If you’re a bodybuilder - or want to look like one - it’s safe to say that you want to be as lean as possible. And no matter how clean your diet is, body fat is always going to be an issue. If you’re gaining, you want to minimize fat gains, and if you’re cutting, you want to accelerate fat loss. While doing cardio for cardiovascular tness is important, the main reason most body- builders do cardio is to get as lean as possible. And certainly cardio can help with fat loss: it’s one of the things I recommend to people.
In the traditional view of things, prolonged, steady-state cardio is the best
means for either reducing or keeping fat gains down, thanks to what is
known as the “Fat Burning Zone”.
This requires some explanation...when we eat food, our bodies convert it
into forms that are used to fuel our activities. Whether we’re sleeping or
training, we burn the same preferred substrates for fuel: carbohydrate and
fat. The only thing that changes is the percentage of each that gets used
for di erent activities.
Both carbohydrates and fat combine with oxygen (O2) during the process
of being broken down to carbon dioxide (CO ) and water. Thus, it’s pos-
sible to measure the contribution that each makes by measuring the ratio
of the gases inhaled vs. those exhaled during exercise. The volume of CO
produced divided by the volume of O consumed is called the Respiratory
Exchange Ratio (RER).
There are some equations involved, and I won’t bore you with them. The
bottom line is: if only fat is being utilized for energy, the RER has a value
of 0.7. If only carbohydrate is being used for energy, the RER works out
to 1.0. An RER value between 0.7 and 1.0 means that a mixture of fat and
carbohydrate is being broken down for energy. It’s also possible to have
an RER larger than 1.0: this means that CO is being produced faster than
O2 is being taken in.
There are a lot of technical details involved, but the main point to remem-
ber is this: as the measured RER approaches 1.0, the percentage of fat be-
ing burned for energy decreases
Why is this important? Because measurements of RER have shown that
higher intensity exercise uses proportionally less fat to fuel the activity than
lower intensity exercise does. One small study on 9 male football players
exercised at di erent intensities came up with the following numbers:
Exercise Level (km/h)
Rest
8
10
12
14
16
18
(From Maiolo et al., 2003)
Mean RER
0.74
0.83
0.88
0.90
0.99
1.00
1.09
As you can see from the data in the table, RER increases with the exercise
level. This means that a greater amount of fat is being utilized at rest and
during lower intensity exercise, while little or no fat is being used at the
highest intensities.
So to tie it all together, the relationship between exercise intensity and
fat oxidation during exercise is behind the idea of the “Fat Burning Zone”
(FBZ). The FBZ is the level of intensity that maximizes the use of fat to fuel
the exercise. The FBZ seemed to be con rmed by research. For example,
one study compared the fat and carbohydrate utilization of ten men, who
cycled either at 33% VO max or 66% VO max on separate days. The study
found that the lower intensity exercise burned more fat (42.4 g vs. 24 g)
and less carbohydrate (142.5 g vs. 188.8 g) than the moderate intensity ex-
ercise.
A 2002 study, “Determination of the exercise intensity that elicits maximal
fat oxidation’” provided additional con rmation. The study monitored the
fat oxidation rates of 18 moderately trained cyclists over a series of pro-
longed, continuous exercise tests using constant work rates. The research-
ers found that the range for optimal fat oxidation was from 55% to 72% VO
2
max. This corresponds roughly to 69% - 80% MHR (maximum heart rate).
This was a small study with a lot of variation between the subjects, however.
A later study using a much larger group of untrained men and women put
“Fatmax” at 48.3% VO2max, or approx. 61.5% MHR
So exercising in the FBZ must be the best way to work o excess body fat,
right?
Maybe not: one problem with the FBZ concept is that only the ratio of the
fat vs. carbohydrate being burned for energy is considered; not the over-
all fat calories or total calories burned. Lower intensity exercise may use
a higher percentage of fat calories, but burns fewer calories overall than
higher intensity work. You can burn just as many total fat calories if you
increase the intensity: the percentage of fat used may be smaller, but since
you’re burning more total calories, it evens out.
I think most people trying to lose fat would agree that burning more calo-
ries is better than burning less.
More importantly, measurements made only during exercise are mislead-
ing, since they don’t account for changes in fat oxidation or metabolism
that occur after exercise. As you increase the intensity, you also increase
the impact that exercise has on post-exercise metabolism. In one study on
college-aged women, for example, fat oxidation during a 3 hour recovery
period was considerably higher after cycling sessions at 75% VO2max that
used the same amount of energy (500 kcal) as longer sessions at 50% VO2
max. The bottom line is that there were no signi cant di erences in total
fat oxidation between the two groups over the measurement period. The
researchers also reported that the rate of fat oxidation in the high intensity
group remained elevated even at the end of the 3 hour recovery period.
Another study on 24-hour energy expenditure (EE) and substrate oxidation
patterns in men, showed there was no advantage to low intensity exercise
with regard to fat oxidation. The researchers concluded:
“Similarly, the di erences in HI and LI exercise, RQ are compensated postexer-
cise leading to similar substrate oxidation patterns over 24 h independently of
the level of exercise intensity.”*
Similar results have been obtained by others. As noted in a 2001 study:
*RQ is an alternate term for RER.
“Furthermore, low- and high-intensity aerobic exercise, matched for energy
expended during exercise, have similar e ects on 24-h nutrient oxidation. We
therefore conclude that low-intensity exercise does not promote greater “fat
burning,” as has been popularized among the lay press.”
Now you might conclude from this that it doesn’t matter which kind of
exercise you do - the end result is the same. But in the 2004 study, for ex-
ample, the high intensity (HI) exercise bouts lasted only 30 minutes each,
vs. 60 minutes for the low-intensity (LI) sessions. So equivalent results were
obtained in half the exercise time. To quote again from the 2001 paper:
“Given that time is a limiting factor for most individuals, we would also suggest
that, if the goal of exercise is to maximize fat oxidation to better regulate body
fat mass, then exercise should be performed at the highest intensity that can
be comfortably maintained. “
So what’s the take home lesson? There’s little point to spending long peri-
ods of time doing low-to-moderate intensity cardio for fat loss...unless you
enjoy that sort of thing. When it comes to fat burning, higher intensity
exercise works just as well as lower-intensity exercise, and you’ll spend less
time doing it. Looks like a bargain to me.
There are also other reasons to raise the intensity of your cardiovascular
exercise that extend beyond simple fat burning, which are discussed in the
next section.
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