Abstract
Several
studies have examined and compared the effect of exercise intensity on
substrate utilization. The purpose of this paper was to provide insight
on these findings. The following recommendations will be based on the
work of Wilson J. and Wilson G. (2005),
Direct Comparisons of Fuel use during Low, Moderate, and High Intensity
Exercises. For a more comprehensive review
on studies and mechanisms on this topic, refer to that article.
References listed on the bottom are additions to this series on exercise
intensity and fuel utilization. For all other sources cited in the body
of this article, but not referenced, refer to the aforementioned
article.
Introduction
Today, an
historical battle exists. Across countries exercise participants
purport the superiority of high intensity interval training (HIIT) which
is short over low to moderate intensity long duration training. One of
the purposes of this article is to analyze the evidence for this claim
and allow the reader to conclude from there. Shawn Phillips, one of the
leading spokesman for HIIT stated that
You knew
deep down, anyhow, that busting your butt burned off more fat than an
exercise that allowed you to read at the same time, didn't you? Well,
research shows our instincts were right…
HIIT
speeds up your metabolism and keeps it revved up for some time after
your workout. The bottom line is HIIT training burns a greater number of
total calories than low-intensity training, and more calories burned
equals more fat lost. What I'm suggesting is you forget about the
"calories burned" readout on the stairstepper or Lifecycle; if you
practice HIIT training, the majority of calories burned will come after
your workout!
The above
statement paints an appealing picture. In reality however, the
scientific evidence suggests that it is unequivocally false (Laforgia et
al., 1997, Gore and Withers, 1990, Freedman-Akabas, 1985).
Laforgia et al. (1997) suggests that a comparison of the excess
calories above moderate intensity exercise ‘for the interval treatment
is of little physiological significance to the energy balance
of athletes because this amount of energy is equivalent to
the kilojoules in only 75 ml of orange juice (1/3rd
cup).’ They further conclude that ‘the major contribution of both
treatments to weight loss was via the energy expended during the actual
exercise. The excess post exercise energy expenditure is therefore of
negligible physiological significance as far as weight loss is
concerned.’
And there are hundreds of studies which support this (again, refer to
the said article for a massive review on this topic). Even for the most
intense exercise sessions, Excess Post Exercise Oxygen Consumption (EPOC)
is always negligible in comparison to the amount of calories metabolized
during physical activity.
Exercise
Intensity and Fuel Utilization
Since he EPOC argument has been dismantled, the next question is, what
exercise intensity optimizes fat utilization?
Evidence suggests that as exercise intensity decreases, there is an
increased reliance on the peripheral adipose depot, with a concomitant
sparing of carbohydrates, particularly intramuscular glucose polymers.
It appears that total fat oxidation becomes optimal within the range of
60% VO2 max.
For instance, Romijn et al. (1995) examined fuel utilization during
25, 65, and 85% VO2 max. To elaborate, Vo2 max can be defined as the
greatest amount of oxygen you can take in, transport, and utilize at the
cellular level for energy.
Results found that over 85 % of the fuel during 25 % V02 max was
provided by fatty acids from adipose, while 7.5 % was provided by
intramuscular triglycerides. Carbohydrate oxidation provided
approximately 7.5 % of the fuel used. Further, muscle glycogen stores
did not contribute significantly to this number, suggesting almost
complete reliance on peripheral glucose.
In the 65 % condition, 50 % of the fuel was derived from lipid
oxidation, in equal measure from adipose and intramuscular stores.
Carbohydrate utilization provided approximately 50 % of the fuel
utilized, with 80 % of this coming from intramuscular glycogen stores.
The lowest percentage of fat metabolism was found in the 85 %
condition, with only 25 % of the fuel coming from lipid oxidation from
equal measures of adipose and intramuscular TGs.
However, total calories utilized increased as exercise intensity
increased. Of those calories, total fat oxidation was highest in the 65
% V02 max condition, while there was no significant difference between
the 25 and 85 % V02 max conditions. Of those fats utilized, the
greatest amount of lipid oxidation derived from adipose tissue came from
the 25 % V02 max condition, followed by the 65 % V02 max condition.
