June 16, 2021 6 min read

The prevalence of overweight persons and obesity in the United States has been rising exponentially in recent years. This increase has been accompanied by a corresponding increase in chronic diseases such as coronary heart disease, diabetes, hypertension, osteoarthritis, and gall bladder disease (1).

And dietary changes along with increased physical activity are recommended in order to slow or reverse this trend. 

When people think of weight loss, sometimes it’s not intuitive that what they should really focus on losing body fat and maintaining or increasing lean mass. 

As the primary objective is loss of body fat; exercise that optimizes metabolism of fat has been prescribed.

In addition to weight control, improvement in fat oxidation has been associated with improvement in insulin sensitivity (2), which may be another mechanism by which exercise reduces the risk of diabetes.

One only needs to go to an exercise equipment store to see guidelines for the “fat burning” zone on treadmills, cycle ergometers, rowers, ellipses, etc.  

But is there really an ideal zone that one should exercise within in order to shed optimal body fat?

A little background on macronutrient metabolism 

The respiratory exchange ratio (RER) is the ratio of carbon dioxide (CO2) produced to oxygen (O2) consumed and allows for the precise determination of percent carbohydrate and fat metabolized under both resting and exercise conditions.

For carbohydrate, the ratio of CO2 and O2 is 6 to 6, producing an RER of 1.0. For fat, the ratio of CO2 and O2 is 16 to 23, producing an RER of 0.70. 

During incremental exercise, lipolysis increases as a result of both the release of free fatty acids from adipocytes and the metabolism of intramuscular triglyceride stores (3).

Here’s an important point to remember: 

As our intensity progresses from light to moderate, the percentage of calories we metabolize as fat declines. However, total fat calories increase because the decline in percent fat calories is countered by an increase in total caloric expenditure. 

As intensity increases from moderate to severe, fat oxidation decreases because glycolytic flux, alterations in pH, redistribution of blood flow away from adipocytes (i.e., fat cells), and enzymatic reactions all have an inhibitory effect on mobilization and metabolism of fat.

The relative decline in fat metabolism coincides with an increase in carbohydrate metabolism to meet the caloric demands of exercise. At RER equal to and greater than 1.0, carbohydrate is supplying 100% of the energy demand.

Some things to consider concerning fat oxidation…

Several factors are known to moderate the relative contributions of fat and carbohydrate to total oxidation, including training status, diet, and sex.

Both longitudinal (4) and cross-sectional studies (5) have demonstrated greater fat oxidation at both the same absolute and relative exercise intensities for trained compared with untrained subjects.

However, it has also been reported that only low-intensity, and not high-intensity, exercise will enhance fat oxidation (6). Consuming carbohydrate before exercise will facilitate carbohydrate oxidation and suppress fat oxidation (7).

In addition, chronic dietary manipulation, such as adoption of a high-fat diet, will favor the oxidation of fat under both rest and exercise conditions.

Finally, for any given absolute and relative exercise intensity, females have higher fat oxidation rates than males (8).

Given that fat oxidation is a desirable objective...

The inevitable question becomes “Should I exercise at an intensity level that optimizes fat oxidation, or is total caloric expenditure the ultimate determinant of fat loss?”

Surprisingly, this fundamental question has not been clearly answered to date, most likely due to the difficulty of precisely controlling caloric intake and expenditure.

Those studies that have been completed generally have controlled for exercise dose, comparing high-intensity, short-duration exercise with low-intensity, long-duration exercise of equivalent caloric expenditure. However, non-exercise physical activity and caloric intake were not controlled, and no definitive conclusion could be reached.

Another prominent question is: Can you enhance aerobic capacity and optimize fat burning at the same time or are the two distinctly different, thus requiring different intensities of training?  

To answer these questions, lets look at a recent, landmark study that compared the fat burning zone with the aerobic zone in competitive endurance athletes (9).

For this study, the heart rate for aerobic zone was 121-156 beats per min (bpm) and for the fat burning zone was 105-136 bpm.

These heart rates corresponded to percent max heart rate of 68-87% for aerobic zone and 59-76% for fat burning zone. 

