Jade Teta ND, CSCS

The status quo in any field will usually be followed to the bitter end, despite better methods and enhanced protocols for success.  the fitness industry is no different.  When it comes to cardiovascular exercise there is the old way (the status quo), long-duration lower-intensity exercise (walking, jogging, biking), and there is the new way, high-intensity short-duration exercise (sprint training and interval exercise).  The truth is both work and neither is super effective without a significant change in diet.  We have said on this blog before, that diet is the most important component in body change. But, the question is which works better to burn fat, higher intensity cardio. or lower intensity? The more nuanced answer would be whichever one you will do frequently enough and consistent enough to see results, but the scientifically correct response is higher intensity cardiovascular exercise. Before, you throw your hands up and revert back to the old way of doing things, you may want to read on because if you still believe you burn more fat with lower intensity exercise, then you have not been paying much attention to exercise research or simply would prefer to remain with what is comfortable rather than trying a new approach that works better.

The Myth of Fat Burning Percentages

A old-schooled personal trainer (one who has stopped reading research for the last 10 years) will tell you that lower intensity cardiovascular exercise like walking for 2 hours daily burns more fat.  They will argue this is because at these lower intensities a higher proportion of fat is used for energy as opposed to carbohydrate or sugar. The key word here is “proportion”.  While it IS true a higher proportion of fat compared to carb. is used in low intensity exercise, higher “total” amounts of both fat and carb. are used in higher intensity exercise. This is usually a point of confusion for many uneducated exercise enthusiasts and may be a sign your trainer is either confused, or just does not know what they are talking about.  Lets hope it is the former. The key terms here are “proportion” vs. “total” amounts of fat.  You may burn a higher porportion of fat doing an activity, but that does not mean you burned more total fat.  Here is an example to explain.  Lets say two people go out and exercise for 40 minutes.  Person A completes a 40 minute brisk walk on a treadmill while Person B goes out and does interval exercises on a track by running the straight away and walking the turns. Person A is exercising at an average intensity of 65% max heart rate (MHR) while Person B spikes above 85% max heart rate (MHR) during the runs and then lowers the intensity during the walks.  We know that person A is burning a higher proportion of fat for energy compared to person B, because the intensity is lower (this is where your trainer is still right).  Lets say this proportion is 60% fat and 40% carbohydrate.  Person B burns a lower proportion of fat compared to carbohydrate of say, 50% fat and 50% carbohydrate.  Now we need to know how many calories each person burned.  They both exercised for the same amount of time, but person B worked out at a greater intensity so had to have burned more calories. Lets use me as an example.  I usually burn about 100 to 200 more calories doing intervals vs. steady state cardio exercise.  So, lets say person B burned 150 calories more during the workout (450 calories for person A and 600 calories for person B).  Now lets calculate the fat burned by each person.  Person A burned 450 calories 60% of which were fat, so 450 X .60 = 270 calories of fat burned.  Person B burned 600 calories of which only 50% were fat, so 600 x .50 = 300 calories of fat (this is where your trainer is wrong).  Who burned more fat?  Person B burned 30 more calories in fat than person A.  Despite person A burning a higher proportion of fat, person B burned more total fat because they did more work.  This demonstrates a common misunderstanding among personal trainers and health care providers.  Low intensity cardio does NOT burn more fat, high intensity exercise does.

The Afterburn

However, the advantage for high intensity exercise does not stop there.  Assuming the intensity of the workout person B did was intense enough, more fat will continue to be burned by them after the workout as well. This is called EPOC, excess post-exercise oxygen consumption, and it refers to the amount of fat and calories burned after a workout is over.  The EPOC of a workout is determined largely by its intensity.  Low intensity workouts have a very small EPOC while higher intensity workouts have a larger EPOC.  Researchers argue the magnitude of this effect, but it is not disputed that higher intensity cardiovascular activity will generate a larger EPOC effect. This is true even when the calorie consumption of a workout is equal or one workout is shorter than the other. A 2001 study in the journal, Medicine and Science in Sports and Exercise looked at standard “aerobic zone” training verse intense interval exercise (alternating extremely high levels of exertion with less intense bouts). The interval group did 2 minutes at 97% max heart rate then recovered for three minutes of low intensity activity. The “aerobic zone” group did 70% max heart rate exercise for the entire exercise session. Both groups used exactly 300 calories during their respective sessions. At the end of the study, the aerobic group lost less body fat compared to the interval group, despite exercising for slightly longer (1).

