The big picture: high-intensity cardio vs. low-moderate steady state cardio

10 Mar

Intro

Walk into the gym and take a look at the cardio section. Chances are you will see a mix of individuals pedaling along on a recumbent bike at a low intensity, folks walking at a brisk pace on the treadmills, and of course – those folks who move lightning fast on the elliptical with absolutely no incline or resistance set on the machine.

The bottom line is that everyone has a different goal. And cardiovascular activity needs to be individualized. However, as a trainer I have noticed a common theme with time. Most of these individuals who do the same cardiovascular exercise routine almost daily have not progressed in body composition as much as one would think with the amount of cardio they are doing.

Maybe improved body composition isn’t the main goal and general cardiovascular and physical fitness is the goal. However, if improved body composition and improved athletic performance and work capacity is a goal – listen up.

Quite often I get asked, “What is the best type of cardio?” The answer to that is……. IT DEPENDS.  It depends on many factors including — training goals, performance goals, training status, and other individualized variables. Most people that ask me this question often want to know the best type of cardio for fat loss or optimal body composition.  

The conversation often leads to the person making this statement, “I have heard that low-intensity cardio burns more fat, so shouldn’t I only do that?”  To better answer that question, we must first understand some physiological principles of fat loss.

The Benefits & Physiological Concepts of Cardiovascular Activity

The benefits of regular cardiovascular exercise are vast — including improved blood pressure, improved blood lipid values, increased insulin sensitivity, and the promotion of feelings of well-being.  Not only is cardiovascular exercise successful at improving health outcomes, but it is often used to enhance sport performance and body composition.

The law of thermodynamics explains that energy cannot be created or destroyed, it can only change form. The food we consume contains energy, known as calories. The calories we consume through food are composed of chemical energy that our body, through many different types of metabolic processes, will transfer into mechanical energy.

The metabolism of energy nutrients (protein, carbohydrates, and fats) result in the production of adenosine triphosphate (ATP).  ATP, often called the energy currency, is important for a multitude of physiological functions. While ATP production is a major priority for our body, not all energy consumed is directly transferred to mechanical energy.

Some of this extra energy is stored as triacyglycerol (fat) in the adipocytes.  This stored fat serves as reserve fuel for our body during a period of fasting or starvation.  You can think of this as our body’s savings account. Much like you keep a savings account for unexpected expenses, your body stores fat so that it can survive if starved.

In times of desperate need of fuel, our body will release stored triacyglycerols to be metabolized for ATP production.  Keep this concept in mind throughout the rest of this article as we discuss the role of cardio exercise on fat loss.

The Issue of Supply and Demand

While reading this article, chances are that you are sitting down.  Even though you are sedentary at the moment, your body is releasing stored fat from your adipocytes into the blood stream.  This fat will be taken to a variety of tissues to be metabolized and converted to ATP.  So actually, at rest you are technically burning fat.  So, if this is true, why is exercise so important?

An increase in physical activity places a greater demand for ATP production.  As certain muscles become active, they need ATP for repetitive contraction.  The exercise intensity will dictate the demand of ATP production.  This is very important to understand.  

As we move from sedentary to highly intense exercise, our body must be able to metabolize substrates (fuel sources) to convert into ATP to be able to maintain the given exercise output.

One thing that needs to be noted is the type of substrate that is metabolized to create ATP.  This chart below shows that the intensity of work will determine the percentage of certain substrates that are used by our body.

 graph (1)

Respiratory exchange ratio (RER) is a ratio of carbon dioxide produced to oxygen consumed.  During steady state conditions, RER is used to commonly used as a means of determining the contribution of fat and carbohydrate to energy expenditure.

At rest, an average individual with a balanced diet has an RER of ~0.80.  The chart above shows that 68% of our energy would come from fat and 32% would come from carbohydrate.  

As an individual engages in physical activity, RER will increase, and the intensity of the activity will determine the amount of increase. 

When exercising near maximal capacity, RER is ~1.00 which means that our body is using carbohydrateas the fuel source.  If this is the case, then this chart is showing that lower intensity activities utilize a greater percentage of fat as fuel.  

This is the reason many “fitness experts” tell people to perform low-intensity steady state cardio for fat loss, and from the information presented above this seems logical.

The Overarching Determinant: Energy Balance and Expenditure

What if I told you the percentage of fat that is burned during exercise isn’t really the most important aspect of cardio for fat loss?  Yes, that is exactly what I am saying.  It doesn’t matter how much fat you burn during exercise. But what does matter is how much fat you burn throughout the day.

