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Fat Burning Zone?

I know we always say, “there is no such thing as the fat burning zone.” This was an interesting study that was recently published and I thought I would share it with those who read my blog:

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

Michelle C. Venables, Asker E. Jeukendrup, Med Sci Sports Exerc. 2008 Mar;40(3):495-502.

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, 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−1). Variables were compared using two-way repeated-measures analyses of variance.

It was shown that fat oxidation rates were increased by 44% after TPCON (0.24 ± 0.01 vs 0.35 ± 0.03 g·min−1, 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 .

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.

Pretty interesting study; I will say a few things:

1) They weren’t looking at fat loss in this study. They were looking to see which protocol (the interval method or the steady state method) elicited the greatest amount of fat oxidation and the greatest increase in insulin sensitivity.
2) The subjects were not athletes by any stretch of the imagination. Rather, they were obese males (who had no major health issues; other than the fact that they were obese). So, this may not apply to your athletic clients. However, those of you who are working with over-weight/obese individuals who need to improve overall health may be able to extract some useful information from this study with regard to programming their cardiovascular exercise (especially at the beginning of an exercise program, when they are very de-conditioned and unable to tolerate high amounts of work).
3) The intensity for the interval-training group was hardly what you would classify as “intense,” 65% of Vo2max. Where as other studies showing greater benefits in the high intensity interval-training group used much higher intensities. Again, this probably will not apply to your athletes.

So what can we take from this study:

1) According to the study, there may very well be a fat burning zone. It would be hard however, to take this study and apply it to healthy athletic people.
2) While they weren’t looking at fat loss in this study (which has been looked at in other studies comparing interval training and steady state cardio), they were looking at markers of overall health (fat oxidation during exercise and improvements in insulin sensitivity).
3) The study showed that the steady state group showed significantly greater improvements than the interval group in all of the tests in question. So, there is some benefit of regular cardiovascular work, despite what the “just do HIIT and don’t ever do steady state because it will make you fat and make you lose muscles and decrease testosterone” camp says.

How does steady state cardio fit into our training programs? That is the million-dollar question, and I will get to that this week.