Why Low to Moderate Intensity Training is not more effective for fat loss
In a recent question on Quora a user asked “Is it better to do intervals or steady jogging for fat burning?” I originally answered the question without doing any research and just commented on my personal experience, and mentioned diet is more important for fat loss than exercise. Recently another user submitted an answer that “your body’s first choice is to burn the fuel it has in greater supply – the fat” Now I knew this was not completely true and had recently been reading some old physiology text books so I commented off the top of my head, but I was wrong as well.
So what is the real answer? The fuel choices that the body uses are determined on how quickly they are needed. This is the difference between aerobic (with oxygen) and anaerobic (without oxygen) as many of you probably already know. When the energy requirement has enough time to get oxygen to the cells where energy is created, more free fatty acids (FFAs) can be used for fuel. The break down (catabolism) of fat (lipids) to FFAs is a complex process and takes longer than using glucose as a fuel source. The highest ratio of FFAs to Carbohydrates (CHO) as a fuel source is when resting which uses about 60-80% FFAs and 40-20% CHO (this varies depending on the source that you read, see references here here and here).
This info means a few different things. Firstly to burn the most fat possible you want your basal metabolic rate (BMR) to be as high as possible. Also your fat to carb fuel ratio will be more fat at low to moderate intensity training (LMIT). What this doesn’t mean is that you will burn more fat in total with lower intesity exercise.
To explain further I will reference a recent research paper by Mark J. Smith, Ph.D. that you can find here. To simplify the explanation although during the training period the majority or all of the fuel used will be CHO, the recovery period after training will burn primarily fat when doing HIIT. A quote from Smith
” I equate it to borrowing money from the bank – one has to pay it back and an increase in oxygen consumption above normal resting levels is needed to do so. When it comes to replenishing the energy stores of the body, it makes sense that if one were to predominantly burn glycogen as a fuel during a work out (a HIIT work out would accomplish this), the body would need to put back those glycogen stores to be ready for a subsequent similar activity. Consequently, it would further make sense for the body to utilize fat while it is accomplishing the glycogen “top‐up.”This is exactly what happens and, in a similar fashion, when the body burns predominantly fat during a work out (a LMICT work out would accomplish this, but with a low total caloric expenditure), the body replenishes the fat while burning glycogen during the recovery. Because EPOC can often last longer than the actual work out time, one would want to be utilizing fat as the fuel of choice during this period to stay lean; SIT and HIIT accomplishes this very well.”
In that paper Smith cites multiple studies that have shown HIIT impacts fat loss greater than LMIT. These studies can be found here:
Although fat loss is the primary topic of this post there are many other benefits to HIIT. Tabata style HIIT has been proven to increases V02 max and overall cardio vascular efficiency. HIIT with resistance (weights, bands, bodyweight) will increase muscle mass, which will also contribute to fat loss.
If you have any corrections or a contradicting opinion I would love to hear it in the comments.
For some reason I forgot to post the most important part of an article, the counter point. I am not sure how I missed this but Lyle McDonald has a great post with some good counter points her http://www.bodyrecomposition.com/research-review/research-review-effects-of-exercise-intensity-and-duration-on-the-excess-post-exercise-oxygen-consumption.html