About a year Pete wrote a great article comparing intervals versus slow, moderate intensity, cardio for fat loss.   It was an attempt to address the age old question of which type of cardio is best for fat loss.  

 

The dogmatic and reflexive answer to that question in the fitness industry is of course higher intensity work can burn fat more efficiently and effectively.  

 

But when Pete summarized results from an important meta-analysis looking at body composition changes, the effects on fat loss weren’t substantially different.  So was this a devastating blow to the high intensity interval zealot crowd? OR Was that review an indication that we need to better understand the nuance of this issue.   

 

In that genre of research, fat loss was measured by individual body composition before and after an interval program or steady state cardio program that was 4 weeks or more in duration.  Essentially, how much lean muscle mass and fat mass do you have before and after the study period.  It’s a crude 30,000 foot view of the issue, which is often helpful in pointing us in a direction, but potentially incomplete or insufficient to definitively resolve the issue.      

 

A recently published meta-analysis took a stab at this same question using a different outcome measure, fat oxidation through indirect calorimetry.  Instead of measuring body composition before and after the research period, these studies included in this review evaluated the ratio of respiratory exchange of oxygen and carbon dioxide.  This gas exchange measurement has been shown to correlate well to fat loss.  

 

All of the participants included in these studies exercised on a bike or treadmill in the lab, wearing a mask, measuring gas exchange during a specific exercise protocol (mostly) 3 sessions per week.  

 

You might argue that measuring the primary outcome measure of body fat seems more significant than respiratory measure that correlate well to fat loss…however the advantage of this type of research is it’s conducted in highly controlled lab environment with real time data.  In the research world more rigorous control over the study design means better confidence in answering the primary question being tested.    

 

18 studies, with over 500 participants, were included in this systematic review and meta-analysis of high intensity interval training (HIIT) and sprint interval training (SIT) versus moderate intensity continuous training (MICT) or no training at all.

 

High intensity interval training was defined to be as inclusive as possible in this review setting a relatively low threshold of 75% or greater using heart rate or VO2.  Sprint interval training is much higher intensity output, think of it as a supramaximal output, that can be sustained for only 30 seconds or less and requires much longer rest intervals in between bouts. 

 

HIIT and SIT were both found effective at increasing fat oxidation compared to no exercise at all (didn’t need the study for that conclusion).  The authors categorized that effect as small but much larger for individuals who are overweight or obese or for people who engage in this type of training for longer periods of time (ie. 12 weeks or longer).  Not surprisingly if you have more fat to lose to start with you have a greater potential for fat loss.  This is the law of diminishing returns at work.  The dose specific response for HIIT and SIT on fat oxidation is also important; 4 weeks of training may give you a fat loss boost, but keep at it for longer and the results are cumulative.

 

9 of the 11 studies that compared HIIT or SIT to MICT found greater fat oxidation with higher intensity work.  The weighted conclusion pointed (although modestly) in the direction of an advantage for HIIT or SIT.

 

And so, in this alternative review, from an alternative body of research, the pendulum now swings back in favour of higher intensity work offering a (slightly) greater advantage.

 

This is in contrast to Pete’s conclusion to this same question just a year ago.  Two high quality systematic reviews with meta-analysis, using slightly different outcome measures, and two different conclusions.  Isn’t dissecting the research fun 🙂

 

From a practical perspective you can argue that adding sprint training or high intensity training offers incremental gains in fat loss over steady state cardio.  And it is a much more time efficient way to get in a great workout.  7 minutes or less, after a warm up, (5 bouts of up to 30 seconds with at least a minute rest between each) and you are on your way.  However, forcing yourself to sprint may not be worth the psychological torment for a minor bump in fat loss.

 

The enjoyment factor is a very real concern.  It’s so significant that Porteguese researchers have developed a questionnaire to help clinicians better understand client exercise preferences. https://link.springer.com/article/10.1007/s12144-021-01718-3 The perfect exercise routine you don’t want to do isn’t going to last very long.  

 

We’ve said this many times before: consistency in your program trumps all.  Do a form of exercise that excites, motivates and/or inspires you.  Because without that foundation you can’t provide a frequent enough stimulus to develop the physical quality you are looking to improve (whether it’s flexibility, strength or cardio).

 

So what’s the best type of cardio for fat loss?  It’s the one you love doing the most; and if you love them all, then HIIT and sprint for the fat loss win!