Exercise for Weight Loss: The Great Intensity Debate

Perhaps no more common or confusing exercise physiology concept exists — particularly when weight loss as a goal enters the equation — than that of Long Slow Distance (LSD) versus High-Intensity Training (HIT).

Some physiological background information…

A truly physically-fit athlete can do a century (100-mile bike ride) with brutal elevation changes using nothing for energy but their body’s stored glycogen or blood sugar. But after the ride, his tank would be empty — he would have used up all his blood sugar. The same athlete, even a real skinny one, has enough stored body fat to do a dozen of these rides back to back, without eating or drinking anything but water, if he could somehow go slow enough or be efficient enough to only burn his body fat for fuel.

There are several problems our fit athlete would have to overcome to burn only his stored body coat for fuel, especially if he is in a competitive race. One, the central nervous system requires sugar to operate so it will be competing with the muscles for an energy source, and two, the speed required in most racing situations is much quicker than the speed which our bodies can turn stored body fat into energy.

[Geek Note: While there are in fact many types of fat in the body, one type — triglycerides — is the one most commonly used for energy production. Triglycerides are retrieved from our stored body fat and from our diet. One triglyceride molecule will eventually give birth to 441 ATP molecules while a glucose (blood sugar) molecule yields only 38 ATP. So, if we can train our bodies to use stored fat instead of blood sugar we can generate over 10 times the energy with each turn of the Krebs Cycle.]

Exercise for Weight Loss

Virtually all normal weight loss comes about purely from creating a calorie deficit. This is a law of thermodynamics and cannot be changed as far as we know. So, when comparing two workouts of equal time, the one which burns more calories will, in fact, be the more efficacious for weight loss.

That being said, we have all heard about the “fat-burning zone”: This mystical place where the cosmic tumblers align and the fat just pours off your body. This idea was popularized because there are in fact intensity levels (e.g., 60-65% of Maximum Heart Rate-MHR) wherein the body is more likely to use stored body fat instead of serum glucose as a fuel source. So, the philosophy goes, if we train in this heart rate zone we will, in fact, burn more body fat.

This is quite true.

Our patients that are competitive endurance athletes (think Ironman and marathoners) spend a great deal of time training in this zone to hopefully improve their performance benchmarks while functioning inside this more efficient metabolic pathway.

For those who hope to lose weight and are exercising to do so, this same strategy may not be as productive. Let’s do a thought experiment:

John weighs 260 pounds. We will have John perform two 30-minute-workouts on a treadmill designed to increase and decrease the speed in a precise manner so as to maintain the subject’s heart-rate within a very narrowly defined range for the period of the study.

Workout A
Fat Burning:60-65% MHR
Workout B
High Intensity:80-85% MHR
Calories burned/minute 9.72 13.72
Fat Calories burned/minute 4.86 5.4
Total Calories burned 291.6 411.6
Total Fat calories burned 145.8 162
Percentage of calories burned as fat 50% 39.36%

As you can easily see, John would have to perform Workout A for 42.3 minutes to burn the same number of calories Workout B burns in 30 minutes. Said another way, Workout A is 41% less efficient from a calorie-burning perspective than Workout B!

Some might validly argue that at the Fat Burning intensity levels they might be able to work out for 90 minutes compared to them only being able to maintain the high-intensity workload for 30 minutes. If this is the case,

and you have the time, then, by all means, do the longer workout! Especially if you are interested in competing in endurance sports.

Bottom Line:

For the purposes of weight loss, and assuming you have no intention of training for an endurance sport, do the most work you can in the period of time you have allotted for your workout; do not worry about staying in the mystical “Fat Burning Zone”.

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