Carb Up to Burn More Fat!? True or False: High CHO Diet Allows for 64% Greater Intra-, Post- & Total Fat Oxidation


The title alone will be considered an atrocity by Taubes’ evangelists: “High-CHO diet increases post-exercise oxygen consumption after a supramaximal exercise bout” (Ferreira. 2016 | title of the paper in the Brazilian Journal of Medical and Biological Research). I mean, more carbs = lower fat oxidation, that’s what they’ve been told for years. And in fact, the title says “after a supramaximal exercise bout” and thus appears to leave enough room for the low-carb run to still burn significantly more total fat. Unfortunately (for Taubes and co.), though, that’s not the case.


If you take a look at the data in Figure 1, you will notice that the rate of fatty acid oxidation was significantly higher both during (EXERCISE) and after the workout (EPOC).


Figure 1: VO2 as a marker of fatty acid oxidation during (EXERCISE) and after (EPOC = excess post-exercise oxygen consumption) in 5 physically active males in response to submaximal exercise on high- vs. low-CHO diets (Ferreira. 2016).


Accordingly, the total amount of fat the subjects burned was larger, not smaller; and the effect size of ES = 1.8 further somehow reminds me of a sentence that I haven’t heard ever since the rise of low-carb dieting: ‘Fat burns in the fire of carbohydrates’.


Don’t be fooled – Increased fatty acid oxidation does not necessarily translate to increased fat loss! If this is not your first SuppVersity article, you probably know that already. For the occasional newcomer, however, it is vital to understand that the lipids you burn during and after your workouts (a) won’t necessarily come from your belly and (b) can be easily restored after the workout even if they came from your more or less abundant adipose energy stores.


Before we get deeper into the discussion of the implications of the results, however, it is imperative to take a look at the methodology of the experiment the scientists from Vitória de Santo Antão, the Universidade de São Paulo, and the Universidade Federal de Lavras have conducted last year.


“[A]fter receiving verbal and written explanations, and signing an informed consent, 5 physically active males (age 31.0±7.7 years, height 180.2±4.3 cm, body mass 77.0±7.7 kg, body fat 13.3±2.9%, V̇O2 peak 48.6±11.5 mL·kg-1·min-1) volunteered to participate in this study […]


In order to produce a large difference in CHO availability, pre-SE endogenous CHO stores were altered by a combination of exercise and diet” (Ferreira. 2016)


Now these “alterations” comprised (a) a glycogen-depletion exercise protocol 48 h before each experimental session. As Ferreira et al. point out, “[t]his protocol consisted of a 90-min cycling at 50% of the difference between LT1 and LT2, followed by 6×1 min exercise bouts at 125% of V̇O2 peak; 1 min recovery was allowed between effort sets” (Ferreira. 2016), and (b) different baseline diets (see light-blue box for more information about the diet – will be updated, if possible).


Figure 2: Experimental design. After the preliminary and familiarization test, and a 7-day period, participants were submitted to a glycogen-depletion exercise protocol (GDEP), followed by 48 h having either a high- or low- carbohydrate (CHO) diet. At the end of the 48-h period, participants returned to the laboratory and performed the test experiment for data collection. After a washout period of 7 days, the process was repeated with participants who had the high CHO diet previously, receiving the low CHO diet, and vice-versa (Ferrera. 2016).


After the standardized glycogen-depletion exercimse protocol, participants followed the sequentially prescribed high- or low- carbohydrate (CHO) diet for 48h. At the end of the 48-h period, participants returned to the laboratory and performed the test experiment for data collection. After a washout period of 7 days, the process was repeated with participants who had the high CHO diet previously, receiving the low CHO diet, and vice-versa (crossover design, Figure 2).