Adipose derived fat oxidation was lowest in the 85 % V02 max condition.
It would be helpful to elaborate on this with an example. Say someone
exercises for 20 minutes at 25, 65, and 85% VO2 max. And say that the
total amount of calories metabolized were 100, 250, and 300 for the 25,
65, and 85% VO2 max conditions, respectively. Now, to calculate the
amount of fat metabolized during each session, simply multiply by the
percentages listed above. For instance, during the 25% VO2 max
condition, 85% of the fuel probably came from fat, which would mean the
person metabolized 85 calories from fat. For the 85% VO2 max condition,
of the 300 calories metabolized, 75 calories of fat were also
metabolized, but an additional 225 calories from carbohydrates were also
metabolized. Finally, in the 65% vo2 max condition, a whopping 125
calories were metabolized from fat during the same period of exercise.
This is 40% more total fat than both the high and low intensity
conditions.
Other studies confirm these results, with deviations.
In summary evidence suggests that 65% +-5 VO2 max is optimal for total
fat oxidation.
Exercise
Intensity and Muscle Catabolism
All body builders have a sworn mortal enemy—cortisol. This hormone acts
to breakdown muscle tissue, and creates a catabolic environment,
contrary to growth.
The question is: what is the effect of exercise intensity on cortisol?
Davies (1973) examined the effect of duration and intensity on plasma
cortisol levels. It was found that as duration and intensity increased,
cortisol increased. However, they found what appeared to be a threshold
for cortisol secretion at 60 % V02 max intensity. Therefore at lower
intensities cortisol is primarily controlled by metabolic need. In this
context Sotsky et al. (1989) investigated the effect of hypoglycemia on
moderate intensity exercise, below 60 V02 max over 50 minutes of cycling
in participants with normal blood glucose levels of 87 mg / dl, and in
participants with low blood glucose levels of 59 mg / dl. No significant
difference in cortisol levels were found in the normal glucose
condition, while a 400 % increase (!) was found in the low glucose
condition. Therefore it appears that under normal dieting conditions
that cortisol secretion may not significantly rise during an hour of low
intensity exercise, suggesting that it is an effective tool for fat
metabolism, without high catabolic effects.
In summary, cortisol increases proportionally with exercise intensity,
with a threshold at 60% vo2 max; meaning that below 60% VO2 max,
cortisol will be kept at bay. Thus, displaying an advantage to very low
intensity exercise. However, this is not the case under a hypoglycemic
condition. During such a condition, cortisol will increase dramatically
no matter what intensity you train at, displaying the importance of
proper nutrition.
Exercise
Intensity and Performance
To briefly summarize this, exercise intensity appears to be the number
one factor for maintaining performance. For instance, if you were to
lower your volume for 4 weeks, but maintain your intensity (i.e. lift
just as heavy) then you would actually improve in various adaptations.
However, if you were to lower your intensity (i.e. drop from 300 to 250
pounds in the squat lift) adaptations—muscular, strength, enzymatic,
etc.—would quickly be lost.
Incidentally, evidence suggests that glycogen stores (the stored form of
glucose) is directly related to being able to maintain exercise
intensity. Thus, if you are glycogen depleted, intensity, and therefore,
adaptations, will drop.
In this context, high intensity cardio sessions, which have been shown
to deplete your body of precious glycogen stores, would seam inadvisable
for performance in the weight room, demonstrating another problem with
high intensity workouts.
High Intensity
Exercise Followed by Low intensity Exercise
During high intensity exercise, lypolysis (the breakdown of
triglycerides into its glycerol and fatty acid components) is potently
stimulated, because catecholamines (fat metabolizing hormones) are
highest during this intensity. However, alpha receptors (receptors
stimulated by catecholamines) inhibit these fatty acids from being
transported by its protein carrier albumen to the musculature that it
may be oxidized. What Wilson J. and Wilson G. (2005) found, was that
when intensity is lowered sympathetic tone lowers proportionally, and a
high rise in plasma fatty acids is seen (Romijn et al., 1993.) For this
reason, we suggest that a combination of high intensity and low
intensity training protocols may be a highly effective technique for fat
metabolism.
