AM, PM or AM+PM "Cardio" And Their Effects on Fat Oxid., Energy Expenditure & Balance in Trained Athletes

Study offers yet another, different look at AM vs. PM and AM/PM cardio.
While it may not sound like it, at first, today's article about a study which sought to examine relation between 24-h fat oxidation and exercise induced transient deficit in energy and/or carbohydrate is closely related to the recently published AM-cardio study (read more). Why?

Well, to elucidate said relation, the scientists conducted three 24-h indirect calorimetry sessions in the course of which their subjects performed 100 min of exercise before breakfast (AM), after lunch (PM) or two sessions of 50 min exercise before breakfast and after lunch (AM/PM).
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For almost three days, the scientists from the University of Tsukuba had their subjects, well-trained endurance athletes, live in a metabolic chamber, where they slept for 7-h from 23:00 to 6:00, and three meals (breakfast at 8:30, lunch at 12:30 and supper at 18:00) were provided.
"Subjects were instructed to remain awake and to keep sedentary position other than prescribed exercise session and bedtime by the protocol. On day 2, subjects performed a session of 100 min exercise beginning at 6:30 (AM), at 16:00 (PM) or two sessions of 50 min exercise at 6:30 and at 16:00 (AM/PM) at 65% of V . O2max using a treadmill (T1201, Johnson Health Tech Japan, Tokyo, Japan). In addition to 100 min running, subjects were instructed to perform 15 min of warm-up activity twice a day (6:15 and 15:45). Subjects were allowed to leave the chamber for 30 min to take a shower (22:00 - 22:30)" (Iwayama. 2015).
On day 3, subjects followed the same protocol of day 2 except from the exercise session, and exited the chamber at 16:00. Twenty-four-hour energy expenditure and nutrients oxidation calculated from 6:00 of day 2 to 6:00 of day 3 were compared among the three experimental conditions. Experimental meals were designed to achieve individual energy balance assuming resting metabolic rate to be 24.0 kcal/kg/day, and physical activity factor to be 1.75 (2464 ± 75 kcal/day) in day 1, 2.48 (3544 ± 127 kcal/day) in day 2 and 1.75 in day 3 (1587 ± 47 kcal for breakfast and lunch), according to the estimated energy requirement for Japanese (MHLW. 2003).
Figure 1: Macronutrient intake (g) during the three days in the metabolic chamber (Iwayama. 2014)
Expressed as a percentage of total energy, the standardized meals consist of 15 % protein, 25 % fat and 60 % carbohydrate (see Figure 1). The contribution of the breakfast, lunch and supper to total 24-h energy intake were 32 %, 34 % and 34 %, respectively.
"All subjects completed 3 trials, and there were no significant differences in body mass, body fat and fat free mass among the trials. During exercise, energy expenditure was similar, but oxidation of fat and carbohydrate were different among the trials (P kleiner 0.01). Fat oxidation during exercise in descending order were exercise performed before breakfast (AM: 481 ± 41), two split sessions before breakfast and after lunch (AM/PM: 279 ± 22) and after lunch (PM: 131 ± 5 kcal/100min). Conversely, carbohydrate oxidation during exercise in descending order were PM (1189 ± 45), AM/PM (992 ± 50) and AM (815 ± 52 kcal/100min) (see Figure 2)" (Iwayama. 2015).
The results also revealed that the accumulated 24-h energy expenditure was similar among the trials (AM: 3540 ± 124, AM/PM: 3525 ± 128, PM: 3487 ± 120 kcal/24-h, P=0.15). Since it was balanced with energy intake, the energy balance over 24-h was zero (AM: +4 ± 74, AM/PM: +19 ± 60, PM: +58 ± 60 kcal/24-h, P=0.15):
  • Figure 2: Energy balance (top) and fat oxidation (bottom).
    Accumulated 24-h fat oxidation in descending order were AM (1142 ± 97), AM/PM (809 ± 88) and PM (608 ± 46 kcal/24-h), and that of AM was significantly higher than that of AM/PM and PM exercise condition (P < 0.01). 
  • Contrary, accumulated 24-h carbohydrate oxidation in descending order were PM (2558 ± 110), AM/PM (2374 ± 114) and AM (2062 ± 96 kcal/24-h) (P < 0.01). 
  • Urinary nitrogen excretion was not significantly different among the 3 trials (AM: 13.1 ± 1.2, AM/PM: 13.3 ± 1.5, PM: 12.5 ± 1.3 g/day, P=0.39).
Although energy intake and expenditure over 24-h was matched in all experimental conditions, time course of relative energy balance was different during the first half of day 2 (6:00-18:00). Thus,  exercise performed before breakfast decreased relative energy balance to -1460 ± 49 kcal, while the corresponding value for the trial with exercise performed after lunch was -219 ± 11 kcal.
Reread the AM-Cardio study here.
Why is it relevant to know the relation between 24-h fat oxidation and exercise induced transient deficit in energy and/or carbohydrate? Well, if we know the metabolic difference between spending energy first vs. trying to get rid of the energy one consumed on a meal before the workout, we could maybe better predict who may benefit from AM cardio and who may not. Despite the results of Schoenfeld's et al.'s recent study there are after all thousands of trainees and trainers who swear by AM cardio. Do you really think all of them are dumb or misled?
While the 24h relative energy balance was positive in all trials, it was significantly reduced in the AM vs. AM/PM and PM conditions, where the subjects had a caloric surpluses of 100.1 ± 49.4 (AM), 347.7 ± 37.9 (AM/PM) and 652.9 ± 42.3 kcal/24h (PM).
Figure 4: The impact of AM cardio on metabolism and energy intake in the study at hand.
Now what's actually interesting and could be practically relevant beyond the results of the study at hand is that (a) the nadir, i.e. the lowest energy expenditure (r = 0.72, P < 0.01) and (b) the 24-h average (r = -0.52, P < 0.01) of relative energy balance were positively and negatively correlated with 24-h fat oxidation.