Below are major findings from this research:

  • There is considerable overlap of the 2 zones when measured both by percent of max heart rate and caloric expenditure. The biggest discrepancy between the 2 zones occurs when comparing fat calories expended at the upper limit of the fat burning zone with the upper limit of aerobic zone. There are considerably less fat calories expended at the upper limit of the aerobic zone compared to the upper limit of the fat burning zone. If the objective is metabolism of fat calories, training at the upper limits of aerobic zone should not be recommended. If total caloric expenditure is the objective, the upper limits of aerobic zone will be the most efficient.
  • In addition to more calories being expended during exercise, caloric expenditure during recovery from high-intensity exercise is greater than recovery caloric expenditure from low-intensity exercise because of the additional energy requirement of ventilation, restoration of adenosine triphosphate phosphocreatine, replenishment of glycogen stores, and body temperature elevation.
  • Also, prolonged exercise at high intensity, as in marathon running, has shown a gradual decrease in carbohydrate oxidation and gradual increase in fat oxidation as glycogen stores become depleted. If fat calories, and not total calories, were a better predictor of weight control, we would expect endurance athletes, who spend a rather large volume of training above the fat burning zone, to have weight control problems. This is clearly not the case.
  • The upper, but not lower limits of the fat burning zone attain guidelines established by the American College of Sports Medicine for training intensity required to enhance aerobic fitness. Individuals may choose to train in the fat burning zone and still obtain improvement in aerobic fitness and VO2max, provided they are in upper end of fat burning zone. 

 

The take home message 

Although a higher percentage of calories come from fat during lower intensity training; more total fat calories are metabolized from higher intensity exercise.

Another problem with lower intensity training is that you will only burn these fat calories for the duration of that particular exercise session with very little expenditure after the exercise ends.

There is a thing called excess post exercise oxygen consumption (EPOC) that determines the extent of calories metabolized after exercise has ended. 

EPOC is much longer following high intensity exercise compared to lower intensity exercise. Research in this area clearly shows higher intensity exercise continues to burn more calories hours after the exercise period has ended, so total fat oxidation will be greater during the recovery period.

Essentially, the bottom line is you’ll get more bang for your buck with higher intensity compared to lower intensity training and be able to optimize aerobic fitness and fat burning potential.

 

 

 

References: 

1.Must, A., Spadano, J., Coakley, E. H., Field, A. E., Colditz, G., and Dietz, W. H. (1999) The disease burden associated with overweight and obesity. JAMA 282, 1523-1529

2.Goodpaster, B. H., Katsiaras, A., and Kelley, D. E. (2003) Enhanced fat oxidation through physical activity is associated with improvements in insulin sensitivity in obesity. Diabetes 52, 2191-2197

3.van Loon, L. J., Greenhaff, P. L., Constantin-Teodosiu, D., Saris, W. H., and Wagenmakers, A. J. (2001) The effects of increasing exercise intensity on muscle fuel utilisation in humans. J Physiol 536, 295-304

4.Friedlander, A. L., Jacobs, K. A., Fattor, J. A., Horning, M. A., Hagobian, T. A., Bauer, T. A., Wolfel, E. E., and Brooks, G. A. (2007) Contributions of working muscle to whole body lipid metabolism are altered by exercise intensity and training. Am J Physiol Endocrinol Metab 292, E107-116

5.Bircher, S., and Knechtle, B. (2004) Relationship between Fat Oxidation and Lactate Threshold in Athletes and Obese Women and Men. J Sports Sci Med 3, 174-181

6.van Aggel-Leijssen, D. P., Saris, W. H., Wagenmakers, A. J., Senden, J. M., and van Baak, M. A. (2002) Effect of exercise training at different intensities on fat metabolism of obese men. J Appl Physiol (1985) 92, 1300-1309

7.Achten, J., and Jeukendrup, A. E. (2003) The effect of pre-exercise carbohydrate feedings on the intensity that elicits maximal fat oxidation. J Sports Sci 21, 1017-1024

8.Venables, M. C., Achten, J., and Jeukendrup, A. E. (2005) Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. J Appl Physiol (1985) 98, 160-167

9.Carey, D. G. (2009) Quantifying differences in the "fat burning" zone and the aerobic zone: implications for training. J Strength Cond Res 23, 2090-2095

Dr. Paul Henning

About Dr. Paul

I'm currently an Army officer on active duty with over 15 years of experience and also run my own health and wellness business. The majority of my career in the military has focused on enhancing Warfighter health and performance. I am passionate about helping people enhance all aspects of their lives through health and wellness. Learn more about me

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