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Endurance Training and Obesity: Effect on Substrate Metabolism and Insulin Sensitivity.

Endurance training and obesity: effect on substrate metabolism and insulin sensitivity

PURPOSE:: Obesity and type 2 diabetes mellitus are disease states associated with hallmark features such as insulin resistance and an impaired ability to oxidize lipids. It has recently been reported that an optimal exercise intensity for fat oxidation (FATmax) exists; we hypothesize that continuous exercise training at this specific intensity can lead to greater improvements in fat oxidation and insulin sensitivity than a eucaloric interval training program. METHODS:: In a counterbalanced, crossover design, eight sedentary, obese, but otherwise healthy male participants performed two 4-wk blocks of endurance training, either at a predetermined intensity eliciting maximal fat oxidation (TPCON) or at 5-min intervals of +/- 20% FATmax (TPINT). During the week preceding the exercise training and 48 h after the final exercise bout, an OGTT, V O2max test, steady-state exercise, and measurements of body composition were undertaken. Diet was controlled the day before all trials (50% carbohydrate, 35% fat, and 15% protein; ~2900 kcal.d). Variables were compared using two-way repeated-measures analyses of variance. RESULTS:: It was shown that fat oxidation rates were increased by 44% after TPCON (0.24 +/- 0.01 vs 0.35 +/- 0.03 g.min, P < 0.05) but not after TPINT, and the whole-body insulin sensitivity index was increased by 27% after TPCON (P < 0.05). These changes occurred despite no change in body weight, body mass index (BMI), waist to hip ratio (WHR), percent body fat (%BF), or V O2max. CONCLUSIONS:: A continuous exercise training protocol that can elicit high rates of fat oxidation increases the contribution of fat to substrate oxidation during exercise and can significantly increase insulin sensitivity compared with a eucaloric interval protocol.

My Comments:

After putting up Steady State vs. Interval Training: An Introduction, I thought this would be an interesting study to examine (before really delving into the topic) since most of what tends to be written is along the lines of intervals are always superior to steady state cardio.

Now, let me make a couple of things clear.

First, I am in no-way anti-interval training, although I expect those with poor reading comprehension may ‘hear’ what I’m saying that way. Rather I’m against the uncritically applied idea that somehow intervals are ALWAYS superior or that steady state is somehow either useless (or, as some claim, detrimental).

Second, this study was not looking at fat loss per se, so some might want to dismiss it out of hand in terms of its relevance to the debate. Maybe, maybe not. Improving health (and insulin sensitivity is one marker of health) is just as important as getting lean, and that is what this paper looked at.

As a bit of introduction, it’s recently come to light that one major cause of insulin resistance in obesity has to do with the accumulation of fat within skeletal muscle. Referred to as intramuscular triglyceride (IMTG), fat stored within skeletal muscle appears to play a big role in how well (or poorly) the muscle can utilize glucose and respond to insulin. I’d note that it’s a touch more complicated than that for reasons I don’t want to get into.

Oddly, studies of endurance athletes usually find not only increased IMTG but improved insulin sensitivity, throwing something of a wrench into the whole idea. Various ideas have been thrown around to try to explain this but, among other things, it appears that there is a defect in the mitochondria in skeletal muscle of obese people that appears to contribute to the IMTG-related insulin resistance; whether this defect causes or is caused by obesity is currently being debated. Of more relevance is the question of whether or not it can be ‘fixed’ with proper training (e.g. to improve mitochondrial functioning).

The point being that the difference between fat people with large amounts of IMTG and trained endurance athletes with high levels of IMTG is probably related to the improved mitochondrial functioning in the endurance guys.

Now, it’s well established that regular exercise improves insulin sensitivity although previous studies have been split as to whether the effect is a function of intensity or volume. Some work says that only high intensity works, other suggests that low-intensity works, I’ve seen some work that it depends on the number of calories burned, one study suggests that it’s the duration of training.

