Burning Calories or Workout Intensity? Which is more important for Fat Loss?

In the many conversations I have with people concerning fat loss and exercise, one question that is frequently asked of me is "What is the best exercise to burn the most calories?" I suppose this is intended to justify countless hours spent doing so-called "cardio"-type activities in the pursuit of losing weight.

However, the real question should be "How do I best mobilize the fat stores for energy?"
A colleague, Dr. Doug McGuff, MD informed me of a couple of recently published research articles that provide compelling answers to these questions. The first article is entitled"Effect of Exercise Training Intensity on Abdominal Visceral Fat and Body Composition"published in Medicine & Science in Sports and Exercise in October of 2008.

Dr. McGuff writes:

"This article looked at twenty-seven middle-aged (51+/- 9 yrs) obese women with the metabolic syndrome comparing either no intervention, low intensity exercise below the lactate threshold, or high intensity exercise above the lactate threshold. High intensity exercise is characterized by producing pyruvate from anaerobic metabolism more quickly than the mitochondria can use it. Pyruvate stacks up, and then gets acted on by lactate dehydrogenase producing lactic acid. When this occurs, you are above the lactate threshold.

The really cool thing this article did is that it adjusted the workout times so that the caloric expenditure was identical in both exercise groups. Exercise time was adjusted so that 400kcal were expended per session. This clearly would show that any changes could not be attributed to burning more calories in a particular session. Actually, the calories burned would favor the low intensity group since they performed more 400kcal sessions per week.

The subjects' body composition was evaluated using single slice CT scan at the L4-L5 disc space and mid-thigh to determine abdominal fat and thigh muscle cross-sectional areas. Percent body fat was assessed by air displacement plethysmography (a Bod Pod).

The results were very clear. The high intensity group significantly reduced total abdominal fat, abdominal subcutaneous fat, and abdominal visceral fat. There were no changes observed in the non-exercise group or the low intensity group. The part not mentioned in the study is the most important part. THIS IS VERY IMPORTANT: THE RESULTS ARE CORRELATED WITH INTENSITY, NOT CALORIES BURNED. Each exercise session was controlled to expend the same number of calories, AND based on total calories burned, the low intensity group had the advantage since they worked out more days per week. Despite this advantage, the low intensity group did not lose fat. This is a key concept: when it comes to losing body fat you cannot achieve it by "burning calories"in an attempt to influence the calories in/calories out equation. Fat will be mobilized when you use an exercise modality that mobilizes glycogen out of the muscle, creating a need for replenishment via the insulin receptors. As insulin sensitivity improves, serum insulin levels drop and body fat can be mobilized. As this scenario is repeated, triglycerides can be tapped from fat cells to supply energy for high intensity exertion, compounding the fat loss effect.

A similar study was conducted in 2004. "The Effects of Exercise Training on Abdominal Visceral Fat, Body Composition, and Indicators of the Metabolic Syndrome in Postmenopausal Women with and without Estrogen Replacement Therapy: the HERITAGE Family Study". Green, JS et. al. Metabolism. 2004 Sep;53(9):1192-6. This study used the same methods for evaluating body composition as the study above. However, the variable studied here is the presence or absence of hormone replacement. There were no significant changes in body composition in either group. The problem with this study is that a low intensity exercise routine was used. Since hormone replacement was the variable studied, it may have been incorrect to conclude it had no value, because it was evaluated in the context of low intensity exercise. If the exercise protocol had been of high enough intensity to actually stimulate results, then perhaps the magnitude of the results would have actually been different with hormone replacement. This illustrates how a study can go awry when it evaluates a given independent variable while using the wrong dependent variables as a backdrop."

It is clear that burning calories in the pursuit of fat loss is a very inefficient and rather ineffective method to lose weight. Much more effective in mobilizing fat is the intensity of effort.

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