Another study from the same journal in 1996 also showed intense exercise burned more fat than low intensity cardiovascular exercise. In this study, the interval group burned more fat during exercise, but they also exhibited increased fat burning effects that persisted for 24 hours after the exercise had ended.  The most interesting thing about this study is that the interval group accomplished this with a workout that was a full 15 minutes shorter than the low intensity session (2).

The most telling study on the effects of high verse low intensity cardiovascular exercise was published in 1994 in the journal Metabolism. The researchers compared 15 weeks of interval exercise with 20 weeks traditional aerobics. The results showed that the aerobic group burned 48% more calories than the interval group (120.4 MJ vs 57.9MJ) over the course of the study.  If the caloric model is true, then we would expect that the aerobic group far exceeded the interval group in fat loss, but that was not the case. Instead, the interval group enjoyed a 9-fold greater fat loss.  If you read this study, you will see that neither group lost a ton of weight, but the weight the higher intensity group lost was almost all fat, while the lower intensity cardio group burned a large amount of valuable muscle in addition to fat. The other interesting thing about this study was that after the study was over researchers measured resting fat usage in both groups by evaluating a key fat burning enzyme called HADH (3-hydroxyacyl coenzyme A dehydrogenase (HADH).  They found this fat-burning enzyme was significantly elevated in the interval group but not the aerobic group. The impact of this study is significant especially when you consider the interval group trained 5 weeks less than the aerobic group and had shorter workouts. The measure of resting fat-burning at the conclusion of the study also gives a hint as to why those using low intensity cardiovascular exercise for weight loss often suffer from rebound weight gain after reverting back to their old dietary habits (3).

Heat Effect

Finally, there is one more advantage to higher intensity exercise verse lower intensity cardiovascular training and that is heat.  Unfortunately, exercise researchers have a difficult time measuring energy consumption during high intensity exercise.  They are unable to take a direct measure and must rely on EPOC to calculate the correct energy usage.  However, EPOC is based on oxygen consumption and researchers have shown much of the energy during exercise is lost as heat and cannot be measured via oxygen consumption.  As a matter of fact, research has shown that this “heat effect” accounts for 40% of the energy used in high intensity cardiovascular exercise. In comparison, the impact is only 10% in lower intensity exercise.  This means that high intensity exercise will use 30% more energy in the form of heat than lower intensity exercise (4).  This fact is completely ignored in calculations of calories burned and EPOC considerations. This is a hugely significant amount of energy that undoubtedly accounts for the much greater impact practitioners of high intensity activity see in terms of results.

Final Thoughts

Ultimately, exercisers should do what works for them and what they enjoy. Much of success in fitness is measured by consistent adherence to the priciples one believes in.  Some people hate the thought of going all out for 20 minutes in an interval sprint workout, while others would rather be stabbed in the eye than go jog for 90 minutes.  The important thing is to understand what the actual science says and why one strategy may work over another.  With lack of time being the number one reason many do not exercise, high-intensity short-duration cardiovascular exercise may not only be more effective, but more doable as well.

References:

1) King, J., Panton, L., Broeder, C., Browder, K., Quindry, J., & Rhea, L. (2001). A comparison of high intensity vs. low intensity exercise on body composition in overweight women. Medicine and Science in Sports and Exercise, 33, A2421.
2) Treuth, M.S., Hunter, G.R., & Williams, M. (1996). Effects of exercise intensity on 24-h energy expenditure and substrate oxidation. Medicine and Science in Sports and Exercise, 28, 1138-1143

3) Tremblay, A., Simoneau, J.A., & Bouchard, C. (1994). Impact of exercise intensity on body fatness and skeletal muscle metabolism. Metabolism, 43, 814-818

4) Scott, Christopher. Misconceptions about Aerobic and Anaerobic Energy Expenditure  Journal of the International Society of Sports Nutrition. 2005;2(2):32-37.