But given the major role of IMTG in insulin resistance, the current paper set out to see if endurance training at an intensity that caused maximal fat oxidation (e.g. ‘burned’ the most fat during activity) would improve insulin sensitivity more than an interval program that was set up to burn an identical number of calories.

Eight obese males with no major health issues were recruited and followed the same training program. After a three day break-in including an exercise test to determine the exercise intensity where they burned the most fat (called FatMax) and a test of insulin sensitivity they all did the continuous program
for 4 weeks, followed by another set of tests, followed by a 6 week washout period. Then another 3 days of testing, 4 weeks of the interval training program and then another 3 days of testing. A variety of measures were taken including an oral glucose tolerance test, body composition, etc. The diet was set to avoid weight loss on either exercise program. This is important because weight loss by itself tends to improve insulin sensitivity so often the effects of the exercise program are mediated by weight loss.

The continuous exercise program consisted of 5 days/week at the intensity shown to elicit FatMax, starting at 30 minutes and increasing by 10′ per week to a maximum of 60′ by week 4. It turned out that this occurred at an intensity of roughly 45% VO2 max which corresponds to about 60% of maximum heart rate.

The interval program consisted of alternating periods of 5 minutes either 20% above or 20% below the intensity of FatMax, again starting at 30 total minute and increasing by 10 minutes to an hour by week 4. Five days per week. So 5 minutes were done at 65% VO2 max or 75% max heart rate or so alternated with 5 minutes at 25% Vo2 max (very low % max heart rate).

Body composition changed in neither group and the average energy expenditure for the workouts were the same.

Of some interest, only the continuous group showed an increase in fat oxidation during exercise, the interval group did not. This meant, to quote the researchers “…the reduction in RER during exercise meant that there was significant increase in the fat oxidation rate during exercise. Rates of fat oxidation increased by 44% after [continuous training] but no change was observed after [interval training].”

Tangentially and with a tone of snarkiness, I’d point out that this pretty much flies in the face of most of the ‘Aerobic exercise is bad because you adapt to it’ arguments. Yeah, they adapted….by burning more fat for fuel with no reduction in caloric expenditure. How awful.

Of more relevance to this paper, since fat oxidation during exercise may or may not relevant to fat loss anyhow, only the continuous group showed increases in insulin sensitivity and this was correlated with the increases in fat oxidation. Given that one adaptation to chronic endurance training is to use more IMTG, and given that only the continuous group increased fat oxidation, this makes sense.

Getting into the discussion, the researchers address some of the observations, especially as it pertains to earlier work. Most of this I addressed above having to do with different study results and intensity, volume and such.

One issue I do want to address is the intensity and duration of the interval group’s training since I suspect this is where most of the pro-interval crowd will find a ‘flaw’ in the study. It’s true that 5 minutes of interval work at roughly 75% of max heart rate is not a tremendously high intensity.

The researchers specifically mention one interval study (by Talanian) which used an interval workout of 10X4 minutes at 90% VO2 max (this is pretty close to maximum heart rate) and found an increase in insulin sensitivity, possibly due to the higher total energy expenditure. The researchers only noted that “..the intensity used by Talanian et .al. would far exceed the tolerance of the obese sedentary male.” True that, no sedentary individual would handle such a workload.

And, yes, I realize that many of the popular fat loss interval programs use much shorter bouts which are more likely to be done. Of course a short interval program of 5X30 seconds hard/30 seconds easy wouldn’t come close to burning as many calories as the study of Talanian (or this one) either.


Summing Up

This study compared improvements in fat oxidation and insulin sensitivity for obese males either performing 30-60 minutes of steady state cardio (at an intensity set to elicit maximal fat oxidation) or an equal calorically burning interval session. Steady state exercise was superior in all parameters. Let’s see if the folks claiming to use ‘science’ to support interval training cite this one.

In the next part of the Interval vs. Steady State series, I get a little bit silly in Pole Vaulting for a Hot Body.

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