The Myth of Cardio

endless cardio will break you!


by Dan Schwartz and Dr S Leventhal

Misinformation abounds when it comes to the right way of doing cardio.  For the last several decades, the prevailing wisdom from doctors and fitness experts was as follows: longer bouts of low to moderate-intensity cardio are the most effective way to burn fat and improve longevity.  From molecular biologists to exercise physiologists, a growing number of scientists now have solid evidence to claim that this may have been a myth of good health.  For this post we turn to the field of exercise science to show how the common line on cardio is flawed, and to highlight High Intensity Interval Training (HIIT) as a more efficient and physiologic way to exercise.

I. Consider the Treadmill

Any frequent gym-goer can attest to the scene: a hoard of iPod-wearing steppers and joggers, slogging it out on Treadmills and Ellipticals for the better part of an hour.  Visually this is striking, not just in the ballet-esque choreography, but in the observation that the most strict adherents seem to have the least change in body composition over time.  Biologically, this phenomenon isn’t so surprising.  After all, the body has evolved through Ice Ages and famine to get the most function using as little energy possible.  In the constant quest for survival, our metabolic machinery adapts when it can, seizing opportunities to minimize energy costs by upregulating storage, i.e., keeping on fat.  This adaptation explains the problem with steady-state exercise: the body adjusts to the same amount of cardio, leading to a switch in metabolic program which slows metabolism to conserve energy!  

This response is validated in looking at the biochemistry of endurance athletes.  Results from a study in Germany show significantly higher levels of cortisol in endurance athletes compared to controls, suggesting that endurance exercise may lead to chronically elevated cortisol [1].  While needed for normal function, cortisol at high levels has been linked with increased appetite and importantly, the accumulation of visceral body fat [2].  

A range of other functions have been to shown to be impacted negatively by steady state cardio.  On the cellular level, endurance cardio causes significant oxidative stress and inflammation, contributing to DNA and lipid damage which underlie a variety of chronic disorders [3].  Muscle mass and bone density, which are critical for mobility and overall health in aging, are also known to take a big hit from endurance exercise.  In short, steady state exercise dampens metabolism and produces changes in biochemistry that actually contribute to the onset of chronic illness.

II. The Benefits of Interval Training

Developed as a training program for athletes in sprint sports, HIIT provides a less time-intensive alternative to endurance cardio that has been shown to produce better results in fat burn and an overall boost in metabolic function.  Here are some of the findings from a recent review in the Journal of Obesity:

1. Catecholamines.  Studies suggest that HIIT produces a greater spike in catecholamines, (i.e., hormones like epinephrine/norepinephrine) which act on receptors in fat tissue to stimulate lipolysis, or the breakdown of fat.

2. Mitochondria.  Looking at the biochemistry of skeletal and total body muscle, researchers have learned that interval-training produces dramatic changes in the makeup of mitochondria, which drive the process of fat breakdown.  By analyzing both quantity and activity of mitochondria, it is now understood that interval training enhances both amount and performance of mitochondrial enzymes to a greater extent than in endurance-based programs.

3. Insulin sensitivity.  A hallmark of metabolic syndrome is the inability of cells to respond to the hormone insulin, which enables sugar to migrate from the bloodstream into cells.  Known as insulin resistance, this phenomenon is associated with a range of chronic diseases from Type II Diabetes to heart disease to sleep apnea.  While the mechanism is not entirely understood, HIIT has been shown to counteract insulin resistance by increasing insulin sensitivity between 23 and 58 percent [4].

HIIT is also demonstrably better when it comes to fat burn.  Comparing interval training to steady pace exercise, researchers at the University of New South Wales found a 3-fold higher difference in fat burn (pounds of fat) in an interval-based workout (12 second sprint, 8 second recovery) that was half the length of the endurance workout (20 minutes versus 40 minutes) [5].  The science is clear: interval training generates a much greater return on a significantly smaller time investment!

III. Getting a Routine

Finding a good interval routine can be easy to integrate into a normal workout (e.g., swimming, biking, running etc). Based on comfort level and motivation, alternate between brief spans of intense exercise (working optimally around 80-90 percent of your maximum heart rate between 30 seconds and 2 minutes) and a recovery period long enough to sustain another sprint.  Keep track of your workout on paper, increasing the length of the sprint over time.  Be sure to consult with your physician before starting a routine.

References and Resources

1. Skoluda, N., Dettenborn, L., et al. Elevated Hair Cortisol Concentrations in Endurance Athletes. Psychoneuroendocrinology. September 2011. Published Ahead of Print.


3. Shojaei, E., Farajoy, A., et al. Effect of Moderate Aerobic Cycling on Some Systemic Inflammatory Markers in Healthy Active Collegiate Men. International Journal of General Medicine. January 2011. 24(2), 79-84.

4. Boutcher, Stephen H. "High Intensity Intermittent Exercise and Fat Loss". Journal of Obesity. (2011).




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