Optimizing the "Fat Burning Zone" : Chronic Endurance Training Boosts Fatty Oxidation - Does More Help More?

You as a SuppVersity reader should know that there is no "instant gratification" with  "doing cardio" and that doing it "in the zone" is totally 90s... 1990s, even ;-)

For decades, the "Fat Burning Zone" has been one of the holy grails of exercise sciences. Then somebody realized that maximizing the ratio of fat : glucose that's are being used as fuel during a workout doesn't really have an effect on weight loss and all of a sudden papers with titles like "Changes in peak fat oxidation in response to different doses of endurance training" (Rosenkilde. 2013) have become a rarity... although, if you look closely, you will realize that this is actually not another investigation into the realms of the "Fat Burning Zone", but an afford to quantify the effect of regular "cardio training" on your bodies ability to oxidize fat, instead of glucose.

Don't worry it's not really about the "fat burning zone"

Luckily Rosenkilde's most recent paper, which happens to be the third spinoff of the high (600kcal/day) vs. medium (300kcal/day energy expenditure from "cardio") training volume that already taught us (you can read more about the exact exercise protocol in the previous SuppVersity articles, below) ...

• Learn more about the "Fallacy of Working Out To Burn Calories"

  how futile it is to work out like a maniac if fat loss is your goal ("Some HIIT For Life & Less LISS For More! How to Burn 27,300 Kcal Extra W/out Losing a Single Extra Pound of Fat!" | read more) and 
• how messed up the die hard belief that "exercise" just makes you hungry actually is and what the effects of endurance exercise on appetite and energy intake are ("Exercise: Does It Really Make You Hungry? The More You Train, The Less Hungry You Are." | read more)

In this second serving of the data, we can now learn whether regular endurance training increases peak fat oxidation in a dose-dependent fashion.

Figure 1: Pre & post respiratory exchange ratio (lower value = higher ratio of fatty acid : glucose oxidation) in sedentary control and 300kcal/day group, left; changes in the expression of mitochondrial enzymes (Rosenkilde. 2013)

As you can see in Figure 1 the outcomes of the experiment were not exactly surprising: While there was a persistent increase in fatty acid oxidation and the expression of the facilitative mitochondrial complexes, i.e. enzymes in the mitochondrial respiratory chain, the daily endurance training volume (MOD: 300kcal/day vs. HIGH: 600kcal/day energy expenditure during endurance training) had no effect on the effect size.

So, if it's not the volume, what determines the increase in fatty acid oxidation?

Rosenkilde have probably asked themselves something similar to the above, when they realized that there were no meaningful differences between the subjects in the medium vs. high dose cardio groups. The statistical analyses the researcher conducted did yet reveal, that

• VO2peak, generally regarded as a marker of cardio-respiratory fitness,
• fat free mass, the weight of everything (incl. bones, organs, etc.) that's not fat, 
• cycling efficiency, the power output at a given VO2 peak, and the
• mitochondrial complexes II–V, enzymes that facilitate the oxidation of fatty acids,

were all associated with higher increases in fatty acid oxidation, while the observed changes in fasting plasma insulin, glucose, FFA, or glycerol had no prognostic value with respect to the increase in fatty acid oxidation.
References: 

• Crane, J. D., MacNeil, L. G., & Tarnopolsky, M. A. (2013). Long-term Aerobic Exercise Is Associated With Greater Muscle Strength Throughout the Life Span. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 68(6), 631-638.
• Harber, M. P., Konopka, A. R., Douglass, M. D., Minchev, K., Kaminsky, L. A., Trappe, T. A., & Trappe, S. (2009). Aerobic exercise training improves whole muscle and single myofiber size and function in older women. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 297(5), R1452-R1459.
• Ozaki, H., Loenneke, J. P., Thiebaud, R. S., Stager, J. M., & Abe, T. (2013). Possibility of leg muscle hypertrophy by ambulation in older adults: a brief review. Clinical interventions in aging, 8, 369.
• Rosenkilde, M., Reichkendler, M. H., Auerbach, P., Bonne, T. C., Sjödin, A., Ploug, T., & Stallknecht, B. M. (2014). Changes in peak fat oxidation in response to different doses of endurance training. Scandinavian Journal of Medicine & Science in Sports.