High Intensity Interval Training, or HIIT for
short, has been promoted as one of the most effective training methods
ever to come down the pike, both for fat loss and for cardiovascular
One of the most popular claims for high intensity interval training is that it burns
“9 times more fat” than conventional (steady state)
cardio. This figure was extracted from a study performed by Angelo
Tremblay at Laval University in 1994. But what if I told you that high intensity interval training
has never been proven to be 9 times more effective than regular
cardio… What if I told you that the same study actually
shows that high intensity interval training is 5 times less effective than steady state cardio???
Read on and see the proof for yourself.
In 1994, a study was
published in the scientific journal Metabolism
Angelo Tremblay and his team from the Physical Activity Sciences
Laboratory at Laval University in Quebec, Canada. Based on the results
of this study, you hear personal trainers across the globe claiming
that “HIIT burns 9 times more fat than steady state
This claim has often
been interpreted by the not so scientifically
literate public as meaning something like this: If you burned 3 pounds
of fat in 15 weeks on steady state cardio, you would now burn 27 pounds
of fat in 15 weeks (3 lbs X 9 times better = 27 lbs).
it’s usually not stated as such, frankly, I think
this is what some trainers want you to believe, because the programs
that some trainers promote are based on convincing you of the vast
superiority of high intensity interval training and the “uselessness” of low
intensity exercise is more effective and time efficient
than lower intensity exercise. The question is, how much more
effective? There’s no evidence that the “9 times
more fat loss” claim is true outside the specific context in
which it was mentioned in this study.
In order to get to
the bottom of this, you have to read the full text
of the research paper and you have to look very closely at the results.
13 men and 14 women
age 18 to 32 started the study. They were broken
into two groups, a high intensity interval training program (HIIT)
and a steady state training program which they referred to as endurance
The ET group
completed a 20 week steady state aerobic training program
on a cycle ergometer 4 times a week for 30 minutes, later progressing
to 5 times per week for 45 minutes. The initial intensity was 60% of
maximal heart rate reserve, later increasing to 85%.
The high intensity interval training group
performed 25-30 minutes of continuous exercise at 70% of
maximal heart rate reserve and they also progressively added 35 long
and short interval training sessions over a period of 15 weeks. Short
work intervals started at 10 then 15 bouts of 15 seconds, increasing to
30 seconds. Long intervals started at 5 bouts of 60 seconds, increasing
to 90 seconds. Intensity and duration were progressively increased over
the 15 week period.
The results: 3
times greater fat loss in
the high intensity interval training group
Even though the
energy cost of the exercise performed in the ET group
was twice as high as the HIIT group, the sum of the skinfolds (which
reflects subcutaneous body fat) in the HIIT group was three times lower
than the ET group.
So where did the
“9 times greater fat loss” claim
Well, there was a
difference in energy cost between groups, so in order
to show a comparison of fat loss relative to energy cost, Tremblay
appeared reasonable to
correct changes in subcutaneous fat for the total cost of training.
This was performed by expressing changes in subcutaneous skinfolds per
megajoule of energy expended in each program.”
subjects did not lose 9 times more body fat, in
absolute terms. But hey, 3 times more fat loss? You’ll gladly
take that, right?
Well hold on, because
Did you know that in this
oft-quoted study, neither group lost much weight? In fact, if you look
at the charts, you can see that the HIIT group lost 0.1 kg (63.9 kg
before, 63.8 kg after). Yes, the HIIT group lost a whopping 100 grams
of weight in 15 weeks!
The ET group lost 0.5
kilograms (60.6 kg before,
60.1 kg after).
Naturally, lack of
weight loss while skinfolds
decrease could simply mean that body composition improved (lean mass
increased), but I think it’s important to highlight the fact
that the research study from which the “9 times more
fat” claim was derived did not result in ANY significant
weight loss after 15 weeks.
Based on these
results, if I wanted to manipulate statistics to promote
steady state cardio, I could go around telling people,
“Research study says steady state cardio (endurance training)
results in 5 times more weight loss than high intensity interval
training!” Or the reverse, “Clinical trial proves
that high intensity interval training is 5 times less effective than
steady state cardio!”
