Have you ever wondered what type of work-out you should be doing to burn the most calories and deplete fat stores?

The answers lie in aerobic and anaerobic metabolic pathways needed to supply energy to working muscles during exercise.  Overall oxygen consumption during exercise, referred to as VO2, and metabolic processes that occur immediately following exercise play a large part in determining the effect on calorie burn.  Exercise that occurs in the anaerobic state is often what we think of as short sprints or any type of brief, intense work.  This is exercise which the body can perform without needing oxygen for the generation of energy.  Aerobic exercise occurs at a relatively lower intensity over a longer period of time and relies on oxygen to supply aerobic metabolic pathways.  Energy is generated through both processes, and the truth is we are using both systems of energy throughout most forms of exercise.

In addition to heart rate and breathing remaining elevated, the body is also working to restore oxygen to blood and tissues in the first few minutes following exercise.  During higher intensity levels of exercise, elevated body temperature and circulating substances (including epinephrine and lactate) require greater oxygen demands post-exercise for recovery.  Simply put, if you want to burn more calories over a longer period of time after exercising without having to do any additional work, high intensity exercise is the way to go!  This certainly supports performing high intensity interval training (aka HIIT) as an ideal workout.

Factors controlling “fuel” selection

Have you heard that low intensity exercise increases the percentage of calories from fat burned?  This is true, however total rate of fat oxidation is a much more important number to consider.  We look at percent of VO2 max to guide exercise intensities.  When exercising at 20% of VO2 max, about 60% of total energy expended would come from fat.   While exercising at  50% of VO2 max, only 40% of your fuel is supplied by fat.  However, by exercising at 50% of VO2 max you actually burn nearly twice as much fat over the same period of time.  When looking to burn the greatest amount of fat as energy, your goal is to reach about 50% of your VO2 max.  This is also approximately 60-65% of maximal heart rate or 120-130 beats per minute for a young adult.  Furthermore, when maintaining this intensity of exercise over a prolonged period of time (i.e. greater than 30 minutes), a gradual shift to greater reliance on fat as an energy source occurs.

How do you know what intensity of exercise correlates with your percent of VO2 max?

If you were on a treadmill and just barely able to respond to your neighbor next to you, then you are likely at or above 50% of your VO2 max.  Ventilation has to increase at this point to meet the work demands placed on your body.  Exercising just below this point is a good way to gauge approximately 50% of your VO2 max.  Alternatively, if you have a heart rate monitor, this is also typically about 60-65% of your max heart rate (220 – age = max heart rate).  It is important to keep in mind that an individual's fitness level and previous training will affect these HR numbers slightly.  Estimates are given in this article.

Hopefully some of this information is helpful in guiding your exercise intensity.  With that said, many other factors guide our choices for exercise, and that’s alright.  Continue to enjoy what you do!

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