Friday, July 5, 2013

The Glucose Repartitioning Effects of Exercise: Moderate Beats High Volume Training When It Comes to Shuttling Glucose Away From Fat and Right into the Muscle

"Are 2h of cardio each day still too little!?  It must be my thyroid! Yeah, that's it. It must be the thyroid!" Could be bro, but if it is probably self-inflicted hypothyrodism
In the context of my dissertations on the unwarranted vilification of insulin as a "fattening agent" (go back to "The "Pro-Insulinogenic" Effects of Non-Nutritive Sweeteners + Mechanisms & Consequences" if you have not read the article already), I presented data from a rodent study to make a point that insulin's fattening effects depend on two closely related and highly familiar factors.

One is the over-consumption of energy that is the norm, not the exception here in the Western Obesity Belt. In conjunction with the lack of glucose depleting exercise this "ensures" that the intra-muscular and hepatic liver stores of the average Westerner are always topped off and the only change to get rid of the glucose that's floating the system of the coke-guzzling convenient generation on a day-to-day basis is to pack it away in the adipose organ.

That being said, it is only logical to assume that working out would help mitigate the problem by restoring the "normal" state of partly if not fully depleted glycogen stores and allowing insulin to do its original "glycogen anabolic". It is this process of insulin induced glucose partitioning towards the emptied glycogen stores of the skeletal musculature and the influence of (a) sedentarism (control), (b) aerobic exercise worth 300kcal/day, and (c) aerobic exercise worth 600kcal/day a group of scientists from the University of Kopenhagen in Denmark (Reichenkendler. 2013) investigated in their most recent study.

300kcal or 600kcal does it even make a difference?

For the experiment on which this paper that is supposed to be published in one of the upcoming issues of the American Journal of Physiology, Endocrinology and Metabolism, the researchers recruited 27 moderately overweight men (BMI: 28.1(1.8); age: 30(6) years; no diabetic relatives, weight stable for the last 6+months), randomized them to one of the three previously mentioned conditions.

According to which group the subjects had been randomized to, the participants either continued their sedentary lifestyle (CON) or performed daily aerobic exercise of 300 kcal/day (MOD) or 600 kcal/day (HIGH) for 11 weeks. The workouts were performed
  • at a higher intensity (>70% of VO2max), three times per week, and
  • at a low-medium intensity on the other three workout day
As you can see this is more of a chronic "general activity" + exercise study, than your average acute (let's save some money) trials and thus highly relevant to the aim of investigating the effect of chronic
"[...] moderate or high dose aerobic physical exercise on insulin-stimulated glucose uptake in individual femoral muscle groups, intra- and retroperitoneal VAT, abdominal (both anterior and posterior) and femoral SAT [subcutaneous adipose tissue] in sedentary, young and moderately overweight men." (Reichenkendler. 2013)
Moreover, the use of DEXA scan and non-invasive FDG PET/CT + hyperinsulinemic, isoglycemic clamp tests by the means of which the researchers assessed the body composition and glucose uptake of the study participants before and after the 11-week intervention ensured that Reichenkendler et al. would get accurate results.
Figure 1: Change in skeletal muscle (left) and body fat (right) glucose uptake from pre- to post-interverion (Reichenkendler. 2013)
Speaking of results, in view of the fact that the participants had not been been engaged in  in regular exercise, before the study was conducted and considering the fact that their maximal oxygen consumption [VO2max] of ≤45 ml O2/kg body mass/min mirrors their sedentary lifestyles, it is actually not really surprising that the unaccustomed exercise led to decreases in abdominal subcutaneous, visceral and leg fat masses in the two intervention groups - what may be surprising, though is the fact that those were not statistically different in the medium vs. high volume group.

On the other hand, only the high, yet not the moderate volume exercise (600kcal/day) led to significant increases in fat free mass in the legs (with the difference between the moderate and high volume achieving borderline significance; p = 0.06).

1:0 for 600kcal/day and high volume training!?

The additional muscle building benefit from the high(er) volume in the study at hand appears to conflict the results of a previous study by Rosenkilde et al. On the other hand, the most important message still remains: Working out twice as much is not going to double your fat loss! If anything it will make you feel miserable. Remember that and spread the word!
The muscle building advantage of the high volume endurance exercise is interesting. After all it appears to contradict data from a previous study by Rosenkilde et al. who observed detrimental effects of doubling the exercise volume from 300kcal/day to 600kcal per day in their study from August 2012 (read more).
If we scrutinize the different protocols, this could be a result of an ostensibly small, but physiologically relevant difference in the exercise prescriptions:

While the subjects in the study at hand had the four low intensity days to recover, the Rosenkilde study did not deliberately implement intensity differences like that - the subjects were just told "you burn 600kcal/day - no matter what!" In view of the still prevalent notion that this would be most beneficial if you achieved it by "training in the zone" (the non-existent "fat burning zone"), it is not unlikely that for Rosenkilde's subjects every day ended up a "high" intensity day and the constant exercise induced stress ended up backfiring.