UPDATE: Exact macronutrient and energy content of the diets: Yes, it is a bummer that the exact macronutrient composition is not mentioned in the article, but I’ve gotten a pretty fast response from the authors who tell me that the low-CHO diet with 10% carbohydrate, 35% lipids, and 55% protein is not a high fat, but a high protein diet, while the high-CHO diet is, more or less, a low fat, low protein diet. Any conclusions about a truly ketogenic (as being in full ketosis 24/7 due to high fat, low carbohydrate and relatively low protein intakes) diet are thus unwarranted – too much protein (and likely gluconeogenesis) going on with only 35% of lipids and 55% of the energy from protein; wha is not surpring, though, is that a diet with the lion’s share of energy being delivered in form of protein is rather ergolytic than ergogenic.


In the test day, participants arrived at 8:00 am in the laboratory after a 12-h overnight fast and rested on a chair during 20 min for the assessment of resting V̇O2 value (Quark b2, Cosmed, Italy). Then, they underwent a 5-min warm-up at 50 W, followed by an SE at 115% of V̇O2 peak until exhaustion, which was assumed when participants were unable to maintain the pedal cadence above 60 rpm. Immediately after the test, they sat comfortably on a chair for 60 min. The V̇O2 peak was measured continuously from the baseline to the end of the 60-min post-exercise period (Quark b2, Cosmed).


You have already seen the results of the O2 analysis (baseline values were similar for both groups) in Figure 1. So, I will stick to the results I haven’t reported, yet:

  • time to exhaustion increases in the high-CHO group (4.4±0.6 vs 3.0±0.6 min, P=0.01, ES=2.4 large, power effect=0.98),
  • total mechanical work was greater in the high-CHO group (76.9±16.5 vs 50.9±9.4 kJ, P=0.001, ES=2.0 large, power effect=0.91) and
  • the V̇O2 measured at exhaustion was slightly higher in high- compared to low-CHO diet (48.6±11.0 and 45.2±11.0 mL·kg-1·min-1, respectively, P=0.004, ES=0.3 small, power effect=0.08)


To summarize it in the authors’ words: “The high-CHO diet increased exercise duration (∼32%) and total mechanical work (∼34%) during a single SE bout. The increased tolerance further led to an increased exercise energy expenditure (i.e., ∼30% VO2 increase)” (Ferreira. 2016).



Things to keep in mind, when interpreting the results: Now, before you start crying “foul play”, let’s address a few issues that must not be overlooked when you’re talking about the practical implications of the study at hand. (A) With the preceding glycogen depletion protocol, we will probably have relatively low glycogen stores in both groups. This, in turn, will have lead to an increase of AMPK in the muscle during both conditions and could thus have produced similar rates of muscular fatty acid oxidation. (B) The “fat advantage” would thus, as the authors hint at in the previously quoted part from their discussion, simply a function of the increased exercise time.


That’s not bad, but it is unlike the result that’s implied in the title of the paper, not in opposition to previous studies which suggest that a high carbohydrate diet will increase the respiratory ratio (RER) and thus reduce the relative amount of fat to carbohydrates subjects (energy from fat:energy from carbs) will burn during the standardized workouts.


Now, from (A) and (B) follows that, unless you’re doing glycogen-depleted cardio workouts (e.g. after leg training or as described in the “Sleep Low”-study from early 2016), there’s only a relatively slim chance that you’d see an effect size of the magnitude Ferreira et al. report in their paper. In view of the fact that the previously explained RER is not a determinant of exercise-induced body fat loss, however, it’s still (with and without glycogen depletion) possible that the ergogenic effects of the high(er) CHO diet and its ability to increase both, the total mechanical work and the workout duration (and thus the total energy expenditure), may still allow you to burn more body fat. Whether that’s indeed the case, and high-carbing alongside glycogen depletion is, as Ferreira et al. speculate “[f]rom a practical standpoint, […] an appealing strategy for a less time-consuming training and weight loss” (Ferreira. 2016), however, requires, as usual, further research | Comment!


Ferreira, G. A., et al. “High-CHO diet increases post-exercise oxygen consumption after a supramaximal exercise bout.” Brazilian Journal of Medical and Biological Research 49.11 (2016).



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