed cardio, and underwent a 4-week aerobic training program that consisted of 1 hour of steady-state treadmill running 3 days/week. The sessions began and ended with a 5-minute warmup and cool down at 50% of the lady’s maximal heart rate, while the 50 minutes in between was spent running at an intensity of 70% max heart rate. They all had customized dietary plans based on the calculated energy balance with the Mifflin-St. Jeor equation minus 500 kcal to impose a deficit. Protein was set at 1.8g/kg to help preserve lean body mass; fat was set at 25-30% total calories; and carbohydrates made up the remainder. Adherence was monitored using MyFitnessPal.

The only difference between the two groups was when a specific meal-replacement shake was consumed. The shake contained 250 kcal of 40g carbohydrates & 20g of protein, and was consumed immediately after the exercise sessions in the fasted group, or immediately before the exercise session in the fed group.

The results

Nothing! Although the 4-week intervention led to significant reductions in body weight and fat mass in both groups, the differences between the groups was not significant.

The caveats

The study was a mere four weeks in duration, which is not a very long time to allow for significant differences to generate, especially when the differences would be minor. This is compounded by the small sample size that further limited statistical power. Moreover, despite non-significance, the fasted group lost more body weight (1.6 vs 1 kg), more fat mass (1.1 vs 0.7 kg), and more body fat percentage (1.3 vs 0.7 %). It really depends on how you look at it. On the surface the answer is quite clear, fasted cardio is on equal grounds with fed cardio. However, if you start to dig I believe that this study provides weak evidence in support of fasted. Of course all we can do is assume at the moment, but at the rates of change in body composition seen between the groups, perhaps 12-16 weeks would have allowed significance to be met.

After chatting briefly with James Kriegar on Facebook, I received some great alternative thoughts on the above. Rather than paraphrase him, I will simply quote him below.

“First, people need to be very careful about taking the slightly better mean change in the fasted group and then saying this study showed a slight edge for fasted cardio. If we're going to look at the data that way, one must also consider that the fasted group started off slightly worse in terms of body fat compared to the fed group. Thus, we would expect the fasted group to have a slightly greater decrease based on that alone. Thus, it is a mistake to infer that the differences would've been magnified with a longer duration study. Also, if we're going to just look at means, it should also be noted that the fasted group had a slightly greater loss of FFM as well.

Second, while there are limitations to statistical significance with small sample sizes, the results were not even close to being statistically significant. We're talking P values of around 0.8 for the differences. Thus, we can't even claim there's a slight trend in any way.”

The bottom line

Despite the results of the study, it must be kept in mind that they are specific to young, non-obese women performing moderate intensity steady-state cardio. We cannot generalize these results to men, overweight/obese persons, or to the competition bodybuilder or physique enthusiast who is trying to achieve extreme leanness. Moreover, fasted training has other benefits. It has been shown to attenuate weight gain and improve glucose tolerance in recreationally active men fed a hypercaloric (+30%) diet. Similarly, although no differences in weight were seen between fasted and fed cardio sessions when subjects consumed an isocaloric diet, the fasted training increased muscular oxidative capacities.


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