If we go by the body composition data this this was not necessarily the case in the study at hand.The increase in fatloss every noob believes would come out of simply doubling your cardio workouts was not present either and that despite the fact that the exercise induced total body glucose disposal rate was increased  only in the high volume group (p = 0.03). In the moderate intensity group this effect did nor reach statistical significance.
Figure 2: The overall greater GLUT-4 response to insulin (signifying improvements in insulin sensitivity) probably explains the advantage of the medium volume training (Reichenkendler. 2013)
Much contrary to the total body glucose intake, where both the liver and the adipose tissue are greatfully sucking up the glucose your muscles are not snatching from under their nose, the total skeletal muscle glucose uptake rate increased in both, the moderate (p = 0.007) and high (p = 0.002) volume groups.

And now for the real surprises...

Did you know that high intensity, muscle damaging workouts reduce the post-exercise glycogen repletion rate and could thus impair the general glucose repartioning effects of exercise? It cannot be said for sure whether an increase in muscle damage was partly to blame for the lower glucose repartitioning effects in the medium vs. high volume groups in the study at hand, but the long-lasting (10 days) defect in glycogen resynthesis Kevin P. O'Really et al. observed in their 1987 study in response to 45min of eccentric cycling should remind you that "go heavy of go home" does not imply that you go home only, when your muscles hurt so much that even going light is not possible any longer.
Much contrary to what you may expect, though, the increase in muscle specific glucose uptake was more pronounced in the moderate (difference to control p = 0.02) than in the high volume group, where only a trend (p = 0.06) was observed.

And while the total glucose uptake rate of femoral adipose tissue did not change significantly in either of the intervention groups, the amount of glucose that ended up in the abdominal subcutaneous fat stores decreased. Just like the muscle specific glucose repartitioning, this effect did yet occur only in the moderate, yet not in the high volume groups.

Similarly the glucose repartitioning away the visceral adipose tissue of the midsection was statistically significantly only for the moderate volume group. For the subjects in the high volume group, the researchers observed a non-significant tendency (p = 0.09) for a decrease in the amount of glucose that was taken up by the visceral fat depots in the abdominal region.

What do we make of these counter-intuitive results: First of all, I would like to emphasize that any form of exercise is going to have a measurable repartitioning effect and whether the latter is "statistically significant" or not may in the end be of secondary importance. Moreover, the overall greater energy expenditure in the high volume regimen resulted in greater improvements in body composition than its low volume counterpart - irrespective of its "non-significant" or "borderline significant" glucose repartitioning effects (so much about "calories don't count", and "it's all about insulin and the fattening carbohydrates" ;-)

These abs were not sculpted by 600kcal/day workouts check out some more promising routines
On the other hand, the results of the study at hand to eventually confirm what the previously cited study by Rosenkilde et al. (click on the image with the mouse) already suggested: Doing more is not necessarily beneficial. Even in the presence of what you may call "light intensity recovery days", the daily hour (on the light days you will need way more than an hour to burn 600kcal) of the ever same, non-challenging exercise is not going to cut it - in the literal sense; or, to put it differently: doing low intensity training just to burn calories everyday is not yield the fat burning, muscle building or maintaining results you are looking for.

If you are looking for better alternatives, check out the fatloss support workouts in the Step By Step Guide to Your Own Workout.

  • O'Reilly KP, Warhol MJ, Fielding RA, Frontera WR, Meredith CN, Evans WJ. Eccentric exercise-induced muscle damage impairs muscle glycogen repletion. J Appl Physiol. 1987 Jul;63(1):252-6.
  • Reichkendler MH, Auerbach P, Rosenkilde M, Christensen AN, Holm S, Petersen MB, Lagerberg A, Larsson HB, Rostrup E, Mosbech TH, Sj√∂din A, Kjaer A, Ploug T, Hoejgaard L, Stallknecht BM. Exercise training favors increased insulin-stimulated glucose uptake in skeletal muscle in contrast to adipose tissue: A randomized study using FDG PET imaging. Am J Physiol Endocrinol Metab. 2013 Jun 25. [Epub ahead of print]