Written by TEAM MD
17 February 2021





Training Intensity and Fat Loss
Should Bodybuilders Do Cardio?



By Daniel Gwartney, M.D.



The image of bodybuilding is sweaty, shaven men curling 60-pound dumbbells and benching 315
to the pace of metronomic grunts, pausing only to flex in front of each mirror passed. Anyone who
has actually competed knows that muscle building is the easiest part of the equation. Lifting is
enjoyable, establishes physical dominance, enhances social rank, and entitles one with a sense
of progression.


The opposing component – cutting body fat to a low, single-digit percentage – is
agonizing, fatiguing, socially restricting, and the slightest failing results in visible regression. Most
recreational “bodybuilders” never compete; not because they cannot gain sufficient size. It is the
degree and duration of sacrifice required to reach a body fat of 5 percent or lower that most
cannot tolerate.

That said, successful bodybuilders have found that it is absolutely necessary to “train smarter, not
harder” when it comes to cutting body fat. Though building muscle is “easier,” it is by no means
easy. Skeletal muscle will not grow by wishing it so; it requires the right combination of resistance
training, rest, and nutrition. Drugs and supplements are relatively ineffective without the proper


When the focus is on “getting cut,” many bodybuilders make the mistake of reducing
calories far too low, or restricting carbohydrates or fat so severely that growth and health is
impaired. Further, they embark upon frantic “cardio” sessions, plunging all that hard-earned
muscle into a catastrophic state of catabolism (muscle breakdown). Ironically, in such a state,
human metabolism freaks out since it is undergoing unnatural stress – resulting in more relative
body fat, not less.

Much of the literature regarding fat loss is contradictory to the pursuit of bodybuilders. For the
general public, the measure of success is weight loss, regardless of the fact that as much muscle
is lost as fat. 1 In research investigating calorie burning, or substrate utilization (whether the
calories burned are from fat or sugar), the focus is primarily upon changes occurring during the
exercise, not throughout the day, including later eating and activity patterns.


Lastly, it is very rare to see discussion on any impact on strength performance or muscle mass over the long term.
Keep in mind that there is a clear crossover effect that occurs when intense resistance-based
strength training is combined with high-volume aerobic training. 2 “Cross training” is used by many
recreational athletes to obtain the best of both worlds. However, for the competitive athlete,
efforts directed toward aerobic training may cannibalize strength/mass gains, and vice versa.

Train Smarter, Not Harder

What then is a bodybuilder to do? Again, train smarter, not harder. It is indisputable that training
at a higher intensity will burn more calories over the same period of time, compared to less
intense exercise. However, high-intensity exercise depletes glycogen (stored sugar) from the
active muscles, as well as the liver.


Circulating sugar is directed toward the working muscles, but
above a low threshold, is unable to meet the energy demand, so it begins burning other sources
of caloric energy. Muscles contain a small amount of stored sugar (glycogen) and intramuscular
fat; muscle can also take up fatty acids from the circulation to burn for ATP production (cellular
energy). At low-to-moderate intensity, roughly 40-50 percent of calories come from sugar, and 50-
60 percent from fat. 3


Muscle wants to burn fat – it contains more energy per gram. However, there is a limit to how
much “fat burning” can take place in muscle. Fatty acid oxidation (burning fat for calories) creates
heat and free radicals that can damage other parts of the cell. 4

A close relative to muscle is brown fat, a specialized cell that is packed with mitochondria (the part of the cell where fat burning

occurs). The function of brown fat is not to store fat, but create heat by burning fatty acids to
maintain a constant core body temperature, especially when at rest.


As there is an oxygen-dependent limit to how much energy can be generated through fat burning,
when exercise intensity exceeds this threshold, only the anaerobic (oxygen-independent) process
can offer any further energy. Under this condition, the percentage of calories derived from fat
drops to as low as 30 percent; worse, 3 to 6 percent of calories are generated by burning protein-
breakdown products. 5


To compare apples to apples, it is more appropriate to compare fat burned
when the exercise is limited by the calories spent, not time spent. Readers who wisely spend their
time are now saying, “So what?” The purpose of this article is to convince those interested in fat
loss, particularly if retaining or building muscle is the primary goal, to perform their cardio at a
lower intensity. To demonstrate, let’s refer to a study published in the International Journal of
Sports Medicine. 6

Researchers in Korea followed three groups of obese, sedentary, middle-aged, Korean women.
Realizing that few bodybuilders arise from such a group, accept that the findings are generally
applicable to healthy, young, adult male athletes.


The groups were assigned to either 1) continue their usual sedentary lifestyle; 2) walk three times per week at a VO 2max of 40 to 50 percent,
2) Building up to 400 calories per session by week five; or 3) jog three times per week at a VO 2max of
70 to 75 percent, building up to 400 calories per session by week five. After 12 weeks, a number
of blood and body measures were performed to see which group derived the greatest
cardiorespiratory benefits, body composition change, and improvement in lipid profile (cholesterol
and triglycerides).

Of course, the more intense exercise created greater improvements in cardiorespiratory fitness; a
gardener will never have the heart and lung function of a track athlete. The groups trained at
specified VO 2max ranges, and that was the measure of cardiorespiratory fitness. Logically, the
harder you train toward a measure, the greater the gain. There is no surprise here.


