Dr. Joe Klemczewski, "The Diet Doc" attempts to shed some light on the above question in this article, "The Right Prescription to Get Shredded.

First, let’s lock on the target: what is the actual goal in terms of physiology?  The vast majority of “fat” in our body (over 80%) is collected in one form and stored in body fat cells.  To get rid of it – using it as energy – is a process called lipolysis.  I’m going to briefly explain the mechanism of this process primarily through exercise (avoiding diet for now), and then the hard part begins: applying those “truths” to the many different circumstances and body types that we find in our great sport.

Activity-related hormones epinephrine and norepinephrine speed up lipolysis greatly.  In other words, when we start working out, we start shuttling out these glycerols and fatty acids (fat) from our body fat cells.  As a matter of fact, research shows that the greatest increase in fat usage starts immediately upon exercise, hits a peak level within 5 minutes, sharply decreases, by the 15-minute mark starts to plateau, and within 30 minutes is back almost to a rate matching a control group.  So, what do we do with the common mantra that carbohydrates (blood sugar) are used exclusively for the first 15-20 minutes of cardio?  In fact, blood sugar is the dominant source of energy used by the cells of our body during the onset of exercise, but in order to provide a seamless transition into longer-term energy stores, the liver (through the action of the hormone glucagon) starts pumping out stored glycogen and as described, body fat cells start releasing glycerols and fatty acids.  It all sounds good so far, but it gets a little tricky when the liver runs out of glycogen.  When this happens, the liver keeps trying to contribute to the energy deficit, but it does so by turning amino acids into glucose.  Of course the liver has a reservoir of aminos available, but still, this is a catabolic event and nothing sends a bodybuilder shrieking to the blender to pound down another protein shake like the thought of catabolism.  Interestingly, though, even during the harshest, longest bouts of exercise, only 3 to 6% of energy is consumed from amino acid use.  As long as fat is available, the body spares protein as if it were the most precious commodity it has.

Nice to know the brain agrees with us on that one.  But, as they say, the devil is in the details.  Before we even address things like duration, frequency, and intensity think about the parameters I just laid out. Even a small percentage of amino acid use can add up if it’s a repetitive occurrence.   I also mentioned “as long as body fat is available”…what happens when the supply is reaching a pretty slim margin?  This is where body type comes into play.  An ectomorph has to have a healthy fear of “too much” cardio since they will be at the higher end of the population in terms of using amino acids for energy.  But, those that lose very slowly need to understand that muscle preservation is their greatest asset – they need to embrace cardio as a very necessary part of their contest prep.

Here is where armchair interpreters of research often start showing that it takes more than throwing some big words around to master the subject.  The rate of lipolysis is virtually unchanged whether we’re at just 25% of our VO2 max or 85%.  That means that whether you’re walking on a treadmill or slamming out 12-mile-an-hour sprints, your body is releasing the same amount of body fat to be used.  Before you say, “Aha!!  I knew that slower, longer cardio sessions were the right thing to do,” you have to differentiate between just releasing fat to be used and actually using it.  If you maintain a slow pace, though the body is releasing much fat to be used, when it’s not used, it simply resynthesizes the glycerols/fatty acids to be re-stored as body fat.  So, we’re still stuck with the questions: Due to a slower rate of usage, should one just perform a longer duration and ultimately use the same amount of calories as someone doing a shorter but harder session?  Which will use more body fat and which will be less catabolic?

Keep in mind that the body has a vested interest in using these fatty acids for energy.  Glucose and glycogen aren’t in endless supply and when activity levels increase, the body needs to turn not only to its larger material source of energy, but it wants to be efficient at it.  As lipolysis is increased, so is the blood flow to the exercising muscles, and so are the chemical processes that convert the fatty acids into usable energy.  Here is yet another twist in the road.  As intensity increases, these glycerols are used at a higher rate – a good thing.  But, when exercise intensity reaches a level where blood flow is necessarily shunted more sharply to the working muscle tissue, blood flow to the available fat stores is restricted tremendously, decreasing the rate of fat that is made available to be used as energy.

If we perform light cardio we release just as much body fat as high-intensity work, thus we’re not risking losing as much muscle, but then due to a slower rate of fat usage, we simply re-store the released body fat.  We can do longer sessions of low-intensity cardio, but after 30 minutes, fat release actually decreases – not increases as conventionally taught.  Trying to keep every variable straight is like trying catch a greased pig.  It’s like the squirrel in the animated movie Ice Age; as soon as you stick every available finger and toe in the leaking wall of ice, another confusing point of physiology springs out of a new crack.  No wonder there isn’t a consensus on the subject.

As I mentioned in the beginning of the article, you would be making a mistake if you take this article as covering every facet of the subject – think of this as just an outline and the book isn’t finished.  Read on for an action plan…

The ease of your body’s ability to burn body fat will affect how much cardio you do.  Being that all of cardio is catabolic, you want to do the least you have to in order to be shredded.  For some that may mean twice a week and for others that may mean twice a day.  Consider two glaring facts: Your body immediately starts releasing body fat with exercise and continues for 30 minutes until the law of diminishing returns virtually eliminates any further benefit.  If we are going at too slow of a pace, the released fat gets re-stored and if we get too high in our intensity, we shunt blood flow away from fat cells.  I believe, therefore, that there are two types of cardio that we can benefit from and still meet our goal of sparing as much muscle as possible.  The first is thirty-minute sessions at a good pace – heart rate sustained at 130-150 beats per minute for most people.  Remember, even at just 25% of our VO2 max we’re going to be releasing all the body fat that we can, but we need a pace that will actually use what is released, but not so much intensity that the body goes into a fight-or-flight mode channeling blood to the muscle tissue systemically and away from the adipose cells and organs.  I also believe the value of super-high-intensity cardio is tremendous but you have to weigh the catabolic effects and the fact that it won’t take long to be counterproductive and decrease the amount of fat actually being released (due to the changing blood flow patterns).  I would recommend using high-intensity sessions 1 to 3 times per week for 15 to 20 minutes to create longer-term fat usage through the increased metabolic effects.  Ectomorphs may have plenty by doing just 10-minute high-intensity sessions, but even endomorphs shouldn’t do more than 2 to 3.  The amount of actual leg muscle recovery necessary should be a limiting factor – you’ll need to recover almost like a leg workout.  The “baseline” 30-minute sessions could be done daily or even twice a day for those who lose weight slowly or have more lower body stores to contend with.

That doesn’t mean that longer, slower cardio is worthless, you just get a fraction of the fat loss after the first 30 minutes.  Breaking up an hour of cardio into two sessions can net more fat loss if the pace is high enough and consistent.  The great thing about human performance research, however, is that we’re still very much in a pioneering phase.  Studies conducted with different variables keep adding to our understanding and more specific information is sure to be discovered.  For now, this is my story and I’m sticking to it.

Dr. Joe Klemczewski