Energy debt = fat oxidation up = total energy balance down = potential fat loss?

In the words of a non-mathematician this means that the more fat you oxidize, the more your total energy expenditure will suffer. In contrast, the nadir of relative carbohydrate balance was negatively correlated with 24-h fat oxidation (r = -0.40, P < 0.05), this is not a surprise (remember: your body has to choose what it wants to burn, carbs or fats).
Figure 4: Association between fat oxidation and average energy balance and nadir of energy balance (Iwayama. 2015).
In terms of the amount of protein that was "lost", the three protocols didn't differ, significantly, though. The urinary nitrogen excretion during the 10-h was not significantly different among 3 trials (AM: 6.9 ± 1.4, AM/PM: 6.9 ± 0.7, PM: 6.4 ± 0.4 g/10-h, P > 0.7) - in that, we must keep in mind, though, that the urinary nitrogen excretion is only a weak proxy of what actually happens to the lean body mass in the long term.
Re-read the results of Aragon's and Schoenfeld's morning cardio study w/ untrained subjects here.
Bottom line: Due to the standardized diet, it's difficult to tell how exactly the results would pan out in practice. What appears to be certain, though, is that the scientists (a) generally overestimated the energy requirements of their subjects and that (b) the AM cardio session was closest to the prediction, therefore the energy overshoot ~100kcal/day was the lowest.

What remains to be seen, though is if doing cardion in the AM can, by increasing the fat oxidation and thus reducing the reliance on carbs eventually help people to eat less compared to those who do their training after lunch.

The results of a 2005 study by Maraki, et al. (2005) would suggest otherwise. In their study, Maraki et al. were able to show that  doing a standardized, albeit shorter workout in the AM did not have different effects on appetite than the same workout in the PM. If that's in fact the case, it's eventually not surprising that Schoenfeld et al. have not found a weight loss advantage for AM cardio in their recent study | Comment on Facebook!
  • Iwayama, Kaito, et al. "Transient energy deficit induced by exercise increases 24-h fat oxidation in young trained men." Journal of Applied Physiology 118.1 (2015): 80-85.
  • Maraki, M., et al. "Acute effects of a single exercise class on appetite, energy intake and mood. Is there a time of day effect?." Appetite 45.3 (2005): 272-278.
  • Ministry of Health, Labour and Welfare of Japan (MHLW). Dietary reference intakes for Japanese. 2005
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