Morning vs. Afternoon Cardio: Early Birds DO Have a Fat Loss Advantage, Authors of New Human Study Conclude

I know, this may be disappointing, but the study at hand won't settle the debate once and for all.

Even though Brad Schoenfeld's often-cited "fasted cardio" study suggests that the long-standing myth that "cardio", i.e. aerobic training at moderate intensities, would burn more body fat if you do it in a fasted state in the AM didn't yield the results proponents of "fasted cardio" hoped for, the myth that there's something to doing your cardio training in the AM is still unrefuted.

In their latest study, scientists from the Sports Medicine Research Center and the Department of Sports and Exercise Medicine at the Teheran University of Medical Science did now try to get to the bottom of the "early is better"-myth.

Based on the observation that exercise can significantly affect your appetite, wich almost 16% of the interventional trials that evaluated the correlation between physical activity and food intake showing that volunteers’ appetite decreases after exercise, and the majority indicating that it remained unchanged, they speculated that the often-cited mechanism, i.e. an increase in fatty acid oxidation with AM training (done in the fasted state) could be irrelevant compared to the effects of early exercise on (obese) subjects' appetite and thus food intake within a given 24h period.
As Alizadeh, et al. point out in the corresponding paper in Clinical Obesity (Alizadeh 2017), "there are few studies that consider the effect of a single session of exercise at different times of day on appetite or food consumption". In view of the "lack of time, as is a common barrier in modern life," the authors do yet believe that "exercise should be undertaken at the best time of day in order to obtain maximal appetite suppression and greater weight loss" (ibid.). To figure out, whether this time is in the morning or the evening they conducted a 6-week study to compare the effect of 6 weeks of morning or evening aerobic exercise on appetite and anthropometric indices in N=48 20–45-year-old female participants with body mass index (BMI) of between 25 and 29.9 kg/m². Inactive patients were chosen "to eliminate the effects of other types of exercise on outcomes" (ibid.) and "to decrease the risk of musculoskeletal injuries during exercise testing and prescribed aerobic exercise" (ibid.).

A total of 25 patients participated in the morning aerobic exercise (ME) as group 1 and scheduled to perform their supervised training sessions within 8–10 AM, while the other subjects had to report at the lab between 2–4 PM. All exercise session included 30 min treadmill running in the VT heart rate (this is when you can no longer talk to someone without breathing heavily).

Figure 1: Baseline demographics, anthropometric measurements and physical characteristics of the study participants; data are expressed as mean ± SD unless otherwise stated. (Alizadeh 2017).

The subjects' rate or perceived exertion (RPE) was recorded in the peak of the exercise session at baseline and in the third and the sixth week of the trial. All participants were asked to complete food record forms 24 h before and after exercise sessions and to fill out an appetite visual analogue scale in order to estimate the level of prospective food consumption; fullness; hunger; satiety; and the desire to eat savoury, sweet, salty and fatty foods before the exercise session and 15 min after at baseline and in the third and the sixth week of the trial. In addition, body analysis and anthropometric measurements were taken at three time-points that have been mentioned before.

Figure 1: Changes in anthropometric variables over the course of the 6-week study (Alizadeh 2017).

As you can see in Figure 1 there was a "weighty" advantage for the AM group. The latter is in line with the scientists' observations that ...

• Table 2: Summary of 24-h food records over time in the morning and evening exercise groups (Alizadeh 2017).

  even though subjects in both groups burned the same amount of energy during their workouts [the patterns of change of RPE, speed and estimated mean heart rate during the exercise session were similar over time between the two groups] and didn't have different post-workout appetite scores,
• the effects on the subjects' energy intakes differed significantly [the subjects in the AM group consumed -17% less energy; most of the reduction came from carbs | Table 2]

Now, all that sounds great, what's not so great, however, is the fact that the scientists didn't find significant changes in body composition. Since they used a pretty unreliable Impedance Analyzer (AVIS33 body composition analyzer, Jawon Medical Co. Ltd, South Korea) to generate the lean mass and fat mass data in Figure 1, it does yet make more sense to focus on the skinfold data to assess the body fat reduction. Here we can see a surprisingly significant inter-group difference (see Figure 1, right), with...

• an increased reduction in abdominal skinfold thickness in the AM (add. 3.8%), and
• an increased reduction in suprailiac skinfold thickness in the PM (add. 5.6%) group,

which (the inter-group difference) is hard, if not impossible to explain. This leaves us with an increased loss of body weight, of which we cannot tell for sure where it was lost - not exactly what I would call convincing evidence in favor of AM cardio.
References:

• Alizadeh, Z., Younespour, S., Rajabian Tabesh, M. and Haghravan, S. (2017), Comparison between the effect of 6 weeks of morning or evening aerobic exercise on appetite and anthropometric indices: a randomized controlled trial. Clinical Obesity. doi: 10.1111/cob.12187