True or False? Muscle Damage From Workouts Can Impair Skeletal Muscle Glucose Uptake and Insulin Sensitivity

I have to admit, I cannot answer the question about the effects of muscle damage on glycemia once and for all, but for the average workout, it's probably irrelevant and the overall effect beneficial.
While I have to admit that I do not know in which of the roughly 2300 published articles I have referred to the studies by Asp et al. (1995), I do know that a new study from the Tatung Institute of Technology and the Department of Sports Sciences at the University of Taipei (Ho 2016) shows that the experimentally verified reduction in GLUT-4 expression in isolated human skeletal muscle cells probably lacks practical relevance.

The reason that I write "probably", is that we do now have one study that suggests beneficial (Ho 2016) and one that found detrimental effects (Asp 1995) - and a couple of reasons why neither of the studies is fully convincing.
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We will get to this issue and the "probably" in the bottom line. Before we do so, however, let's briefly take a closer look the study design. The purpose of the study was to determine the magnitude of muscle damage and blood glucose responses during an oral glucose tolerance test (OGTT) after eccentric training in healthy junior athletes (16.3 ± 0.5 years of age) with no history of musculoskeletal disorders who were instructed to perform ...
  • 5 sets of barbell front squat for 20 repetitions at 30 pounds a day and 
  • 100-meter downhill sprinting consisted of 5 repetitions...
... for 3 consecutive days, each. Glucose tolerance (by the means of an oral glucose tolerance test) and creatine kinase levels (CK-MB = skeletal muscle specific) were measured on day 4 and 5, i.e. one and two days into the three-day recovery phase.
Figure 1: Plasma levels of muscle creatine kinase (Ho 2016); as a SuppVersity reader you shouldn't be surprised to see "high-" and "low-responders" as discussed in a 2014 article on CK elevations w/ exercise.
The exercise challenge resulted in what the scientists call a "wide spectrum of muscle creatine kinase (CK) surges in plasma" (Ho 2016), 48 h after the last bout of exercise (see Figure 1), which is in line with the previously discussed genetic component of CK responses (learn more in "Rhabdo & Liver Failure or Just an Intense Leg-Workout?"). For the statistical analysis, the participants were then divided into two groups according to the magnitude of CK increases (low CK: +48% ± 0.3; high CK: +137% ± 0.5, P < 0.05).
Figure 2: Oral glucose tolerance test (OGTT), blood glucose levels and area under the curve - Pre: before exercise challenge; Post: ca. 24h after exercise challenge. *Significantly different against Pre, P < 0.05 (Ho 2016).
As you can see in Figure 2, both groups, showed comparable decreases in blood glucose levels in OGTT, suggesting that this muscle-damaging exercise does not appear to decrease but rather improve glycemic control in men.
So, it's 'hit it hard and don't care about muscle damage'? Not so fast... as previously pointed out, the study at hand shows that muscle damage has a less sign. effect on your muscles' ability to suck up glucose than the earlier studies by Asp et al. suggested. The reason(s), it does not fully refute the notion that muscle damage may promote temporary insulin resistance are...

Figure 3: While there was a sign. reduction in GLUT4 (top) and a correspondingly reduced glycogen resynthesis (bottom) in exercised (full circles) vs. control leg (open circles) in the Asp study, it's not likely that the subjects' suffered from physiologically relevant insulin resistance. 
(A) the, in my humble opinion, questionable time-frame within which the glucose tolerance test was conducted (24h post) - while it is true that it takes time for the CK levels to rise and testing them 48h after the last workout may be sensible, there's no logical reason not to test the effect on glucose uptake immediately after the workout;

(B) the nonreflective use of CK-MB as a marker of muscle damage that becomes obvious when the scientists divide their subjects into two groups - what the authors believe to be a division in subjects with high and low muscle damage is (as discussed previously) rather a division in genetic high- and low-responders (an analysis of the correlation between CK and glycemia, as you would expect it in view of the research question, is missing);

(C) the probably relatively low(er) degree of muscle damage which is, as one can speculate, significantly less pronounced than in the Asp study, in which the subjects did one-legged eccentric exercise on a motor-driven device and saw ~2-times greater mean elevations in CK

The good news, however, is that the effect on the GLUT4 expression and glycogen flux in the Asp study (Figure 3) looks more relevant than it actually is - the sign. difference between the pre- and the late muscle glycogen measurements also raise the question how accurate the latter was actually measured; after all, the fractional velocity of the glycolytic flux (not shown in Figure 3) changed only non-significantly from 34.1 ± 3.6% to a short-lived minimum of 29.8 ± 2.8% on day 1 after the muscle damaging leg workout in the Asp study. Eventually, it's thus unlikely that hard workouts will turn you into a type II diabetic (i.e. have health relevant effects on your glucose levels) - that harder is not necessarily better (for glucose management), however, is also something worth remembering, I guess | Comment on Facebook!
  • Asp, Sven, Jens R. Daugaard, and Erik A. Richter. "Eccentric exercise decreases glucose transporter GLUT4 protein in human skeletal muscle." The Journal of physiology 482.3 (1995): 705-712.
  • Ho, Chien-Te, Machiko Otaka, and Chia-Hua Kuo. "Improving glucose tolerance by muscle-damaging exercise." Journal of Traditional and Complementary Medicine (2016).
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