Does this necessarily mean the high-intensity exercise will result in greater loss of subcutaneous
(under the skin) or visceral (inside the abdomen) fat? Nope, let’s cut to the chase. Both low-
intensity and high-intensity exercisers reduced waist circumference (girth), and dropped fat from
the subcutaneous and visceral areas. However, it was the low-intensity group that lost the
greatest (though not statistically significant, due to small group size) amount of abdominal fat
(subcutaneous and visceral). 6


Surprising, considering the high-intensity group had greater
reduction in body mass index that was statistically significant. Body composition was not
recorded, but the suspicion is that the high-intensity group may have lost some lean mass. The
exercising groups both showed improvements in the lipid profile (higher HDL, lower free fatty
acids) as well as reduced insulin resistance (better blood sugar control). Again, the numbers
tended to be better for the low-intensity exercisers, but statistical significance was not reached.


Should Bodybuilders Do Cardio?

Why do bodybuilders do any cardio? Not necessarily for cardiorespiratory fitness – that is not an
essential component of success in this sport. It is to reduce subcutaneous fat. Older bodybuilders
approaching or surpassing mid-life may become more interested in cardiovascular benefits, but
that is a matter of preventative health, not competitive advantage. By the way, the control group
in the Korean study continued to be obese and sedentary.

This flies in the face of all those days of coaching harassment, as most bodybuilders have a
sports background to some degree. How can better results be obtained by working less intently?
The findings in this study agree with many others, and the practical experience of many
bodybuilders. For other athletes, it is a different story as cardiorespiratory fitness is an element of
their sport in most cases.


Several studies have shown that corrected for calories expended (burned), low-intensity exercise is equivalent to high-intensity exercise for weight loss, abdominal fat, or overall body fat. 7-9 fat, or overall body fat.

Also, it appears that exercise duration, not intensity, may be the factor
that correlates best with improving insulin sensitivity. 7,10 The Korean study demonstrated such
improvements, measured as a decrease in insulin resistance (HOMA-IR). 6


Some may question why the high-intensity group did not burn more calories after exercise
compared to the low-intensity group – resulting in greater fat loss, a finding in other studies. 11
Keep in mind, these groups were not on controlled diets, and it has been documented that high-
intensity exercise often results in greater energy intake (eating) during the remainder of the day. 12
Also, the potential for exercise-related fatigue may reduce later-day activity. 13 Both of these
effects were seen in this study, though again, food intake did not quite reach statistical

For the bodybuilder, the only place for high-intensity exercise may be during glycogen-depletion
routines. As reported in Muscular Development, training legs in a state of partial glycogen
depletion resulted in greater fat burning throughout the day. 14 It is unclear if the same would work
for other muscle groups.

For once, exercise research is giving support to the idea of taking it easy to get ahead. If you ask
most competitive bodybuilders, they will tell you most mistakes were a consequence of overdoing
it. Judges at the competitions can tell in a glance which competitors have catabolized lean tissue
by restricting calories or carbohydrates; or by overworking their muscles through marathon
workouts and excess cardio.

What’s the take-home message? If you do not need to build your cardiorespiratory function any
further to maintain a state of health, need to reduce body fat (especially subcutaneous fat), and
have to preserve existing muscle mass – avoid high-intensity aerobic training. Instead, be
consistent and intelligent in following a regular practice of low-to-moderate intensity aerobic


Take advantage of the only time of the day that one is truly in a fasted state, right after
waking up, to maximize the mobilization of stored fat from the fat cells. Even better, go for a walk
outdoors. You will meet your neighbors, likely inspiring them to begin their own fat-loss efforts.


1. Coxon A, Kreitzman S, et al. Rapid weight loss and lean tissue: evidence for comparable body
composition and metabolic rate in differing rates of weight loss. Int J Obes, 1989;13 Suppl 2:179-81.

2. Nader GA. Concurrent strength and endurance training: from molecules to man. Med Sci Sports Exerc,
2006 Nov;38(11):1965-70.

3. Brooks GA. Importance of the “crossover” concept in exercise metabolism. Clin Exp Pharmacol Physiol,
1997 Nov;24(11):889-95.

4. Ricquier D, Bouillaud F. Mitochondrial uncoupling proteins: from mitochondria to the regulation of energy
balance. J Physiol, 2000 Nov 15;529 Pt 1:3-10.

5. Gibala MJ. Regulation of skeletal muscle amino acid metabolism during exercise. Int J Sport Nutr Exerc
Metab, 2001 Mar;11(1):87-108.

6. Cho JK, Lee SH, et al. Randomized controlled trial of training intensity in adiposity. Int J Sports Med, 2011

7. Houmard JA, Tanner CJ, et al. Effect of the volume and intensity of exercise training on insulin sensitivity.
J Appl Physiol, 2004 Jan;96(1):101-6.

8. Nicklas BJ, Rogus EM, et al. Exercise blunts declines in lipolysis and fat oxidation after dietary-induced
weight loss in obese older women. Am J Physiol, 1997 Jul;273(1 Pt 1):E149-55.

9. Slentz CA, Aiken LB, et al. Inactivity, exercise, and visceral fat. STRRIDE: a randomized, controlled study
of exercise intensity and amount. J Appl Physiol, 2005 Oct;99(4):1613-8.

10. Bajpeyi S, Tanner CJ, et al. Effect of exercise intensity and volume on persistence