What Exactly Is The Metabolic Effect Of Exercise?
by Jade Teta on April 5, 2012
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There has been an ongoing debate about what many in the health and fitness field call the “metabolic effect” or “metabolic after-burn”. The idea behind this concept is that exercise of sufficient intensity elevates the efficiency of fat burning for many hours after exercise. We have compiled more recent data to add to the debate. Based on this mounting evidence, it is becoming more clear that 1) traditional aerobic exercise is not very effective for fat loss, and 2) intensity of exercise may be the real key to body change.

Aerobic vs. Anaerobic exercise

Because it can still be a confusing topic, it is useful to define aerobic and anaerobic exercise. Stated simply, aerobic exercise is exercise below a certain intensity threshold while anaerobic exercise occurs at intensities above that threshold. The important distinction to make here is if you are “anaerobic” you are also aerobic. In fact, your aerobic capacity is maxed out. However, the reverse is not true. Aerobic exercise does not include a significant contribution of anaerobic metabolism.
More evidence against aerobic exercise

A March 2011 study in the International Journal of Obesity followed 300 women over the course of a year (1). The intervention was compared roughly 3 hours per week of moderate-intensity aerobic exercise with a control group who did no exercise. Both groups were asked to make no change in their dietary practices. The results showed the exercise group lost just over 5 pounds of fat while the control group achieved a loss of 1 pound of fat. 4 pounds of fat was all that separated the exercisers from the non-exercisers after 12 months of exercise. A bad result by anyone’s standards, and far from the body change people perceive they will derive from exercise.

Another study in April of 2011 looked at 399 obese and sedentary women (2). These subjects were randomized into a diet intervention, an exercise intervention, or a diet plus exercise group. They too were followed for a 12-month period. The exercise program was standard, moderate intensity, aerobic exercise is done 45 minutes 5 days per week. The results showed that the exercise plus diet program provided about a 4-pound fat loss advantage. If our math is right, that means the addition of close to 200 hours of aerobic exercise added to the diet resulted in just 4 pounds of fat loss! A disappointing result has given the time and effort.

These two studies add to review studies by Melanson et al in 2008 and Miller el at in 1997. These two studies showed the exact same results (3-4). The latter review looked at hundreds of studies spanning a quarter decade of research on aerobic exercise. The results were similar to those found above. Aerobic exercise provides an almost insignificant advantage to fat loss over diet by itself.

3 parts to the Metabolic Effect

Part of the reason aerobic exercise may be so ineffective is that it provides no real metabolic advantage beyond the calories burned during activity. Higher intensity exercise may have a unique advantage in this regard. Dr. Christopher Scott of the University of Southern Maine points out that to fully account for the effects of exercise three things should be considered: calories burned aerobically during exercise, calories burned aerobically after exercise (called EPOC), and anaerobic calories during exercise (5). While EPOC is often thought of as synonymous with the metabolic effect, all three of these factors and likely others provide a more complete explanation.

These mechanisms were parsed out in a 2005 study comparing a purely aerobic challenge with anaerobic sprints. 3.5-minutes of jogging was compared to three 15-second sprints (5). The aerobic calories burned during each exercise bout were 29 calories versus 4 calories for the jog and sprints respectively. When the EPOC component was added in, the totals went to 36 calories for the jog and 39 calories for the sprints. Finally, when the anaerobic contribution was added, the total changed dramatically. The numbers for the jog did not change, remaining at 39 calories. But the sprint totals took a huge jump to 65 calories demonstrating substantial benefits to energy use.
Intense exercise delivers

A February 2011 study by Knab et al. is the latest study to show intense exercise provides a caloric advantage and an after-burn (6). In this study vigorous cycling for 45 minutes elevated post-exercise metabolism for 14 hours. This equated to a 37% rise in resting metabolism and burned an additional 190 calories after the workout.

Another more recent study showed that resistance training can drastically enhance the metabolic after-burn for up to 72 hours (7). In this study, participants used a 10 rep max (a weight that could be lifted 10 times but not 11) on ten different exercises which they completed for either 1 set or 3 sets.

The metabolic rate was measured at baseline and then again at 24, 48, and 72 hours after the session. At 72 hours there was still a significant metabolic advantage from the weight training sessions. The interesting thing about this study is that the metabolic elevation occurred for both 1 set and 3 set protocols showing even very short bouts of exertion can induce significant metabolic perturbations.

Finally, Trapp et al. looked at a direct comparison between old school aerobic exercise and high-intensity sprint type exercise. The 15-week study was published in the International Journal of Obesity in 2008 (8). Forty-five women between 18 to 30 years old were divided into three groups: a high-intensity sprint group, a moderate-intensity aerobic group, and a no-exercise control.

The sprint group repeated 20 cycle sprints each lasting for 8s with a 12s rest in between. The aerobic group peddled in the aerobic zone for 40 minutes. The results showed a loss of 2.5 pounds of fat in the sprint group while the aerobic group showed a non-significant trend towards fat gain.

Comparing the results of this study to the year-long aerobic interventions above should make us stop and pause. Aerobic exercise was only able to deliver a 4 pound fat loss in 12 months while sprint training accomplishes 2.5 pounds of fat loss in just 15 weeks? And this with a workout that takes a third less time compared to most aerobic exercise interventions (20 minutes vs. 60 minutes).

Final thoughts:

Fitness is evolving, and debates will continue. These new studies provide more insight into the existence of the metabolic effect and what it means for fat loss and body change. While all exercise has benefits, it may be time to make a distinction between what is healthy exercise and what is fat loss exercise. It seems we are finding the two are not always the same.
Friedenreich, et al. Adiposity changes after a 1-year aerobic exercise intervention among postmenopausal women: a randomized controlled trial. International Journal of Obesity. March 2011;35(3):427-35.
Foster-Schubert, et al. Effect of Diet and Exercise, Alone or Combined, on Weight and Body Composition in Overweight-to-Obese Postmenopausal Women. April 2011. Published online ahead of Print.
Melanson, et. al. Exercise improves fat metabolism in muscle but does not increase 24-hr fat oxidation. Exercise and Sport Sciences Reviews. 2009;37(2):93-101.
Miller, et. al. A meta analysis of the past 25 years of weight loss research using diet, exercise or diet plus exercise intervention. International Journal of Obesity. 1997;21:941-947.
Scott, et. al. Misconceptions about aerobic and anaerobic energy expenditure. Journal of the International Society of Sports Nutrition. 2005;2:32-37.
Knab, et al. A 45-Minute Vigorous Exercise Bout Increases Metabolic Rate for 14 Hours. Medicine and Science in Sports and Exercise. 2011 Feb 8.
Heden, et al. One-set resistance training elevates energy expenditure for 72 h similar to three sets.
European Journal of Applied Physiology. Volume 111, Number 3, 477-484, Mar 2011
Trapp, et al. The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women International Journal of Obesity. 2008;32:684-691