In other words, the most important aspect is daily energy expenditure, and in this case maximizing calories burned from exercise. 

The goal for your cardio should be to burn as many calories as possible in the time that you spend exercising.  Remember, the intensity of the exercise will determine the amount of energy expended.  

The following scenario will demonstrate how low-intensity cardio is not as optimal as most people think.

A 200lb male wants to lose weight.  He has two options of cardio to promote fat loss.

1)    30 minutes of walking on a treadmill at 3.5 mph with 3% grade

vs.

2)     20 total minutes of high-intensity interval training (HIIT):

5 minute warm-up and cool down using a treadmill set at 3.5 mph and 0% grade.

Ten 20-second sprints at 10 mph and 0% grade followed by 40 seconds of walking at 3.5 mph at 0% grade.

Which is more effective?

Option 2.  Don’t believe me?  Let’s take a look why.

1)      Walking on a treadmill would elicit ~245 calories burned in 30 minutes.

2)      Performing the HIIT, the same individual would burn ~160 Kcals in 20 minutes. 

EPOC: The “afterburn effect”

But wait, walking on a treadmill at a steady state burns more calories than HIIT. Yes, at face value. Your treadmill will likely read 245 calories burned at the conclusion of your cardio session, but there is a forest beyond the trees here.

One thing that I haven’t yet mentioned, and most people don’t know about is excess post-exercise oxygen consumption (EPOC). 

EPOC refers to the time after exercise where the body is trying to get back to its resting physiological state.  Think of after you get done walking or running.  You don’t immediately go back to normal respiratory function. Depending on the intensity of the exercise, your body is still trying to recover.  Again, intensity plays a significant role in determining the degree of EPOC.

0

Mike doing work on the weighted push sled. High intensity cardio at its finest.

Let’s take a look at this graph.

graph2

The yellow on the picture above is the EPOC.  Notice that the high intensity cardio has a significantly higher EPOC than the low intensity cardio. 

This because it takes a much longer period of time to return all physiological systems back to baseline after working near maximal capacity.During this recovery period, you still have an elevated heart rate, ventilation, and metabolism. 

This elevated metabolism can last for hours, which plays a significant role in weight loss.  Because our metabolism stays elevated, we are now burning more calories at rest throughout the remainder of the day.  At the end of the day, a greater amount stored body fat will have been released and catabolized for energy production.  

As you can see, the amount of fat you burn during exercise is not as important as total calories burned throughout the day.  Fat loss will occur regardless if you shift your focus to expending as many calories in the amount of time that you have available.  

Remember, focusing on the substrate oxidized, whether it is primarily from fat or carbohydrate, during the cardio session is not as important as the total calorie expenditure. Big picture, folks — always keep it in mind!

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3 Responses to “The big picture: high-intensity cardio vs. low-moderate steady state cardio”

  1. Kyle Radaker May 16, 2016 at 8:55 pm #

    Your example above makes perfect sense and is theoretically accurate. However, doesn’t the issue return back to what kind of energy one is burning while recovering to their baseline physiological state?
    If someone’s RER is below .85 while recovering, for example, they would be burning more fat in the EPOC state. However, if above .85, they would be burning more carbohydrate.
    And in the case of having an elevated RER during recovery and/or at rest, we could hypothesize the need for a base training phase until the RER returns to a higher fat burn during recovery!
    So your initial statement questioning which is best, and the answer “it depends” is correct when answering a steady state training mode versus HIIT as well.

    • Cornerstone Powerlifting & Performance May 17, 2016 at 2:06 am #

      Hey Kyle,

      “It depends” is almost always going to be an appropriate answer when it comes to individual physiological responses to training. In regards to your statement about having greater fat oxidation during recovery following a base training phase, this is what we would expect to occur. Aerobic training increases mitochrondrial density, hemoglobin and myoglobin, blood plasma volume, and lipolytic enzymes and transporters. These adaptations to training will allow for greater fat utilization and during recovery, I would expect this to have an enhanced effect following an intense bout of exercise. If the exercise is very intense, CHO will become the primary substrate utilized, but those that are more aerobically trained, will have a quicker switch back to fat utilization as the energy source during recovery.

      As with most training-related responses, they vary among individuals based on their genetics, training status, and other factors.

      Thanks for the comment and taking the time to read the article.

Trackbacks/Pingbacks

  1. Cardio! | femmefitale - June 6, 2014

    […] (for a more scientific explanation, check out Fit and Fueled’s article on cardio here.) […]

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