Fasted Cardio Before Breakfast Increases 24h Fat Oxidation by Almost 50% over Doing AM+PM Workouts, But This Does Not Necessarily Mean That You Will Lose 50% More Fat

Can fasted cardio switch on your "fat burning mode. And, more importantly, will it help you lose body fat faster?
Before you get overtly excited about this being the study to finally solve the "fasted cardio" conundrum (i.e. answer the question "Will fasted cardio promote fat loss?"), I want to warn you. To answer this question, we will have to wait for Brad Schoenfeld's latest study on the chronic effects of fasted cardio to be released.

The study at hand, which was conducted by researchers from the Graduate School of Comprehensive Human Sciences at the University of Tsukuba can only inform us about the acute (24h) metabolic effects of cardio at different timepoints and with different amounts of food in the tummy (Iwayama. 2014).
High intensity interval training aka HIIT is more than just an alternative for steady state.

Never Train To Burn Calories!

Tabata = 14.2kcal /min ≠ Fat Loss

30s Intervals + 2:1 Work/Rec.

Making HIIT a Hit Part I/II

Making HIIT a Hit Part II/II

60s of HIIT per week suffice!
In the experiment the nine young male endurance athletes, whose mean age was 23.2 ± 2.7 years of age (height: 168.4 ± 5.7 cm | weight: 59.5 ± 1.2 kg | body fat: 11.6 ± 0.6 %) and who had a relatively high cardiovascular fitness (O2max was 71.7 ± 6.4 ml/kg/min), performed three different types of exercise at different times in the day during a 42h stay in a metabolic chamber. 
  • 100 min exercise before breakfast (AM),
  • 100 min exercise after lunch (PM) or
  • two sessions of 50 min exercise before breakfast and after lunch (AM/PM),
All the exercise sessions were performed at the same relatively low intensity of only 65% of the subjects' individual VO2max and repeated the day after under different caloric conditions. Thus, the actual experiment that was designed to investigate the effects of "cardio" on empty vs. full stomach, the effects of the duration and the time of the day, once in energy balance (2464 ± 75 kcal/day in day 1) and once in an energy deficit (3544 ± 127 kcal/day in day 2). The macronutrient composition (15 % protein, 25 % fat and 60 % carbohydrate) and timing of the meals was identical for all conditions. The contribution of the breakfast, lunch and supper to total 24-h energy intake were 32 %, 34 % and 34 %, respectively.
Figure 1: 24h fat and glucose oxidation during the two trials (fed condition) measured pretty reliably
in a metabolic chamber (Iwayama. 2014)
The scientists observed that the 24h energy expenditure was similar among the trials (that's why all three bars in Figure 1 have the same height). The 24h fat oxidation, on the other hand, was significantly higher in the 100 min trials and highest, when the 100 minutes of cardio were performed fasted in the AM (1142±97, 809±88 and 608±46 kcal/24-h in descending order of its magnitude for AM, AM/PM and PM, respectively).
Figure 2: Relations of 24-h fat oxidation to average (r = -0.52, P < 0.01) and nadir (r = -0.72, P < 0.01) of relative energy balance were plotted on panel (left) and (right), respectively (Iwayama. 2014).
If you take a look at Figure 2, you will also see that the increase in fatty acid oxidation correlated with the nadir of energy balance (Figure 2, right). In other words: The higher your cardio induced energy debt (as in doing cardio on empty), the greater the increase in 24h fatty acid oxidation.

Since the overall energy expenditure was identical in all trials, you should not be surprised that the increase in fatty acid oxidation in the AM group went hand in hand with a decrease in 24h glucose oxidation. A decrease of which the data in Figure 1 indicates that it largest in the AM (2062±96 kcal/24-h) trial, and hardly different for the PM (2374±114 kcal/24-h) compared to the AM/PM trial (2558±110 kcal/24-h), in which the fatty acid oxidation had been the lowest.
For people with type II diabetes interval training and thus burning more glucose than fat during the workout would be the preferred method. At high intensities you will burn a lot of fat after the workout, anyway (learn more). And even at low intensities, interval training is more effective than "steady state". Only recently, scientists from the University of Copenhagen have been able to show that at intensities that were as low as 40% VO2max during active rest and 70% VO2Max during the intervals, interval training is more beneficial than steady state cardio training with identical energetic costs (Karstoft. 2014).
Fat oxidation up, glucose oxidation down: At first sight, a 47% difference in fatty acid oxidation sounds like a huge advantage, but the fact that you oxidize fat, not carbohydrates does not necessarily mean that this fat (a) comes from your belly and other areas, where you may want to lose it, and (b) will not be restored after the workout.

In a caloric deficit, on the other hand, one could cautiously argue that burning more fat for fuel could lead to a slightly faster depletion of the adipose energy stores and thus slightly (not by 50%!) accelerate your weight loss. Plus: As long as you burn fat for fuel, your chances of running low on blood sugar and experiencing hunger pangs and cravings will be significantly reduced.

For the average physical culturist, there is yet another significant benefit: All the glucose / gylcogen you don't use during your cardio workouts will be available for your strength training sessions, where you will need them to lift heavy and thus promote lean mass retention  | What are your experiences with "fasted cardio"? Comment on Facebook!
  • Iwayama, K. "Transient energy deficit induced by exercise increases 24-h fat oxidation in young trained men." Journal of Applied Physiology Published (2014). Ahead of print.
  • Karstoft, Kristian, et al. "Mechanisms behind the superior effects of interval vs continuous training on glycaemic control in individuals with type 2 diabetes: a randomised controlled trial." Diabetologia 57.10 (2014): 2081-2093.
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