Mind you, THIS IS THE
SAME STUDY THAT IS MOST OFTEN QUOTED TO SUPPORT
If I said 5 X greater
weight loss with steady
state, I would be telling the truth, wouldn’t I? (100 grams
of weight loss vs 500 grams?) Of course, that would be misleading
because the weight loss was hardly significant in either group and
because interval training IS highly effective. I’m simply
being a little facetious in order to make a point: Be careful with
statistics. I have seen statistical manipulation used many times in
other contexts to deceive unsuspecting consumers.
advertisements for a popular fat burner claim that use of
their supplement resulted in twice as much fat loss, based on
scientific research. The claim was true. Of course, in the ad, they
forget to tell you that after six months, the control group lost no
weight, while the supplement group lost only 1.0 kilo. Whoop de doo!
ONE KILO of weight loss after going through a six month supply of this
“miracle fat burner!”
Back to the
HIIT story –
there’s even more to it.
In the ET group, there
were some funky skinfold and circumference
measurements. ALL of the skinfold measurements in the ET group either
stayed the same or went down except the calf measurement, which went
The girths and skinfold
measurements in the limbs
went down in the HIIT group, but there wasn’t much difference
between HIIT and ET in the trunk skinfolds. These facts are all very
easy to miss. I didn’t even notice it myself until exercise
physiologist Christian Finn pointed it
out to me. Christian said,
you look at the changes
in the three skinfold measurements taken from the trunk, there
wasn’t that much difference between the steady state group
(-6.3mm) and the HIIT group (-8.7 mm). So, much of the difference in
subcutaneous fat loss between the groups wasn’t because the
HIIT group lost more fat, but because the steady state group actually
gained fat around the calf muscles. We shouldn’t discount
simple measurement error as an explanation for these rather odd
pointed out that the two test groups were not evenly
matched for body composition at the beginning of the study. At the
beginning of the study, the starting body fat based on skinfolds in the
HIIT group was nearly 20% higher than the ET group. He concluded:
while this study is
interesting, weaknesses in the methods used to track changes in body
composition mean that we should treat the results and conclusions with
One beneficial aspect
of HIIT that most trainers forget to mention is
that HIIT may actually suppress your appetite, while steady state
cardio might increase appetite. In a study such as this, however, that
can skew the results. If energy intake were not controlled, then some
of the greater fat loss in the HIIT group could be due to lowered
Last but not least,
I’d like to highlight the words of the
researchers themselves in the conclusion of the paper, which confirms
the effectiveness of HIIT, but also helps put it in perspective a bit:
a given level of energy
expenditure, a high intensity training program induces a greater loss
of subcutaneous fat compared with a training program of moderate
is obvious that high intensity
exercise cannot be prescribed for individuals at risk for health
problems or for obese people who are not used to exercise. In these
cases, the most prudent course remains a low intensity exercise program
with a progressive increase in duration and frequency of
In conclusion, my
intention in writing this article wasn’t to
be controversial, to be a smart-alec or to criticize high intensity interval training. To the
contrary, additional research has continued to support the efficacy of
HIIT for fat loss and fitness, not to mention that it is one of the
most time efficient ways to do cardiovascular training.
I have recommended high intensity interval training for years in my Burn
The Fat, Feed The
program, using a 1:1 long interval approach,
which, while only one of many ways to do HIIT, is probably my personal
favorite method. However, I also recommend steady state cardio and even
low intensity cardio like walking, when it is appropriate.
for writing this article
1. To encourage you to
question where claims come
from, especially if they sound too good to be true.
2. To alert you to how advertisers might use research such as this to
exaggerate with statistics.
3. To encourage the fitness community to swing the pendulum back to
center a bit, by not over-selling the benefits of HIIT beyond what can
be supported by the scientific research.
4. To encourage the fitness community, that even as they praise HIIT,
not to condemn lower and moderate intensity forms of cardio.
As the original
author of the 1994 HIIT study himself pointed out, HIIT
is not for everyone, and cardio should be prescribed with progression.
Also, mountains of other research has proven that walking (GASP! - low
intensity cardio!) has always been one of the most successful exercise
methods for overweight men and women.
There is ample
evidence which says that obesity may be the result of a
very slight daily energy imbalance, which adds up over time. Therefore,
even a small amount of casual exercise or activity, if done
consistently, and not compensated for with increased food intake, could
reverse the obesity trend. High intensity interval training gets the job done fast, but that
doesn’t mean low intensity cardio is useless or that you
should abandon your walking program, if you have the time and if that
is what you enjoy and if that is what’s working for you in
your personal situation.
The mechanisms and
reasons why high intensity interval training works so well are numerous. It goes
way beyond more calories
burned during the workout.