In addition to this, results show that fat oxidation is increased
proportionally to concentrations of fatty acids. For instance, they have
done studies where they have fed participants fat closely to an event,
and this increased fat oxidation during exercise. Thus, this rapid rise
in FFA's should enhance fat oxidation.
To give an example, performing a 10 minute high intensity cardio
session, followed by a 30 minute low to moderate intensity cardio
session, could prove highly efficacious for fat metabolism.
Practical Applications
Evidence suggests that as exercise intensity decreases, there is an
increased reliance on the peripheral adipose depot, with a concomitant
sparing of carbohydrates, particularly intramuscular glucose polymers.
It appears that fat oxidation becomes optimal within the range of 60%
VO2 max. However, training below this range (>50% VO2 max) may be
beneficial, if optimal muscle sparring is a priority, as catabolic
hormones are minimized during low intensity cardio; so long as you are
in a euglycemic state (e.g. your blood glucose levels are stable).
An
analysis of scientific literature demonstrates that the maintenance of
adaptations are intensity specific. The reader is therefore, cautioned
to avoid over reliance of high intensity, glycogen depleting protocols.
High
intensity training may prove beneficial if used properly. For example,
its potent stimulation of whole body lipolysis during exercise leads to
a rapid influx of plasma free fatty acids after intensity is lowered. In
this context, it is postulated that performing a notably short, high
intensity session, followed by a long duration, low to moderate
intensity workout, may optimize lipid oxidation.
Amidst
hypertrophic growth cycles, in which there is a caloric surplus, short,
high intensity workouts may elicit a supplementary anabolic stimulus.
This is attributed to preferential recruitment of type II fibers, which
have the greatest capacity for growth, as well as an increase in
anabolic hormones.
That’s all fine
but…How the heck do I calculate my VO2 max!?!?
Glad you asked!
Well, it is not practical for most to directly measure oxygen
consumption during all exercise sessions. Therefore, an alternative
method is needed.
What results show, is that there is a very close correlation between
heart rate reserve (HRR) and vo2 max (Shepard & Astrand, 2000, p.g.,
14).
To elaborate, HRR is calculated by subtracting your maximum heart rate
by your resting heart rate. A classic example of maximum heart rate is
the Karvonen formula, which is 220 minus your age. Say you have a 20
year old, with a resting heart rate of 60 beats per minute, this
individuals maximum HR would be 200 (220-20) and his HRR would be 140
(200-60).
So, say this individual wants to train at 65% of his VO2 max. He simply
would times his HRR by .65, which would be 91 beats per minute. To
translate this to a normal heart rate, simply add the resting heart rate
to the heart rate reserve. This would add to 150 beats per minute, or
75% of his maximum heart rate.
VO2 Max
Calculator
Does this still not make sense?
No problem! Just use our quick and easy to use VO2 max calculator,
created by Adam “Old School” Knowlden. Simply plug in your age, and
desired VO2 max, and a formula will pop up for what heart rate you
should train at. Click below to use this:
VO2 Max Calculator
Sincerely,
Jacob Wilson
President Abcbodybuilding / The Journal of HYPERplasia Research
Trainer@abcbodybuilding.com
Gabriel “Venom” Wilson
Executive of Bioenergetic Research
Venom@abcbodybuilding.com
And
Adam “Old School” Knowlden
oldschoolabcbbing@gmail.com
President
of Biomechanical Engineering/
Editor and Writer, The Journal of Hyperplasia Research
References
Shepard & Astrand (2000). Endurance in Sport. 2nd addition.
The Encyclopedia of Sports Medicine. Blackwell science. p.g., 14
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Bodybuilding Company. All rights reserved. Disclaimer
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